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HVAC Design Criteria and Guidelines
DIRECT DIGITAL CONTROLS FUNDAMENTALS
Section
Topic
1
Introduction
2
Fundamentals of HVAC Control
3
Input/Output Basics
Input Devices
Output Devices
4
Final Control Elements
Control Valves
Control Dampers
5
Direct Digital Controllers and Systems
Controllers and Control Loops
DDC Systems and Control
Architecture and Networks
6
DDC Communication and Protocols
7
The DDC Design Process
The Design Process
Specific DDC Design Concerns
8
Commissioning HVAC Systems Controls
Appendix
Glossary of DDC Terms
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HVAC Design Criteria and Guidelines
Section 1: INTRODUCTION
Direct digital control (DDC) systems allow HVAC design engineers to easily accomplish things that would
not have even been dreamed of 25 years ago. But, if a designer is not careful, the capabilities of DDC also
can allow him or her to do dumb things very quickly and with great precision. For example, the same control
network that allows us to integrate and optimize the performance of a campus chilled water system served
by multiple chiller plants with thousands of tons of capacity at each location also can burn up one or more
chiller starters when it attempts to cycle a starter seven or eight times a minute because of a control loop
program error.
Many HVAC engineers find the ever-changing DDC technology difficult to understand and work with. This is
not surprising considering that most HVAC designers have a mechanical engineering background with no
real expertise in electronics, computers, networks, communications protocols, etc. Fortunately, you do not
have to be a computer expert to obtain good control systems for your HVAC processes. The real key is to
understand and then fully describe how the HVAC systems, subsystems, and individual components are
intended to function. This takes us back to HVAC fundamentals, which do not change much over time, and,
more importantly, you are well-acquainted with them. The trick, then, comes in your clearly communicating
process requirements in the contract documents.
HVAC design engineers must treat controls design seriously…it is, after all, the key to intended and effective
HVAC systems operation. There are several elements to this:
1. Controls design cannot be properly performed if put off until the end of the HVAC design
process. An HVAC system’s mechanical and controls designs are interdependent. Therefore,
temperature controls design is an iterative process that must be performed as part of the overall
HVAC design process, beginning in the Schematics Design phase. Leaving the controls design to
the end of the project will greatly increase the chance that neither the HVAC system nor the
controls will work well.
2. Detailed operational sequences and control point definitions are the most important components
to control system design. It should be obvious that a controls contractor cannot reasonably be
expected to extrapolate an HVAC system’s intended operation sequence from the mechanical
plans and specifications alone. This is important for two reasons:
a. The cost of a DDC system is primarily determined by the point quantity and types of
points (analog points are much more expensive than digital points). While a controls
contractor can reasonably extrapolate a points list from a well-written operation sequence,
making this effort part of the bid process increases risk that the winning contractor will not
provide the intended system.
b. The process of developing point definitions provides the engineer with an excellent
check of the operation sequence and the rest of the control system design.
Without a well-engineered and well-specified control system design, the contractor cannot reasonably be
expected to provide what is intended.
Section 2: FUNDAMENTALS OF HVAC CONTROL
Every HVAC process is controlled by three elements: a sensor or other input device, a controller, and a
controlled or output device (see Sections 3 and 4).
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HVAC Design Criteria and Guidelines
The sensor measures the controlled medium or other control input in an accurate and repeatable manner.
Common HVAC sensors are used to measure temperature, pressure, relative humidity, airflow state. etc.
Other variables may also be measured that impact the controller logic. Examples include other
temperatures, time-of-day, or the current electrical demand condition. Additional input information (sensed
data) that influences the control logic may include the status of other parameters (airflow, water flow,
current) or safety (fire, smoke, high/low temperature limit, or any number of other physical parameters).
Sensors can be the first, as well as the weakest link in the chain of control. (See Section 3 for a more
detailed discussion.)
The controller processes data that is input from the sensor, applies the logic of control, and causes an
output action to be generated. This signal may be sent directly to the controlled device or to other logical
control functions and ultimately to the controlled device. The controller’s function is to compare it’s input
(from the sensor) with a set of instructions such as setpoint, throttling range, and action, then produce an
appropriate output signal. This is the logic of control. It usually consists of a control response along with
other logical decisions that are unique to the specific control application. How the controller functions is
referred to as the control response. Control responses are typically one the following:
1. Two-position control compares the value of an analog or variable input with the programmed
instructions and generates a digital (two-position) output. The instructions involve the definition of
an upper and lower limit for the measured variable. The output changes its value as the input
crosses these limit values. There are no standards for defining these limits. The most common
terminology used is setpoint and differential or throttling range. The setpoint, plus (or minus) half
the differential, indicates the point where the output “pulls-in,” “energizes”, or is “true.” The output
changes back, or “drops-out”, after the input value crosses through the value equal to the setpoint
minus (or plus) half the differential.
Two-position control can be used for simple control loops (temperature control) or limit control
(freezestats, outside air temperature limits). The input can be any measured variable including
temperature, relative humidity, pressure, current, liquid level, etc.
Time can also be the input to a two-position control response. This control response functions like a
time clock with pins. The output “pulls-in” when the time is in the defined “on” time and drops out
during the defined “off” time.
A common application of this type of control is the residential heating system. With a setpoint of
70° F and four-degree differential, the thermostat (the "controller" will energize the heating system
when the space temperature falls to 68° F and turn it off when the temperature rises to 72° F in the
space. Because of thermal lag, the actual operating differential will be somewhat larger than the
controller differential. The result of this control is shown in the following figure:
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HVAC Design Criteria and Guidelines
2. Floating control is a control response that produces two possible digital outputs based on a
change in a variable input. One output increases the signal to the controlled device, while the other
output decreases the signal to the controlled device. This control response also involves an upper
and lower limit with the output changing as the variable input crosses these limits. Again, there are
no standards for defining these limits, but the terms setpoint and deadband are common. The
setpoint sets a midpoint and the deadband sets the difference between the upper and lower limits.
When the measured variable is within the deadband (or neutral zone), neither output is energized
and the controlled device does not change, it stays in its last position. For this control response to
be stable, the sensor must sense the effect of the controlled device movement very rapidly.
Floating control does not function well where there is significant thermodynamic lag in the control
loop. Fast airside control loops respond well to floating control. An example of floating controls is
shown in the following figure:
Proportional control response produces an analog or variable output change in proportion to the
change of a varying input. In this control response, there is a linear relationship between the input
and the output. Setpoint, throttling range, and action typically define this relationship. There is a
unique value of the measured variable that corresponds to full travel of the controlled device and a
unique value that corresponds to zero travel on the controlled device. The change in the measured
variable that causes the controlled device to move from fully closed to fully open is called the
throttling range. It is within this range that the control loop will control, assuming that the system
has the capacity to meet the requirements.
The action dictates the slope of the control response. In a direct acting proportional control
response, the output will rise with an increase in the measured variable. In a reverse acting
response, the output will decrease as the measured variable increases. The setpoint is an
instruction to the control loop and corresponds to a specified value of the controlled device, usually
half-travel. An example is shown in the following figure:
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HVAC Design Criteria and Guidelines
With proportional control, the final control element moves to a position proportional to the deviation
of the value of the controlled variable from the setpoint, in a linear manner. The final control
element is seldom in the middle of its range because of the linear relationship between the position
of the final control element and the value of the controlled variable. The setpoint is typically in the
middle of the throttling range, so there is usually an offset between the control point and the
setpoint.
In digital control logic, proportional control can be represented mathematically as follows:
V = (K x E) + M
where
V
K
E
M
= output (control) signal
= proportionality constant (gain) = sensor span/throttling range
= deviation (control point – setpoint)
= value of the output when the deviation is zero, usually the output value at the
middle of the output range
3. Proportional plus integral (PI) control involves the measurement of the offset or “error” that can
occur with proportional control over time. This error is integrated and a final adjustment is made to
the output signal from the proportional part of this model. This type of control response will use the
control loop to reduce the offset to zero. A well set-up PI control loop will operate in a narrow band
close to the setpoint. It will not operate over the entire throttling range.
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HVAC Design Criteria and Guidelines
PI control loops do not perform well when setpoints are dynamic, where sudden load changes
occur, or if the throttling range is small.
In digital control logic, proportional plus integral (PI) control is represented as follows:
V = (K x E) + M + (K x T1) E dt
where
V
K
E
M
T1
K/T1
dt
= output (control) signal
= proportionality constant (gain) = sensor span/throttling range
= deviation = (control point – setpoint)
= value of the output when the deviation is zero
= reset time
= reset gain
= differential of time (increment in time)
Reset of the control point is not instantaneous. Whenever the load changes, the controlled variable
changes, producing an offset. The proportional controller makes an immediate correction, which
usually still leaves an offset. The integral function of the controller then makes control corrections
over time to bring the control point back to setpoint.
Integral windup can occur with PI controllers. This is an excessive overshoot condition caused by
the integral function making continued correction while waiting for feedback on the effects of its
prior correction(s). Integral windup can occur when the controlled system is off, the heating or
cooling medium fails or is not available, or one control loop overrides or limits another. DDC
systems generally must have control software to prevent integral windup.
4. Proportional plus integral plus derivative (PID) control adds a predictive element to the control
response. In addition to the proportional and integral calculation, the derivative or slope of the
control response will be computed. This calculation will have the effect of dampening a control
response that is returning to setpoint so quickly that it will “overshoot” the setpoint.
In digital logic, proportional plus integral plus derivative (PID) control is represented as follows:
V = (K x E) + M + (K x T1) E dt + (K x TD x dE/dt)
where
V
K
E
M
T1
K/T1
dt
TD
KTD
dE/dt
= output (control) signal
= proportionality constant (gain) = sensor span/throttling range
= deviation = (control point – setpoint)
= value of the output when the deviation is zero
= reset time
= reset gain
= differential of time (increment in time)
= rate time (time interval by which the derivative advances the effect of proportional
action)
= rate gain constant
= derivative of the deviation with respect to time (error signal rate of change)
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HVAC Design Criteria and Guidelines
Adding the derivative function to create PID control is a labor intensive and time consuming
process to implement, requiring "tuning" and retuning of the control loop to eliminate instability.
Few HVAC processes require PID control and applying it for those that don't need it can create
more problems than it solves...add the derivative function only after proportional, and then PI,
control fail to work satisfactorily.
You must evaluate the specific requirements associated with each control loop to select the proper control
mode:
1.
2.
3.
4.
What degree of accuracy is required?
What amount of offset, if any, is acceptable?
What types of load changes are anticipated (size, rate, frequency, and duration)?
What are the system characteristics (time lag, reaction rate)?
The following can be used as a general guide to selecting control modes for HVAC processes:
Control Application
Space Temperature
Fan Static Pressure
Control Mode
P (Floating control is acceptable only for
simple comfort air-conditioning applications
with a wide throttling range)
PI
PI (cooling), P (heating)
P
PI (with wide throttling range and fast reset
rate) or PID if PI proves unstable
PI
Water Pressure
P
Humidity
Dewpoint Temperature
P (PI if throttling range is 5% or less)
P (PI if throttling range is 2F or less)
Mixed Air Temperature
Coil Discharge Temperature
Hot Water Supply Temperature
Airflow or water flow
A controlled device is a device that responds to the signal from the controller, or the control logic, and
changes the condition of the controlled medium or the state of the end device. (See Sections 3 and 4 for
more detailed discussion of these elements.)
Section 3: INPUT/OUTPUT BASICS
A digital input (DI) typically consists of a power supply (voltage source), a switch, and a voltage-sensing
device (analog-to-digital converter). Depending on the switch’s open/closed status, the sensing device
detects a voltage or no voltage condition, which in turn generates a logical/binary 0 or 1, on or off, alarm or
normal, or similar "either-or" defined state.
A digital output (DO) typically consists of a switch (either mechanical as with a relay, or electronic as in a
transistor or triac) that either opens or closes the circuit between two terminals depending on the binary
state of the output.
An analog input (AI) is a measurable electrical signal with a defined range that is generated by a sensor and
received by a controller. The analog input varies continuously in a definable manner in relation to the
measured property.
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HVAC Design Criteria and Guidelines
The analog signals generated by some types of sensors must be conditioned by converting to a higher-level
standard signal that can be transmitted over wires to the receiving controller. Analog inputs are converted to
digital signals by the analog-to-digital (A/D) converter typically located at the controller. Analog-to-digital
conversion is limited to a small range of DC voltage, so that internal or external input circuitry must change
the character of non-compatible signal types to a DC voltage range within the limits of the A/D converter.
There are basically three types of analog input signals; voltage, current, and resistance.
Common voltage signals used in the controls industry are 1-5 Volts Direct Current (VDC), 2-10 VDC, 3-15
VDC, 0-5 VDC, 0-10 VDC and 0-15 VDC. The 4-20 mA signal has become the industry’s most commonly
used signal for use with analog and digital controllers. Resistance measurement is most commonly
associated with direct inputs from temperature sensing devices, such as thermistors and RTD's. RTD
nominal resistances are typically 100 , 500, 1000  or 2000. Common thermistor nominal resistances
are 2252 , 3k , 10k, 20 k or 100 k .
An analog output (AO) is a measurable electrical signal with a defined range that is generated by a controller
and sent to a controlled device, such as a variable speed drive or actuator. Changes in the analog output
cause changes in the controlled device that result in changes in the controlled process. Controllers first
produce a digital output that is then converted to an analog signal. The analog circuitry is typically limited to
a single range of voltage or current, such that output transducers are required to provide an output signal
that is compatible with controlled devices using something other than the controller's standard signal.
There are four common types of analog outputs; voltage, current, resistance, and pneumatic. Voltage,
current, and resistance ranges are the same as for analog inputs. Common output pneumatic ranges are 020 psi and 0-15 psi.
Inputs and outputs can also be used in special configurations, such as accumulating points, pulse width
modulated (PWM) signals, and tri-state or floating points.
Accumulating points are typically associated with inputs and are special in that during each scan
the controller adds the input point value to the accumulated value. Accumulating points may have
either analog or digital input.
One of the most common applications of accumulating points is with turbine-type flow meters,
which generate a pulse or change of input state with each rotation of the turbine rotor. The total
number of pulses is proportional to the volume of fluid passing through the meter. The number of
pulses per unit of time is proportional to the flow rate during that time interval. Accumulating points
are also used to determine energy quantities, such as kilowatt-hours from a power sensor and
MBtu from flow and temperature sensors.
PWM signals are based on the amount of time a digital output circuit is closed over a fixed time
base (usually 2.85 to 25.6 seconds). This amount of time can range from 0 to 100 percent of the
time base, providing an analog value for each time period that represents the time base of the
signal.
A tri-state (or "triac") signal consists of two digital signals used together to provide three
commands. This type of signal is commonly used to operate a damper or valve actuator in a
modulating fashion, but may also be used with a transducer to generate an analog signal. If both
digital outputs are "off", the actuator does not move. Output 1 "on" will cause movement in one
direction; output 2 "on" will cause movement in the other direction. The fourth possible signal (both
outputs "on") is not used in tri-state operation. The concept was initially developed to allow electric
controls consisting of single pole, double throw switches with a center-off position to control
actuators in a modulating fashion. Modulating operation is achieved by this action because the
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HVAC Design Criteria and Guidelines
actuators being controlled drive slowly so the change in position is proportional to the amount of
time the output remains energized.
Input Devices
Digital input devices: A switch, in one form or another, is the most common device used to
complete a DI circuit.
A switch is an electrical device used to enable or disable flow of electrical current in an electrical
circuit. Switches may be actuated in a variety of ways, including movement of two conducting
materials into direct contact (mechanical), or changing the properties of a semi-conducting material
by the application of voltage (electronic).
Switches are typically rated in terms of voltage, voltage type (AC or DC), current carrying capacity,
current interrupting capacity, configuration, and load characteristic (inductive or resistive). Also
specified are applicable ranges of ambient conditions over which the ratings are valid. Current
carrying capacity (or current rating) is the maximum current that may continuously flow through the
closed switch contacts without exceeding the maximum permissible temperature.
Process property sensing (flow, level, etc.) switches are also rated by parameters such as
adjustment range, accuracy or repeatability, and deadband or differential. The range of a control
switch is specified by upper and lower process values between which the switch has been
designed to operate. The accuracy or repeatability of a control switch is a value typically measured
in process units or percent of range that represents the expected maximum deviation from setpoint
at which the switch will operate under test conditions. The switch differential or deadband is the
change in process value required to cause the state of the switch to change. For example, a
pressure switch that makes at 10 psig and breaks at 8 psig has a 2 psig differential.
Switch contacts are characterized in much the same way as relay contacts.
The following figures illustrate the most common contact configurations, using industry standard
terminology and symbols:
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HVAC Design Criteria and Guidelines
Manual (or "hand") switches are used as digital input devices and in hardwired electrical control
circuits associated with digital outputs. Hand switches come in numerous sizes, shapes, and
configurations. Common switch types include rotary, selector type, toggle, and pushbutton.
Selector and toggle switches are almost always maintained contact type. Pushbuttons may be
momentary or maintained contact type. Selector switches can have key operators to prevent
tampering.
Limit switches convert mechanical motion or proximity into a switching action. Limit switches are
most commonly used in DDC control systems to provide position status feedback to the controller
for valve and damper positions (an "end" switch).
Industrial Limit Switch
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HVAC Design Criteria and Guidelines
Mercury Limit Switch
Proximity Switches
Temperature switches (also called thermostats, aquastats, or freezestats depending on their
application) are commonly used in DDC systems to provide a digital input when a process medium
temperature rises or falls to a setpoint temperature.
These temperature switches use a bonded "bimetal" strip consisting of two dissimilar metals with
different thermal coefficients of expansion. When the temperature changes, the metals expand or
contract at different rates causing the strip to bend. Various configurations such as coiled elements
are used to increase the thermal movement to cause two contacts to come together or separate.
Some configurations use the bimetallic principle to change the orientation of a bulb containing liquid
mercury so that the mercury flows into contact with two electrodes, completing the circuit.
Fluid thermal expansion temperature switches use the thermal expansion of a fluid to cause
displacement of a bellows, diaphragm, bourdon tube, or piston to open or close a set of contacts.
Fluid system based temperature switches can be connected to a remote fluid containing bulb by a
capillary tube, allowing the switch element to be remote from the sensing bulb.
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HVAC Design Criteria and Guidelines
Remote Bulb Thermostat
The freezestat is commonly used to prevent water or steam coils in air handling units from freezing.
Freezestats use a fluid that is a saturated vapor at the switch setpoint temperature. This fluid is
confined within a long capillary tube. The tube is installed in a serpentine fashion over the area of
the air stream to being monitored. If any point along the tube falls below the saturation temperature,
the vapor begins to condense causing a rapid change in pressure in the system and actuating the
switch mechanism.
Electronic temperature switches use the same sensing technologies are used for analog
temperature sensing to electronically operate a set of output contacts.
Freezestat
Humidity switches, or humidistats, are used in DDC control systems to provide a digital input when
a process or space humidity level rises or falls to a setpoint level. Common applications are high
limit safety interlocks for humidifiers, space or process humidity alarms, and simple on-off humidity
control.
Mechanical humidistats use a hygroscopic material such as animal hair, nylon or other plastic
material that changes dimension with changes in moisture content. The dimensional change is
amplified via a mechanical link to causing a switch to operate. Mechanical humidistats are rapidly
being replaced by electronic humidistats that use thin film capacitance or bulk polymer resistance
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HVAC Design Criteria and Guidelines
analog humidity sensing technologies combined with electronic switching circuitry to produce a
switching action at an adjustable set point.
Flow switches are used to provide a digital input to DDC controls systems when a fluid flow rate
has risen above or fallen below the setpoint value. Common applications include safety air and
water flow interlocks for electric heaters and humidifiers, chiller safety interlocks, and burner safety
interlocks. Numerous technologies are available, but the most common types used in DDC systems
are mechanical and differential pressure types.
Mechanical flow switches operate on the principle that the kinetic energy of a flowing fluid creates a
force on an object suspended in the flow stream. The magnitude of the force varies with (the
square of) the velocity of the fluid. Various configurations are used to transfer this force into
operation of a switch. Common configurations include paddles or sails, pistons or discs.
Differential pressure type flow switches operate on the principle that a difference in pressure is
always associated with fluid flow, or the principle that the total pressure of a flowing fluid is always
greater than the static pressure. These differences in pressure can be accurately predicted for a
given situation and related to the fluid flow rate.
Differential Pressure Airflow Switch
Level switches are used in DDC control systems to provide a digital input when the fluid level in a
tank, vessel, or sump has reached a predetermined height. Common applications include cooling
tower sump level control and monitoring, steam condensate tank level, storm water and sewage
sump level monitoring and control, and thermal storage tank level monitoring. Numerous
mechanical and analog technologies are currently available. Some analog technologies include
capacitance, ultrasonic, and magnetostrictive-based devices in combination with solid-state
electronics to provide a switching action based on level. More commonly used technologies include
devices that employ the use of a float (integral, rod and float, submersible), conductivity probe, or
differential pressure mechanism.
Integral float type level switches typically combine an metal or plastic float attached to the arm of a
submersible rotary switch mechanism, or a float that encloses a magnet which slides on a hollow
rod enclosing one or more reed switches.
Submersible float switches utilize an encapsulated integral float type switch or mercury switch
suspended on a fluid tight cord in the vessel being monitored. When the level is below the cord
attachment, the float hangs down and the switch is in its normally open or closed position. When
the fluid level rises, the float rises above the cord attachment point, changing the float orientation.
When the float has position has inverted sufficiently, the internal switch changes position.
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HVAC Design Criteria and Guidelines
Conductivity probe-type level switches work for conductive liquids only and use the liquid itself to
conduct low level electrical signals between two or more electrodes to operate higher level
electronic switching devices such as transistors or triacs.
Pressure switches are used in DDC systems to provide status indication for fans, filters, and
pumps, and to provide flow and level status indication by virtue of the predicable relationships
between pressure and these values. Typical mechanical pressure switches use a piston, bellows,
bourdon tube or diaphragm and a magnetic or mechanical linkage to convert the forces resulting
from the measured pressure into repeatable motions used to operate one or more switches. Low
pressure switches commonly used to measure air pressures in the range of 0.05 inches water
column to 1 psig typically use a flexible diaphragm. Piston, bourdon tube, and bellow type switches
are available for higher pressures ranging from 1 to over 100 psig.
Vibration switches are used to provide a signal when vibration levels in rotating machinery such as
fans, reach unsafe levels. Vibration switches are commonly applied on large cooling towers and air
handling unit fans for safety reasons.
Moisture detecting switches are commonly used to detect moisture under raised floors, in piping
and tank containment areas, and in the drain pans of air handling units to alert system operators
before damage or flooding occurs. Most moisture detecting switches are instruments of the float
type or conductivity type. Float types are adapted to actuate at very low fluid levels. Conductivity
types may consist of point sensitive probes located very close to the bottom of a low point or sump
where water will collect, or they may be ribbons or strips with wires separated by a non-conductive
material, such that when any portion of the ribbon is exposed to liquid moisture, the electrical circuit
is completed and the switch mechanism activates.
Current sensing relays are used in DDC systems to monitor the status of electrical devices. The
devices typically have one or more adjustable current set points. Common applications include fan
and pump on/off status feedback. Current switches can detect broken fan belts if properly adjusted.
Current relays can also be used for phase monitoring.
Digital input devices can be used as direct DDC inputs and the control logic written to define the
function of each device in relation to other devices in the control of a specific equipment item. For
example, the input from a freezestat must indicate "closed" before a start relay will close to start a
fan. This is called software interlock.
However, with software interlock, a failure of the DDC system means that the HVAC equipment
control circuit is no longer interlocked or protected by its safety devices. Therefore, it is more
common to used hardwired interlock for safety or limit controls.
The most common use of hardwired interlock is the start/stop circuit for fans, pumps, and primary
heating and cooling equipment, such as boilers and chillers. Appendix A includes hardwired
interlock circuits where deemed appropriate to ensure the proper performance of safety and/or limit
controls.
Analog input devices: There are numerous AI devices utilized in HVAC control. The main
categories of devices include (but are not limited to) the measurement of temperature, humidity,
dew point, pressure, flow (liquid, air), liquid level, light level, electrical attributes (voltage, current,
phasing, power), energy, occupancy, position, and gas concentration.
One of the most common properties measured in the HVAC control world is temperature. Human
comfort, computer room requirements, and a host of other considerations make temperature
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HVAC Design Criteria and Guidelines
measurement necessary to HVAC control strategies.
technologies exist for use with DDC control systems:
Several temperature measurement
Resistance Temperature Detectors (RTD's) operate on the principle that the electrical
resistance of a metal changes predictably and in an essentially linear and repeatable
manner with changes in temperature. The resistance of the element at a base
temperature is proportional to the length of the element and the inverse of the cross
sectional area. RTD's are commonly used in sensing air and liquid temperatures in pipes
and ducts, and as room temperature sensors. DDC systems may accept RTD inputs
directly, or a transmitter with voltage or current output may be used.
The accuracy of a RTD sensor is typically expressed in percent of nominal resistance at
0°C. RTDs are relatively accurate when compared to other sensing devices and have
good stability characteristics.
RTDs are constructed in thin film, thick film, totally supported and "bird-cage"
configurations. They can be made from many materials, some of which include platinum,
tungsten, silver, copper, nickel, nickel alloys and iron. Currently, the most common RTDs
(used in the HVAC field) are constructed in film type configurations with platinum, nickel,
or nickel iron.
Since the resistance of the sensor is the property being measured, the resistance of all
elements of the circuit, including the sensor leads, affects the measurement. With RTD's,
and particularly those with lower base resistance values, the resistance of long leads can
amount to several percent or more of the sensor circuit. This can result in significant error.
One option for correcting this problem is to locate a transmitter at the sensor. The other
way is to compensate for the lead resistance by the method of wiring.
Thermistors are commonly used for sensing air and liquid temperatures in pipes and ducts
and as room temperature sensors. The term "thermistor" evolved from the phrase
‘thermally sensitive resistor’. Thermistors are temperature sensitive semiconductors that
exhibit a large change in resistance over a relatively small range of temperature. There
are two main types of thermistors, positive temperature coefficient (PTC) and negative
temperature coefficient (NTC). NTC thermistors are commonly used for temperature
measurement.
Unlike RTD's, the temperature-resistance characteristic of a thermistor is non-linear, and
cannot be characterized by a single coefficient. Manufacturers commonly provide
resistance-temperature data in curves, tables, or polynomial expressions. Linearizing the
resistance-temperature correlation may be accomplished with analog circuitry or by the
application of mathematics using digital computation. The lead resistance of most
thermistors is very small in comparison to sensor resistance.
Solid-state sensors are available for space, duct and pipe applications. These sensors
provide a milli-volt level voltage signal used in a two-wire configuration or a micro-amp
level current signal used in a three-wire configuration.
Thermocouples are available for space, pipe and duct application. Thermocouples operate
on the principle that when two dissimilar metals are joined at both ends and one of the
ends is at a different temperature, a voltage that is proportional to the temperature of the
junction is produced. This principle requires that the leads be made of the same metals in
order to achieve reasonable measurement accuracy. The signal level from a
thermocouple is in the milli-volt range such that transmitters are often used to overcome
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HVAC Design Criteria and Guidelines
the effect of the leads. Although in widespread laboratory and industrial use,
thermocouples are not widely use in commercial HVAC control applications.
The following table compares the most common temperature measurement technologies applicable
to DDC control systems for HVAC. The comparisons made are general in nature and not intended
to be all inclusive for each sensor type.
Advantages
Thermistor
Resistance
Large
resistance
change
with
temperature.
RTD
Resistance
Linear
resistance
with temperature.
Solid State Sensor
Voltage or Current
Linear high level
output
vs.
temperature.
Good stability
Rapid
time.
response
High
resistance
eliminates problems
with
lead
resistance.
Simple
rugged.
Low cost.
Wide
range
operating
temperature.
Thermocouple
Voltage
Widest operating
range.
and
of
Low cost
No external power
supply required.
Interchangeable
over
a
wide
temperature range.
Low cost.
Good stability.
Interchangeable.
Non-linear.
Disadvantages
Limited
operating
temperature range.
Interchangeable
over only narrow
temperature ranges.
Subject
to
inaccuracy due to
self heating.
Current
required.
source
Small
resistance
change
with
temperature.
Limited
operating
temperature range.
Lower stability.
Slower
time.
response
Power
required.
Subject
heating.
to
Subject
heating.
self
Transmitter or 3- or
4wire
leads
required for lead
resistance
compensation.
Some types easily
damaged by shock
or vibration.
External
power
required.
16
circuit
source
Non-linear.
supply
to
self
Newer technology
with few vendors,
less
standardization.
Reference junction
temperature
compensation
required.
Radio frequency or
electronic
noise
can
affect
low
signal level.
HVAC Design Criteria and Guidelines
RTD's, thermocouples, thermistors, and solid-state temperature sensors are all small devices with
similar mounting techniques used for all of the types. Sensors for pipe and duct mounting are
commonly sheathed in a stainless steel sheath of 1/8” to 1/4" diameter (larger and smaller
diameters are available). Sensors for liquid piping systems may be mounted with direct immersion
into the fluid or installed in a thermowell to allow removal without draining the piping system and to
reduce the likelihood of leaks. Sensors installed in wells should be installed with a heat transfer
compound filling the space between the sensor and the well to insure good thermal contact
between the measured fluid and the sensor.
In measuring the temperature of air in large ducts, it is often desirable to use an averaging element
because the air temperature may vary significantly over the cross section of the duct. RTD and
thermistor sensors have been developed that accomplish this using multiple sensors installed in a
single flexible tubular element. The element is typically arranged in a serpentine fashion to obtain
representative measurements over the entire cross sectional area of the duct. Very large ducts or
air handling unit casings often require multiple sensors that are customarily wired in parallel-series
arrangements. Averaging elements are commonly applied downstream of mixing dampers and
large heating or cooling coil banks.
In general, it is better to use precision thermistors (10,000 ohms) for room sensing and 1000 ohm
platinum RTDs for HVAC unit temperature sensing.
Sensors for outdoor air applications should be located in normally shaded areas to prevent the
heating effects of solar radiation. These sensors are usually provided with a shield or hood to
reduce the effects if exposed to direct sunlight and prevent direct contact with precipitation.
In some cases, it is desirable to enclose sensors in aspirated cabinets to prolong their life and
reduce maintenance. Aspirated cabinets typically include a filtered air intake and an exhaust fan to
provide positive airflow through the enclosure.
By far the most common measurement of humidity in the HVAC industry is relative humidity (RH).
Relative humidity sensors are used in DDC control systems to measure humidity in spaces and
ducts. Commonly applied sensor types include thin-film capacitance, bulk polymer resistance, or
the integrated circuit type that combines a sensor (commonly of the capacitance type) and some of
the signal conditioning circuitry to form a solid-state device.
Thin film capacitance sensors operate on the principle that changes in relative humidity
cause the capacitance of a sensor made by laminating a substrate, electrodes, and a thin
film of hygroscopic polymer material to change in a detectable and repeatable fashion.
Because of the nature of the measurement, capacitance humidity sensors are combined
with a transmitter to produce a higher-level voltage or current signal.
Capacitance type relative humidity sensor/transmitters are capable of measurement from
0-100 % relative humidity with application temperatures from -40 to 200 °F. Capacitance
sensors are affected by temperature such that accuracy decreases as temperature
deviates from the calibration temperature. Sensors are available that are inter-changeable
within plus or minus 3% without calibration. Sensors with long term stability of <±1% per
year are available.
Bulk Polymer Resistance sensors use the principle that resistance change across a
polymer element varies with relative humidity and is measurable and repeatable. As with
capacitance humidity sensors, polymer resistance sensors are combined with transmitters
to produce a higher-level voltage or current signal.
17
HVAC Design Criteria and Guidelines
Bulk polymer resistance humidity sensor/transmitters are commonly capable of
measurement from 0-100 % relative humidity with application temperatures from -20 to
140 °F. Resistance sensors are affected by temperature such that accuracy decreases as
temperature deviates from the calibration temperature. Bulk polymer resistance humidity
sensors are not commonly interchangeable. Sensors with long term stability of <±1% drift
per year are available.
It is common practice when measuring relative humidity to combine a temperature sensor and
transmitter into the same device as the humidity sensor. Adding a microprocessor makes it
possible to calculate and transmit dew point temperature or enthalpy. Commonly, these devices
can be configured to output calculated humidity ratio, wet bulb temperature, and absolute humidity
as well as dew point.
Pressure is measured in DDC systems in order to control the operation and monitor the status of
fans and pumps. Space pressure is sometimes measured and used for control. Pressure is also the
basis of many flow and level measurements. Diverse electrical principles are applied to pressure
measurement. Those commonly used with DDC systems include capacitance and variable
resistance (piezoelectric and strain gage).
Capacitance pressure sensors, as illustrated below, typically use a capacitance cell
consisting of a diaphragm exposed to the pressure medium separated from another plate
by a fill fluid. When the applied pressure deflects the diaphragm, the capacitance
characteristic of the sensing element changes. The capacitance cell is excited by a high
frequency source. The frequency changes as the capacitance of the cell changes. This
frequency shift is converted to the output signal by the transmitter electronics.
Capacitance transmitters are available configured for either differential or gauge pressure
measurement. Usual outputs are voltage or current.
Capacitance transmitters are available with ranges from a few inches water column (in.
wg) to thousands of pounds per square inch (psi).
Variable resistance technology includes both strain gage and piezo-resistive or
piezoelectric technologies.
18
HVAC Design Criteria and Guidelines
Traditional strain gages are constructed of a wire filament bonded to a substrate. The
resistance of the wire changes in proportion to the strain in the substrate, which is
transmitted to the wire through the bond. Strain gauges are applied to diaphragms or other
mechanical pressure elements and change resistance in response to strains induced in
the element by the applied pressure. When arranged to form a Wheatstone bridge circuit,
an analog voltage signal is produced that is proportional to applied pressure.
Piezo-resistive sensors operate on the principle that certain semiconductor materials,
such as silicon, change resistance with stress or strain. These piezo-resistive elements
are implanted on a solid-state chip that is attached to a mechanical sensing element or
used as the sensing element. When the piezo-resistive elements are arranged to form a
bridge circuit (as with the wire filament strain gage sensor), an analog voltage signal is
produced that is proportional to the applied pressure.
Piezo-resistive type sensors have a sensitivity of approximately 100 times greater than a
wire strain gage. Also, other strain gages must usually be bonded to a dissimilar force
sensing material with different composition and thermal characteristics. The wire strain
gage sensor is subject to degradation from failure of the bond to the force sensing
element, thermal effects and plastic deformation of the force-sensing element. In contrast,
the silicon based piezo resistors may be integral with a silicon wafer that serves as the
force-sensing element. This eliminates many of the inherent problems with thermal effects
and bonding. Silicon has very good elasticity throughout the typical operational range and
normally fails only by rupturing.
Strain gage and piezo-resistive transmitters are available with ranges of a few inches
water column (in. w.c.) to thousands of pounds per square inch (psi).
The major considerations for the installation of a pressure element in a fluid system should include
the following:








sensor location (pipe mounted, tank mounted, remote).
isolation of the sensing element from undesirable and potentially.
damaging transient pressures, such as those resulting from water hammer and
turbulence.
temporary isolation from the pressure source for maintenance and release of
trapped pressure when removing the sensor for maintenance or for setting zero
during calibration.
over-range protection for differential pressure instruments.
protection from process temperature outside of the range of the sensor
application.
venting trapped, non-condensable gases in liquid sensing piping.
draining trapped liquids from gas.
Pressure snubbers or dampeners are used to reduce the magnitude of pressure transients. These
can be a sintered metal element with small openings, a small orifice fitting, a high-pressure drop
valve (such as a needle valve), or a pressurized gas filled container mounted on the sensing piping.
The designer must incorporate valves to provide isolation, venting, drain, and pressure relief for
pressure instruments installed in piping systems.
Flow measuring devices are used in DDC systems to monitor air and liquid flow rates. Typically,
airflow-measuring devices are used to monitor and control the output of fans, dampers, and
19
HVAC Design Criteria and Guidelines
associated equipment used to control outside airflow, terminal unit airflow, and space pressures.
Liquid flow is commonly measured to maintain required flows in boilers, chillers and heat
exchangers, and to control and monitor energy production and use (requires temperature
measurement also).
Numerous reliable technologies are available for use with DDC systems. Some technologies have
been applied to both air and liquid flow measurements as their principles of operation hold true in
either application. Other technologies lend themselves to being airflow or liquid flow specific.
Flow rate is typically obtained by measuring a velocity of a fluid in a duct or pipe and multiplying the
by the known cross sectional area (at the point of measurement) of that duct or pipe. Common
methods for measuring airflow include hot wire anemometers, differential pressure measurement
systems, and vortex shedding sensors. Common methods used to measure liquid flow include
differential pressure measurement systems, vortex shedding sensors, positive displacement flow
sensors, turbine based flow sensors, magnetic flow sensors, ultrasonic flow sensors and ‘target’
flow sensors.
"Hot Wire" or thermal anemometers operate on the principle that the amount of heat removed from
a heated temperature sensor by a flowing fluid can be related to the velocity of that fluid. Most
sensors of this type are constructed with a second, unheated temperature sensor to compensate
the instrument for variations in the temperature of the air. Hot wire sensors are available as single
point instruments for test purposes, or in multi-point arrays for fixed installation. Hot wire type
sensors are better at low airflow measurements than differential pressure types, and are commonly
applied to air velocities as low as 50 feet per minute.
Differential pressure measurement technologies can be applied to both airflow and liquid flow
measurements. Sensor manufacturers offer a wide variety of application specific sensors used for
airflow and pressure measurements, as well as ‘wet-to-wet’ differential pressure sensors used for
liquid measurements. Both lines offer a wide variety of ranges.
For airflow measurements, differential pressure flow devices in common use in HVAC systems
include Pitot tubes and various types of proprietary velocity pressure sensing tubes, grids, and
other arrays. All of these sensing elements are combined with a low differential pressure transmitter
to produce a signal that is related to flow velocity.
20
HVAC Design Criteria and Guidelines
The Pitot tube measures both static pressure (SP) and total pressure (TP). The difference between
these two values is the velocity pressure (VP). Velocity pressure is a function of the air velocity
that can be computed on the following basis:
V = 4005 (VP)1/2
where
V = Velocity (fpm)
VP = Velocity pressure (in. wg)
The need to sense multiple points in the cross section of a duct gave rise to averaging type sensors
with arrays of pressure sensing points. This is called an airflow monitoring station, as shown in the
following figure, and is commonly used in HVAC applications.
Some differential pressure based flow stations include transmitters that have the capability to
electronically extract the square root of the measured pressure and provide an analog signal that is
linear with respect to velocity, whereas others provide an analog signal that is proportional to
measured pressure and depend upon the DDC system to calculate the square root and, therefore,
velocity.
Velocity range is limited by the range and resolution of the pressure transmitter used. Most
differential pressure type stations are limited to a minimum velocity in the range of 400 to 600 feet
per minute. Maximum velocity is only limited by the durability of the sensor.
In recent years, flow arrays have been developed for installation in the inlets of centrifugal fans,
making total airflow measurement much more convenient.
For water flow measurements, differential pressure flow devices in common use in HVAC systems
operate either by measuring velocity pressure (insertion tube type), or by measuring the drop in
pressure across a restriction of known characteristic (orifice, flow nozzle, Venturi).
21
HVAC Design Criteria and Guidelines
Insertion tube type flow sensors are usually constructed of a round or proprietary shaped tube with
multiple openings across the width of the flow stream to provide an average of the velocity
differential across the tube and an internal baffle between upstream and downstream openings to
obtain a differential pressure. Insertion tube type meters have a low permanent pressure loss and,
with proper installation and associated pressure instruments, are satisfactory for many common
applications. Insertion tube flow sensors are available that can be installed and removed through a
full port valve so that installation and service are possible without draining the section of piping in
which they are installed.
A concentric orifice plate meter is the simplest and least expensive of the differential pressure type
meters. The orifice plate constricts the flow of a fluid to produce a differential pressure across the
plate. The result is a high pressure upstream and a low pressure downstream that is proportional to
the square of the flow velocity. An orifice plate usually produces a greater overall pressure loss
than other flow elements. An advantage of this device is that cost does not increase significantly
with pipe size.
Concentric Orifice Plate Meter
Venturi tube meters exhibit a very low pressure loss compared to other differential pressure meters,
but they are also the largest and most costly. They operate by gradually narrowing the diameter of
the pipe, and measuring the resultant drop in pressure. An expanding section of the meter then
returns the flow to very near its original pressure. As with the orifice plate, the differential pressure
measurement is converted into a corresponding flow rate. Venturi tube applications are generally
restricted to those requiring a low pressure drop and a high accuracy reading. They are widely
used in large diameter pipes.
Venturi Tube Meter
22
HVAC Design Criteria and Guidelines
Flow nozzle meters may be thought of as a variation on the Venturi tube. The nozzle opening is an
elliptical restriction in the flow but with no outlet area for pressure recovery. Pressure taps are
located approximately 1/2 pipe diameter downstream and 1 pipe diameter upstream. The flow
nozzle is a high velocity flow meter used where turbulence is high (Reynolds numbers above
50,000) such as in steam flow at high temperatures. The pressure drop of a flow nozzle falls
between that of the Venturi tube and the orifice plate (30 to 95 percent).
Flow Nozzle Meter
Vortex shedding flow meters operate on the principle (Von Karman) that when a fluid flows around
an obstruction in the flow stream, vortices are shed from alternating sides of the obstruction in a
repeating and continuous fashion. The frequency at which the shedding alternates is proportional to
the velocity of the flowing fluid. Vortex flow meters provide a highly accurate flow measurement
when operated within the appropriate range of flow. Vortex meters are commonly applied where
high quality water, gas, or steam flow measurement is desired.
Positive displacement meters are used where high accuracy at high turndown is required and
reasonable to high permanent pressure loss will not result in excessive energy consumption.
Applications include water metering such as for potable water service, cooling tower and boiler
make-up, steam condensate, and hydronic system make-up. Positive displacement meters are also
used for fuel metering for both liquid and gaseous fuels. Common types of positive displacement
flow meters include lobed and gear type meters, nutating disk meters, and oscillating piston type
meters. These meters are typically constructed of metals such as brass, bronze, cast and ductile
iron, but may be constructed of engineered plastic, depending on service.
Due to the close tolerance required between moving parts of positive displacement flow meters,
they are sometimes subject to mechanical problems resulting from debris or suspended solids in
the measured flow stream. Positive displacement meters are available with flow indicators and
totalizers that can be read manually. When used with DDC systems, the basic meter output is
usually a pulse that occurs at whatever time interval is required for a fixed volume of fluid to pass
through the meter. Pulses may be accepted directly by the DDC controller and converted to flow
rate, or total volume points, or a separate pulse to analog transducer may be used. Positive
displacement flow meters are one of the more costly meter types available.
Turbine and propeller type meters operate on the principle that fluid flowing through the turbine or
propeller will induce a rotational speed that can be related to the fluid velocity. Turbine and
propeller type flow meters are available in full bore, line mounted versions and insertion types
where only a portion of the flow being measured passes over the rotating element. Full bore turbine
23
HVAC Design Criteria and Guidelines
and propeller meters generally offer medium to high accuracy and turndown capability at
reasonable permanent pressure loss. Turbine flow meters are commonly used where good
accuracy is required for critical flow control or measurement for energy computations. Insertion
types are used for less critical applications. Insertion types are often easier to maintain and inspect
because they can be removed for inspection and repair without disturbing the main piping. Some
types can be installed through hot tapping equipment and do not require draining of the associated
piping for removal and inspection.
Magnetic flow meters operate based upon Faraday's Law of electromagnetic induction, which
states that a voltage will be induced in a conductor moving through a magnetic field. The
magnitude of the induced voltage E is directly proportional to the velocity of the conductor V,
conductor width D, and the strength of the magnetic field B. As shown in following figure, magnetic
field coils are placed on opposite sides a pipe to generate a magnetic field.
Magnetic Flow Meter
As a conductive liquid moves through the field with average velocity V, electrodes sense the
induced voltage. The distance between electrodes represents the width of the conductor. An
insulating liner prevents the signal from shorting to the pipe wall. The only variable in this
application of Faraday's law is the velocity of the conductive liquid V because field strength is
controlled constant and electrode spacing is fixed. Therefore, the output voltage E is directly
proportional to liquid velocity, resulting in linear output. Magnetic flow meters are used to measure
the flow rate of conducting liquids (including water) where a high quality low maintenance
measurement system is desired. The cost of magnetic flow meters is high relative to many other
meter types.
Ultrasonic flow sensors measure the velocity of sound waves propagating through a fluid between
two points on the length of a pipe. The velocity of the sound wave is dependant upon the velocity of
the fluid such that a sound wave traveling upstream from one point to the other is slower than the
velocity of the of the same wave in the fluid at rest. The downstream velocity of the sound wave
between the points is greater than that of the same wave in a fluid at rest. This is due to the
Doppler effect. The flow of the fluid can be measured as a function of the difference in time travel
between the upstream wave and the downstream wave. Ultrasonic flow sensors are non-intrusive
and are available at moderate cost. Many models are designed to clamp on to existing pipe.
24
HVAC Design Criteria and Guidelines
A target meter consists of a disc or a "target" which is centered in a pipe. The target surface is
positioned at a right angle to the fluid flow. A direct measurement of the fluid flow rate results from
the force of the fluid acting against the target. Useful for dirty or corrosive fluids, target meters
require no external connections, seals, or purge systems. Target flow meters are commonly used
to for liquid flow measurement and less commonly applied to steam and gas flow.
Target Flow Meter
All airflow sensors work best in sections of ducts that have uniform, fully developed flow. All airflow
sensing devices should be installed in accordance with the manufacturers recommended straight
runs of upstream and downstream duct in order to provide reliable measurement. A number of
manufacturers offer flow straightening elements that can be installed upstream of the sensing array
to improve undesirable flow conditions. These should be considered when conditions do not permit
installation with the required straight runs of duct upstream and downstream from the sensor.
As with airflow, all liquid flow sensors work best when fully developed, uniform flow is measured. To
attain fully developed, uniform flow sensors should be installed in accordance with the
manufacturers recommended straight runs of upstream and downstream pipe in order to provide
the most reliable measurements.
With most liquid flows measured for HVAC applications, density changes with pressure and
temperature are relatively small and most often ignored due to their insignificant effect on flow
measurements. When measuring the flow of steam or fuel gases, unless temperature and pressure
are constant, ignoring the effect density changes with varying temperature and pressure will often
result in significant or gross errors. For this reason, it is common to measure the temperature and
pressure, in addition to the flow, and electronically correct the result for the fluid density. This
correction may be done using an integral or remote microprocessor based "flow computer" or it
may be made in the DDC controller with suitable programming.
Selection of fluid flow meters is based on four factors: pressure drop, accuracy, turndown, and cost.
Turndown defines the flow range over which the meter is accurate. For example, a turndown of 4
indicates that the minimum flow at which accuracy is maintained is 25% of the meter's range or
rated maximum flow. At a turndown of 30:1, the meter is accurate down to about 3% of its flow
range. The following table summarizes these factors for each flow measurement technology:
25
HVAC Design Criteria and Guidelines
Relative
PD
Very low
Typical
Accuracy
1%
Air/liquids
Liquids/steam
Liquids/steam
Liquids/steam
Air/steam/
liquids
Liquids
Low
High
Low
Medium
Medium
5%
3%
3%
3%
0.5%
High
0.1%
Turndown
50-12,000 fpm
velocity
4:1
4:1
4:1
4:1
20:1 air/steam
30:1 liquids
100:1
Steam/liquids
Low
1%
30:1
Liquids
Dirty liquids
Very low
Very low
30:1
20:1
Liquids/steam
Medium
1%
1-5%
depending
on liquid
1%
Turbine: High
Prop: Low
High
Moderate
20:1
Low
Technology
Hot wire
Fluid(s)
Air
Pitot tube
Orifice plate
Venturi tube
Flow nozzle
Vortex shedding
Positive
displacement
Turbine or
propeller
Magnetic
Ultrasonic
Target
Relative
Cost
Moderate
Moderate
Low
High
High
High
Low to Moderate
The accuracy of most meters is defined in terms of percent of full range. Therefore, it is important
that designers select flow meters that have a maximum rated flow relatively close to the anticipated
HVAC process flow. Often, this introduces a pressure drop trade-off that must be evaluated.
The turndown of differential pressure meters (Pitot tube, orifice plate, Venturi tube, and flow nozzle)
can be improved to between 10:1 and 16:1 by using dual transmitters, one for the low range of flow
and one for the high range.
Liquid level measurements are typically used in DDC systems to monitor and control levels in
thermal storage tanks, cooling tower sumps, water system tanks, pressurized tanks, etc.
Numerous sensing technologies are available. Common technologies applicable to HVAC system
requirements are based on hydrostatic pressure, ultrasonic, capacitance and magnetostrictivebased measurement systems.
Level measurement by hydrostatic pressure is based on the principle that the hydrostatic pressure
difference between the top and bottom of a column of liquid is related to the density of the liquid
and the height of the column. For open tanks and sumps, it is only necessary to measure the
gauge pressure at the lowest monitored level. For pressurized tanks it is necessary to take the
reference pressure above the highest monitored liquid level. Pressure transmitters are available
that are configured for level monitoring applications. Pressure instruments may also be remotely
located, however this makes it necessary to field calibrate the transmitter to compensate for
elevation difference between the sensor and the level being measured.
Bubbler type hydrostatic level instruments have been developed for use with atmospheric pressure
underground tanks, sewage sumps and tanks, and other applications that cannot have a
transmitter mounted below the level being sensed or are prone to plugging. Bubbler systems bleed
a small amount of compressed air (or other gas) through a tube that is immersed in the liquid, with
an outlet at or below the lowest monitored liquid level. The flow rate of the air is regulated so that
the pressure loss of the air in the tube is negligible and the resulting pressure at any point in the
tube is approximately equal to the hydrostatic head of the liquid in the tank.
26
HVAC Design Criteria and Guidelines
Ultrasonic level sensors emit sound waves and operate on the principle that liquid surfaces reflect
the sound waves back to the source and that the transit time is proportional to the distance
between the liquid surface and the transmitter. One advantage of the ultrasonic technology is that it
is non-contact and does not require immersion of any element into the sensed liquid. Sensors are
available that can detect levels up to 200 feet from the sensor. Accuracy from 1% to 0.25% of
distance and resolution of 1/8" is commonly available.
Capacitance level transmitters operate on the principle that a capacitive circuit can be formed
between a probe and a vessel wall. The capacitance of the circuit will change with a change in fluid
level because all common liquids have dielectric constant higher than that of air. This change is
then related proportionally to an analog signal suitable for DDC analog inputs. Resolution of 1/8"
and accuracy of 1% to 0.25% of span are available.
Magnetostrictive level transmitters operate on the principle that an external magnetic field can be
used to cause the reflection of an electromagnetic wave in a waveguide constructed of
magnetostrictive material. The probe is composed of three concentric members. The outermost
member is a protective, product-compatible outer pipe. Inside the outer pipe is a waveguide, which
is a formed element constructed of a proprietary magnetostrictive material. A low-current
"interrogation" pulse is generated in the transmitter electronics and transmitted down the
waveguide creating an electromagnetic field along the length of the waveguide. When this
magnetic field interacts with the permanent magnetic field of a magnet mounted inside the float, a
torsional strain pulse, or waveguide twist, results. This waveguide twist is detected as a return
pulse. The time between the initiation of the interrogation pulse and the detection of the return
pulse is used to determine the level measurement with a high degree of accuracy and reliability.
Accuracy and resolution of 1/16" or better are available from some manufacturers.
Light level sensors are used in DDC systems are typically used to turn on night lighting when light
level drops below a setpoint level and/or to turn off indoor or outdoor lighting when ambient levels
are sufficient. Light level sensors can be used to control the output of dimmable fluorescent lighting
to setpoint level. Accuracy of ±1% of reading is common.
Monitoring of electrical attributes is performed by DDC systems to determine status or condition of
HVAC system components, determine power and energy consumption of various components, and
implement usage and demand control strategies to reduce building energy costs. The two most
common electrical measuring devices used for DDC are current transducers and power measuring
devices:
Current transducers are used to monitor current flow to motors, heaters, or electrical
distribution systems. Their input may be used for demand limiting purposes, control, or
energy accounting. The sensing element of a current transducer is typically a current
transformer. It transforms the current being monitored into a higher voltage, lower current.
Additional circuitry reduces this voltage to the desired level. Current transducers may have
line and load terminals for the monitored current, or they may be arranged as a coil that
the current carrying conductor passes through. With this arrangement, the load conductor
induces the current in the transformer via the electromagnetic field surrounding the
conductor. Current transformers and transducers are available with solid or split cores.
The split core device may be installed without disconnecting the power conductor provided
that there is sufficient slack in the conductor and room in the enclosure. Accuracy of ±0.5
% of full scale is readily available.
Power Monitoring Devices:
include:
Commonly monitored characteristics of a power system
27
HVAC Design Criteria and Guidelines







Power Demand (kW)
Power Consumption (kW per hour)
Voltage (Volts)
Current (Amps)
Frequency (Hertz)
Power Factor
Reactive Power (kVAR)
Transducers are available to provide a standard voltage or current input to a controller
based on measured frequency, reactive power, or power factor. Available devices for load
protection are available that monitor three phase voltages and provide a relay signal to
disconnect loads if the power supply becomes unsuitable for continued operation due to
conditions such as phase loss, phase imbalance, low or high voltage, or phase reversal.
There are other methods of monitoring building demand and consumption. One of the simplest
methods is to obtain a pulse signal output from the utility company's metering equipment. This can
be input directly to a controller with pulse input capability, or a pulse to analog signal transducer
may be used. The pulse represents a set number of kilowatt-hours. Average demand is calculated
using a rolling time average of the number of pulses over the stipulated time period. Average
demand is typically calculated for billing purposes over a 5, 15, or 30 minute period. Power
consumption and demand may also be calculated using current transformers to measure current
flow and voltage transducers to measure voltage on the selected load or system. The DDC
controller calculates the demand from these values, and integrates this value over time to
determine power use.
Occupancy sensors are commonly used in DDC systems to initiate 2-position control of lighting
and/or room air-conditioning equipment. Sensors turn lights and air conditioning equipment off (or
to reduced levels) when no occupants are detected. Occupancy sensors may be designed to
detect motion or differences in background infrared radiation and the radiation emitted from a
human occupant. Many occupancy sensors used for lighting also incorporate photocells or other
light sensitive devices to reduce lighting when ambient light is sufficient.
Position sensors and transmitters are used in HVAC system controls where the feedback of
position is necessary for precise control of system components, such as valves and dampers, or
where monitoring of position is necessary or desired. Position transmitters commonly operate using
a slidewire or rotary potentiometer to provide a variable resistance that changes with linear or
rotary position.
With the increased interest in indoor air quality and the need to monitor potentially dangerous
gases, gas concentration measurements have become increasing more prevalent in DDC system
design. Many devices are currently available for use in HVAC applications:
Carbon monoxide detectors are used to operate ventilation equipment to prevent carbon
monoxide levels from becoming unsafe. They are also used to warn facility owners and
occupants of unsafe levels in garages, loading docks, tunnels, and other areas where
vehicles are operated. Solid state sensing technology is most commonly used. Single or
multiple sensing point versions are available that can provide contact closures at one or
more set levels and/or analog signals that are proportional to carbon monoxide
concentration.
Carbon dioxide concentration inside of buildings has been related to general ventilation
adequacy and is commonly monitored by DDC systems as a measure of indoor air quality
28
HVAC Design Criteria and Guidelines
and ventilation adequacy. It is also measured by DDC systems and used to control
outdoor air fans and dampers to keep the concentration below set levels. The most
commonly used sensing technology is Non-Dispersive Infra-Red (NDIR). This is based on
the principle that carbon dioxide gas absorbs infrared radiation at the 4.2 µm wavelength.
Attenuation of an infrared source can be related to the gas concentration in air in the
range of 0-5000 parts per million with a general accuracy of plus or minus 150 ppm or 50
ppm over narrower ranges.
Refrigerant gas detectors are used in the control of emergency ventilation systems to
evacuate hazardous concentrations of refrigerant gas in refrigeration machinery rooms
and other enclosed areas. Gas specific detectors are available to detect individual
refrigerant types, including CFC's, HFC's, HCFC's and ammonia. The most commonly
used sensor types are infrared (IR), photo-acoustic, and solid state sensing technologies.
Single or multiple sensing point versions are available that can provide contact closures at
one or more set levels and/or analog signals that are proportional to refrigerant
concentration.
Output Devices
Digital output devices: DO devices are used to provide two-position control (open/close, on/off,
etc.) of valves, dampers, electric motors, lighting and external signaling devices, such as alarm
bells and indicator lights. Digital outputs may also be used to control analog devices using tri-state
or pulse width modulation (PWM). PWM is accomplished by monitoring a timed closure of a set of
contacts. The amount of time the contacts are closed is proportional to a level of performance for
the controlled device.
The most common digital output devices are relays, contactors, starters, and two-position
actuators.
A relay is a device where power applied to a coil or input terminal causes the path between pairs of
separate, additional terminals to either allow electrical current flow, or stop current flow. Contactors
and starters are essentially relays designed for interrupting and applying power to larger loads (i.e.,
integral horsepower motors) and significant resistance loads (i.e., lighting and heaters). The most
common types of relays are standard instantaneous control, latching, and timing. Contactors and
starters can be considered common types of heavier duty relays with and without load protection.
Standard control relays are electromechanical or solid state. Electromechanical control
relays use a magnetic coil and armature to cause contacts to open or close when current
is applied to the coil. Solid state relays use semi-conducting devices (such as transistors
or triacs) that become electrically conductive between output terminals when a voltage is
applied to the input.
Relays are typically used to switch AC and DC control signals with voltages from 0 to 600
volts and typically have contact ratings of less than 20 amps. Control relays come in
numerous sizes and shapes. Relays used on printed circuit boards for pilot duty can be
made very small, with the largest dimension under 1/2 inch (12.5 mm). Modular, miniature
and sub-miniature rail mounted plug-in type relays are often used in shop or fieldfabricated control panels because they are less costly and easy to mount and replace.
Latching relays are a variation of the standard instantaneous control relay where the
contacts change position when initially energized, but do not revert to the normal state
(when the input signal is removed) until a separate reset signal is applied. Latching relays
29
HVAC Design Criteria and Guidelines
may have mechanical latches using a set and reset coil, or they may latch magnetically.
Latching relays are also available with manual reset latches.
Time delay relays are a variation of the standard instantaneous control and latching relay
where a fixed or adjustable time delay must occur following a change in the control signal
before the switching action occurs. Common time delay relay configurations include on
delay, off delay and on/off delay.
Two-position actuators are used to control the linear or rotary motion of a final control element (see
Section 4) to one of two positions, usually open or closed. The two most common types of twoposition actuators are the solenoid type and rotary type:
The solenoid actuator consists of a coil wound around a fixed core and a movable core
that is usually enclosed in a non-magnetic case. When the coil is energized, the movable
core is attracted to the fixed core, causing a rapid linear motion. Solenoid actuators are
most commonly applied to small valves for control of water and air flow in pipe and tubing.
Solenoid valves are available in pilot-operated models, where fluid pressure of the fluid
being controlled actually provides the motive force for operating the valve. The solenoid is
used to control the internal flow of the pilot fluid within the valve, causing the operation of
the valve. Non-pilot type solenoid valves open and close very quickly and may cause
water hammer when used for controlling flow in liquid systems. Pilot-operated valves may
be designed for slower opening and closing time to reduce this tendency.
Solenoid valves are use for on/off control of pneumatic control air supply and are referred
to as electric/pneumatic (EP) relays. Two-state, on/off control of pneumatic dampers and
actuators is almost universally accomplished using the electrical signal to operate a
solenoid valve that turns the air supply to the pneumatic actuator on or off.
Rotary actuators typically are based on rotary electric motors combined with a gear train
that may be reversible, or combined with a spring, such that the position is reversed by the
energy stored in the spring when the motor is de-energized. Spring-return actuators are
commonly applied where a device must be returned to a safe or normal position when the
power supply or control signal fails. Linkages, rack and pinion configurations, cams and
various other mechanisms are used to convert the rotary actuator motion to linear motion
when applied to devices (such as globe-type control valves) requiring linear motion for
actuation.
Analog output devices: AO devices are used to provide modulating control of sequencers, final
control element actuators (see Section 4), electric motors (through variable speed drives), and
silicon controlled rectifiers
Sequencing of multiple on-off devices based on a single analog output from a control loop is
required for cooling towers with multiple two-speed fans, multi-stage electric heaters, and multistage refrigeration systems. This sequencing can be accomplished within the DDC controller or it
may be accomplished externally using a discrete sequencing device. These devices have two or
more relay or digital outputs that are adjusted to spread the signal range that they turn on and off.
For example, a two-stage sequencer might be adjusted so the stage one relay turns on at 37.5%
analog signal level and off at 12.5%. The stage two relay would be adjusted to turn on at 87.5%
analog signal and off at 67.5%. More advanced sequencers may incorporate adjustable inter-stage
time delays, minimum on and off times, etc.
Variable frequency drives (VFD's) are used to vary the speed of HVAC motors in order to control
the output of driven equipment.
AC variable speed drives operate on the principle that the
30
HVAC Design Criteria and Guidelines
synchronous speed of an AC induction motor is directly proportional to the frequency of the AC
power supplied to the motor. In the US, the standard frequency at which AC power is distributed
and motors are rated is 60 cycle per second (hertz). Virtually all AC variable speed drives currently
manufactured use solid state components to accept AC power at standard distribution voltages and
60 hertz frequency (50 hertz in Europe) and output a variable frequency power supply to the
controlled motor(s). Commonly available drives have provisions for external on/off control by a
contact closure, analog speed feedback signal for monitoring, and accept a standard analog
voltage or current signal for speed input. Many drives are available with one or more drive status
alarms. Some are also available with digital communication interfaces that allow detailed status and
fault monitoring by DDC control systems.
The following items should be considered for any variable speed drive application:
1. Normally, NEMA Design B squirrel cage induction motors with continuous duty rating
are used.
2. Multiple motor loads can be controlled from a single AC variable speed drive, however
the manufacturer's guidelines must be followed regarding operation if some or all motors
are not connected. This applies in particular to drives with current source-type inverters.
3. PWM inverters usually cause motors to produce more noise than normal.
4. Any type of inverter produces a current waveform that contains harmonics that do not
produce any additional torque, but do cause additional heating in the motor windings. This
will typically produce 5% - 15% additional heating load and must be considered when
operating motors controlled by drives near full load conditions.
5. With current source inverters, an open circuit (such as a disconnected load due to a
broken belt or coupling) will cause an excessive voltage rise in the inverter. Unless
appropriate protection is provided, this condition may cause inverter failure.
6. Jerky shaft motion can result with any inverter type at low speed (typically below about
10 hertz) due to badly distorted waveforms at these frequencies. Some PWM drives are
available that are optimized for operation at low speed and can reduce this effect.
7. It is important to consider the torque vs. speed characteristic of the load to be imposed
on the drive. Most HVAC applications are for centrifugal machines (pumps, fans and
compressors) and are described as "variable torque" because the torque is low at low
speed and rises according to the cube of the motor speed. Infrequent applications for
HVAC, such as positive displacement pumps, may have constant torque characteristics.
Silicon controlled rectifiers (SCR's) are used to regulate an AC power supply to a resistive
electrical load, such as an electric heater, to provide continuously variable output. SCR's accept
standard analog control signals (usually voltage or current) and regulate the output of their load
proportionally. With microprocessor-based controls, SCR's can be used in combination with
sequenced contactors to provide vernier control that is continuous in proportion to the input signal,
but does not require control of the entire load by a SCR and thus reduces the cost.
Actuators (also called "operators") are one of the most important components of DDC systems
today. Actuators impart movement to the final control elements, dampers and valves, that are the
key to control of HVAC processes.
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HVAC Design Criteria and Guidelines
Valve and damper "action" is defined as "normally open" or "normally closed"…the position of the
valve or damper when its actuator is exerting 0 torque due to loss of control signal. In most
applications, the "failure mode" of a coil or heat exchanger is defined by the "normal" valve
position, open or closed. With valves, heating applications always utilize normally open control
valves so that heat is applied even if the control system fails, For cooling, the valve mode is
optional: in more northerly climates, normally closed cooling valves are typically utilized, while
normally open valves are used in warmer climates,
To have a valve or damper return to its normal position in the event of a control signal or power
failure, the use of a spring return type actuator is required. All pneumatic actuators, inherent to
their design, are spring return type. However, most electronic/ electric types are not, they stay in
their last position until a control signal is applied. Therefore, designers must specify the need for
spring return when this type of action is needed with electric/electronic actuators.
For DDC system installations, electric/electronic actuators tend to be preferred over their pneumatic
counterparts because:
1. Electric/electronic actuators easily can be interfaced directly with standard DDC
outputs. The cost and complexity of an additional transducer element
is not required in most instances.
2. Concentric shaft-mounting arrangements virtually eliminate non-linearity, hysteresis,
and play in the linkage system, making electric actuators as precise as, if not more precise
than, their pneumatic equivalents.
3. Elimination of the requirement for a compressor and compressed air system can
reduce first cost and ongoing operating costs significantly.
Modulating electric/electronic actuators must have drive motors rated for at least 1,200 starts/hour
and 60,000 full stroke cycles to help ensure long life. A positioning circuit accepts an analog
control signal (typically 0-10 V or 4-20 mA). The actuator then interprets this control signal as the
valve position between the two limit switches (maximum closed position and maximum open
position). To achieve this, the actuator has a position sensor (usually a potentiometer), which
feeds the actual rotation position back to the positioning circuit. In this way the actuator can be
positioned along its stroke in proportion to the control signal.
Other common problems with electric/electronic actuators include:
1. Torque ratings are limited for spring return type actuators.
2. Speed of movement can be as low as 4 seconds/mm that, in rapidly varying control
applications, may be too slow. (This is particularly true for cooling tower bypass valves.)
3. Their "plastic" enclosures and mounting assemblies are not particularly robust and
tend to fail, requiring actuator replacement. The designer must pay careful attention to the
operator construction at the time of vendor submittals... always request a sample of each
type and size of actuator to be used.
There are applications for which pneumatic actuators remain the better choice:
1. Installations that employ large dampers or valves with high operating torques. In many
cases, the actuating power of a pneumatic actuator simply cannot be matched.
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HVAC Design Criteria and Guidelines
2. Installations in which an actuator may need to respond quickly to a sudden change in a
process. A slow actuator often is the difference between recovering with a minimal
deviation from setpoint and dynamic instability, safety trips, and damage to a system.
Getting an actuation speed of 10 seconds or less with an electric actuator can be
expensive, if not practically impossible.
One of the most common applications in which the limitations of electric/electronic actuators can
lead to operating difficulties is control of a condenser water bypass valve. The problem tends to
appear at startup during the fall, winter, and spring, when tower basins are full of relatively cold
(typically, 40 to 60F) water. With large chillers and very short bypass circuits, the heat rejected by
the chiller quickly overwhelms the relatively small thermal flywheel represented by the recirculation
loop. Once setpoint temperature is achieved, if the valve cannot position itself to redirect flow over
the tower faster than the chiller can raise the temperature in the recirculation loop, the condenser
temperature will spiral out of control, and the chiller will trip off on high head pressure. Complicating
matters is that water temperature tends to step up in increments, rather than ramp up, as the
system warms up. There is not much opportunity for water to mix in the recirculation loop; so with
each pass through the chiller, the water in the loop tends to ratchet up by whatever the condenser
temperature rise is. This is a frustrating and difficult problem is best addressed during design by the
following:
1. Ensure that a fast actuator is supplied. Ideally, use a pneumatic actuator. If electric
actuation must be used, specify the required full range actuator speed required. Because
faster actuators are more expensive than their slower counterparts, they will not be
typically provided with competitive-bid procurement.
2. Design the control sequence so that the chiller is held at minimum capacity until the
condenser system is warmed up and the bypass valve is in control.
Valve actuators, no matter the type, must be selected for tight shut-off against the maximum
system pressure which that can be developed to prevent "valve lift" due to system pressure,
resulting in unintended flow through the coil or heat exchanger. For water systems, the maximum
possible system pressure is equal to the pump "cut-off" pressure, the pressure produced by the
pump at (or near) a zero flow condition. For steam systems, the maximum possible pressure is
equal to the maximum anticipated steam pressure at the valve.
Section 4: FINAL CONTROL ELEMENTS
The manipulated variable in most HVAC systems is usually a fluid (air, water, and steam, typically), whose
flow rate is changed in response to a change in the controlled variable’s condition. The devices that
manipulate these fluids are steam and water (or other liquid) control valves and air dampers, which are
called the final control elements.
The importance of the correct selection of control valves and dampers to proper control system operation
cannot be over-emphasized.
Control Valves
There are four valve characteristics that must be selected by the designer: required valve configuration and
operating mode, the type of valve required for the application, the valve rangeability, and the flow coefficient
(Cv).
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HVAC Design Criteria and Guidelines
Water control valves may be 3-way or 2-way configuration. Three-way valves are utilized when the water
system is designed for constant flow, while 2-way valves are used in variable flow systems. A standard
globe or butterfly valve is inherently a 2-way valve. A 3-way globe valve simply has a second port added
while 3-way action using butterfly valves requires two valves and a pipe tee.
Three-way control valves may be the "mixing" or "diverting" type. With mixing valves, the valve has two
inlets and one outlet, allowing two input flows to “mix” to create a single output flow, while diverting valves
have one inlet and two outlets, splitting the single input flow to two output flows. Diverting valves, due to the
pressures involved, require more robust construction and are more expensive than mixing valves, which are
typically preferred for HVAC applications.
The type of control valve selected for water flow control applications may be a globe valve or a butterfly
valve. The actuator torque required to operate a globe-type control valve is fairly constant from the full
closed to full open positions. But, for butterfly valves, the total valve operating range must be limited to a
range from 0 degrees (closed) to 70 degrees (80% open). The dynamic torque required to move a butterfly
valve starts fairly high as it necessary to "unseat" the wafer from the annular seal. The torque requirement
falls off quickly as the valve approaches 25 degrees of rotation (about 30% open), increases very rapidly to
peak at 75-80 degrees rotation, and then drops suddenly at approximately 85 degrees rotation. This makes
flow control unstable beyond 70 degrees of stem rotation.
Water valves are available with three flow characteristics, "quick opening", "linear", and "equal percentage":
1. Quick opening control valves are selected for two-position (open-closed) control. These valves
allow as much as 90% of their design flow rate when only 20% open, so there is little or no delay in
flow as the valve opens or closes.
2. Linear control valves have flow rates that are in direct proportion to the stem travel of
percentage open. These valves are used only to control loads with little variation and where very
"tight" control is required.
3. Equal percentage control valves are designed to be “slow opening”, allowing only about 10%
flow when 50% open. This characteristic is exactly counter to the capacity vs. flow relationship for
typical coils and heat exchangers, where 10% flow produces 50% heat transfer capacity.
Therefore, coupling an equal percentage control valve with a typical coil or heat exchanger results
in an essentially linear control function, as shown in the following figure, which is ideal for
proportional temperature control.
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HVAC Design Criteria and Guidelines
Rangeablity (also called "turndown") is the measure of the ratio of the maximum controllable flow rate
through the valve to the minimum controllable flow rate. The rangeability should be as large as possible and
never less than 30:1. Large rangeability reduces potential control problems under light load conditions.
The Cv is the flow rate through the valve resulting in a 1 psig pressure drop, and is used to define the valve
size to obtain the desired design pressure drop at the design (maximum) flow rate.
Water valves for modulating control duty must be selected to provide a wide-open pressure drop equal to or
greater than the full load pressure drop through the coil or heat exchanger being controlled. Thus, the
pressure drop through the valve, not through the coil, will control the actual flow rate (the valve with have
authority over the coil). Once the required pressure drop is established, the required water control valve
flow coefficient can computed as follows:
Cv = Q / (PD0.5)
where
Q =
PD =
the coil or heat exchanger peak flow rate (gpm)
pressure drop required through the wide-open control valve (psig), equal to or
greater than the coil or heat exchanger full flow pressure drop.
For applications where the Cv is greater than 160, the rangeability of a 70 degree rotation butterfly valve will
be better than the rangeability of a globe valve with the same C v. Since butterfly valves have very low wide
open pressure drop, valves in throttling duty will almost always be smaller than line size and must be
installed with concentric reducers to equalize the water pressure on the valve wafer.
Water valves for two-position control duty, no matter the design, may be line size or (more typically) one size
smaller. Butterfly valves for two-position duty use their full 90 degrees of rotation.
Steam control valves are always globe type. For modulating control duty, valves should be selected for a
wide-open pressure drop equal to 80% of the pressure differential (in psig) between the steam supply and
the condensate return to establish authority. The required flow coefficient can then be computed as follows:
Cv = (Q x V0.5) / (63.5 xPD0.5)
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HVAC Design Criteria and Guidelines
where
Q = the coil or heat exchanger peak steam flow rate (lb/h)
PD = pressure drop required through the wide-open control valve (psig)
V = specific volume of the steam at the supply pressure (cf/lb)
Steam valves for two-position control duty may be line size or one size smaller.
For steam preheat coils, a special control problem exists called steam stall. When, capacity is controlled by
a modulating steam control valve, as the load reduces, steam flow is reduced to reduce coil capacity. But,
reducing flow also reduces the coil inlet pressure (since the control valve acts as a pressure reducer when
throttling) and, at low flow conditions, the entering steam pressure may fall below the “back pressure”
created by the steam trap, condensate piping, and any condensate "lift"….steam flow stops (“stalls”) and coil
flooding occurs, potentially resulting in coil freezing. Any coil over-sizing will accelerate the stall problem.
There are two designer options to eliminate the steam stall potential:
1. Incorporate a dual control valve configuration with a modulating control valve (sized for 75-80%
of the flow) and a two-position control valve (sized for 20-25% of the flow) piped in parallel and
operating in sequence. At low loads, the modulating valve is closed and temperature control is
maintained by open/close control of the two-position valve. When the two position valve is open, it
provides enough steam pressure to "push" any condensate out of the coil.
2. Use a single modulating control valve and install a pressure-powered condensate pump/trap,
which eliminates the trap, condensate piping, and any lift losses as back pressure.
Control dampers
A modulating control damper can be thought of as an “air control valve” and for the inherent damper
characteristics (i.e., airflow vs. percent open) to be nearly linear. This typically requires that the damper be
an opposed blade type, sized so that its wide-open pressure drop is 8-10% of the total system pressure
drop.
The pressure drop through a wide-open opposed blade damper is very low, as shown in the following figure:
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HVAC Design Criteria and Guidelines
HVAC control dampers typically have a free area ratio of 0.70-0.80 (unless they are very small where the
free area ratio can be reduced to 0.50-0.60).
The most common application of control dampers in air systems involves dampers for controlling air mixing
for an AHUs economizer cycle. To size these dampers, the first step is to evaluate the “system” in which the
damper is installed. For an economizer cycle application, each damper has its own system defined as the
outdoor air, return air, and relief air flow paths and each damper is selected based on the individual system
or path pressure condition. ASHRAE Guideline 16 defines the methods for sizing these dampers.
With an airside economizer cycle, mixing of the outdoor and return air streams to maintain uniform air
temperature and eliminate the potential for coil freezing due to "stratification" is very important. The
following approaches help to ensure complete mixing of the return and outdoor air streams:
1. Arrange the intersection of the return and outdoor air paths 180 degrees from each other so that
the two air streams meet “head on” to promote mixing.
37
HVAC Design Criteria and Guidelines
2. If the two air streams must meet at 90-degrees from each other, utilize parallel blade dampers,
arranged to provide near "head on" airflow paths. (For systems with low damper velocities,
additional mixing baffles may also be required.)
3. If the two air streams enter the mixing plenum parallel to each other on the same side, install
baffles to force the two air steams to mix and/or utilize parallel blade dampers installed vertically to
provide near "head on" air flow paths.
The second most common application of control dampers in HVAC systems is for airflow control in VAV
systems, a common source of control problems in the field. However, most "control" problems with VAV
terminal units (TUs) actually result from failure by the designer to properly design VAV TUs and their
installation, resulting in poor temperature control:
1. Failure to define both maximum and minimum air flow rates for each TU and/or failure to select
the terminal for minimum inlet velocity of 600 fpm (0.03” VP). The following table summaries
typical TU airflow ranges on this basis:
Inlet
Connection
Size
(Diameter)
4”
6”
8”
10”
12”
14”
16”
CFM
Range
(min-max)
55-200
120-400
205-700
310-1100
475-1550
640-2100
840-2800
2. Failure to select pressure independent TUs.
3. Sizing supply ductwork to TU smaller than inlet connection size.
4. Designing TU with poor supply duct arrangements: inadequate straight duct (at least equivalent
to 4 duct diameters) or using long flex duct runs that reduces the available duct static pressure.
5. Failure to apply fan-powered TUs correctly:
a. Utilize series arrangement TUs with constant volume operating mode.
b. Primary air flow must always be equal to or less than TU fan air flow.
c. Manufacturer’s minimum downstream pressure loss requirement may be a problem for TUs
without reheat coils.
One common control problem is the failure by the designer to require software interlock between each AHU
and the TUs it serves so that all controls associated with the AHU are de-energized when the AHU is
stopped.
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HVAC Design Criteria and Guidelines
Section 5: DIRECT DIGITAL CONTROLLERS AND SYSTEMS
Controllers and Control Loops
The key element in any DDC system is the microprocessor-based digital controller, illustrated by the
following figure:
The microprocessor performs several functions. First, it accepts the input signals, either binary or analog,
that define the condition or value associated with the measured variable. Then, based on the control logic
defined by the stored program(s), computes an output condition or value for the manipulated variable.
Analog input and output signals must be converted to and from digital values by "converters" that are part of
the controller electronics. Other functions, such as communications with other controllers, time-keeping,
data storage, etc. are also performed by the controller microprocessor.
In general, there are two classifications of controller: primary control units and secondary control units.
Primary controllers (often referred to as "building level controllers") typically have the following features:









Real-time accurate clock function
Full software compliment
Larger total point capacity
Support for global strategies
Buffer for alarms/messages/trend & runtime data
Freeform programming
Downloadable database
Higher analog/digital converter resolution
Built-in communication interface for PC connection.
Secondary (or "application") controllers typically have the following features:







Not necessarily 100% standalone
Limited software compliment
Smaller total point count
Freeform or application specific software
Typically lower analog-to-digital converter resolution
Trend data not typically stored at this level
Typical application is terminal equipment or small central station equipment.
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HVAC Design Criteria and Guidelines
A DDC "system", then, consists of multiple microprocessor-based controllers with the control logic dictated
by software. Most systems distribute the software to remote controllers to eliminate the need for continuous
communication capability, i.e. each controller can "stand-alone".
Control "loops" are at the heart of a DDC system’s control logic. A "closed" control loop, as shown in the
following figure, must have an output (typically an AO or DO point), an input (typically an AI or DI point), a
setpoint, and a defined control sequence of operation ("logic") resident in the controller.
SETPOINT
CONTROLLER
OUTPUT SIGNAL
FINAL CONTROL
ELEMENT
INPUT
SIGNAL
MANIPULATED VARIABLE
HVAC PROCESS
CONTROLLED
VARIABLE
SENSOR
DISTURBANCES
For any HVAC process control, there is a controlled variable (temperature, humidity, pressure, etc.) and a
corresponding manipulated variable (fan speed, water flow, airflow, etc.) The control loop then, based on
the variation between the setpoint and the actual condition resulting from a "disturbance" (based on the
"feedback" input provided by a sensor), provides an output signal to the final control element (valve, damper,
switch, etc.) that changes the manipulated variable to return the controlled variable to its setpoint
condition…very simple in concept, sometimes no so simple in execution.
Multiple sensor inputs may be incorporated in order to have an HVAC process control respond to multiple
conditions. For example, hot water supply temperature setpoint for a boiler may be boiler may be "reset" as
a function of outdoor air temperature.
A control loop may be "open". In an open control loop, the output has no effect on the input. For example,
while a boiler (the output) is turned on when the outside air temperature (the input) is below 55ºF (the
setpoint), the boiler’s operation clearly has no effect on the outside air temperature. An open control loop
often can use a DI point or even a software variable as its input. An example is AHU start/stop (the output)
based on a schedule (where the time of day is the input and the start time is the setpoint).
Each control loop is maintained within a direct digital controller, which is, in essence, a microprocessor
computer. Every DDC controller consists of the following components: an analog-to-digital input converter
which translates analog inputs into digital input signals; a digital-to-analog converter that translates digital
output signals into analog output; and a microprocessor with program (usually ROM) memory in which the
control logic resides, RAM or working memory, and one or more communications ports. These controllers
may be "application specific", designed for one HVAC control application such as controlling an AHU, or be
more general in nature, capable of being programmed for any type of HVAC process control.
DDC Systems and Control
To understand DDC systems, it first necessary to understand the underlying concepts and terms that apply.
40
HVAC Design Criteria and Guidelines
The term points is used to describe data storage locations within a DDC system. Data can come from
sensors or from software calculations and logic. Data can also be sent to controlled devices or software
calculations and logic. Each data storage location has a unique means of identification or addressing.
Data in a DDC system can be classified three different ways, by data type, data flow and data source.
Data type is classified as digital, analog, or accumulating. Digital data may also be called discrete
data or binary data. The value of the data is either 0 or 1 and usually represents the state or status
of a set of contacts. Analog data are decimal numbers and typically have varying electrical inputs
proportional to temperature, relative humidity, pressure, or some other common HVAC sensed
variable. Accumulating data are also numeric, decimal numbers, where the resulting sum is stored.
This type of data is sometimes called pulse input.
Data flow refers to whether the data are going into or out of the DDC component/logic. Input points
describe data used as input information and output points describe data that are output information.
Data Source: Points can be classified as external points if the data are received from an external
device or sent to an external device. External points are sometimes referred to as hardware points.
External points may be digital, analog, or accumulating and they may be input or output points.
Internal points represent data that are created by the logic of the control software. These points
may be digital, analog or accumulating. Other terms used to describe these points are virtual
points, numeric points, data points and software points.
Global or in-direct points are terms used to describe data that are transmitted on the network for
use by other controllers. These points may also be digital, analog, or accumulating.
There are basically three common approaches used to program the logic of DDC systems. They are line
programming, template or menu-based programming, and graphical or block programming:
Line programming-based systems use BASIC or FORTRAN-like languages with HVAC
subroutines. A familiarity with computer programming is helpful in understanding and writing logic
for HVAC applications.
Menu-driven, database, or template/tabular programming involves the use of templates for
common HVAC logical functions. These templates contain the detailed parameters necessary for
the functioning of each logical program block. Data flow (how one block is connected to another or
where its data comes from) is programmed in each template.
Graphical or block programming is an extension of tabular programming in that graphical
representations of the individual function blocks are depicted using graphical symbols connected by
data flow lines. The process is depicted with symbols as for electrical schematics or pneumatic
control diagrams. Graphical diagrams are created and the detailed data are entered in background
menus or screens.
Architecture and Networks
HVAC designers, for the most part lacking any background in electronics or information technology (IT),
have some difficulty understanding DDC networks and how DDC system components communicate with
each other and their operators. For a practical matter, detailed understanding of these elements is not
necessary in order to design and specify a DDC system. However, a basic understanding is necessary.
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HVAC Design Criteria and Guidelines
A network is the “infrastructure” over which the connected devices are connected to each other. These
connections involve equipment like routers, switches, bridges and hubs using cables (copper, fiber, and so
on) or wireless technologies (Wi-Fi).
A network communications protocol consists of a standard set of rules that define characteristics for
transporting data between network devices (see Section 6).
Networks are defined by separate specific functions (called “layers”), as follows:
Physical: Cable or media standards. Electrical characteristics, length limitations, how computers or
other devices mechanically and electrically connect.
Data Link: Format of data on the network and how it flows. Standards describe how data is
packaged into "packets", including things like maximum length and what type of address is included
for the sender and receiver. Other standards describe how a device can gain access to the
physical layer, what to do if there is contention, how to tell if there is a transmission error, how to
mark packets to keep them in sequence, etc.
Network: Provide routing and related functions that enable multiple physical network segments to
be combined into an inter-network. This layer provides the standards for logical naming and
addressing of devices so you can route to them even if they are not physically connected to the
same network.
Transport:
Provide reliable process-to-process communication. This layer implements
"connections", which require that data flow in sequence, that errors be detected and corrected, and
that data transmissions be acknowledged, if desired - some applications like web servers don't
check if you actually got the data. At this level, the network also provides "addresses" for different
types of services to make sure that data received by a computer goes to the correct application
process.
Session: Concept of tying together multiple transport streams into a single "session".
Presentation: Defines data format conversion, compression, encryption, etc.
Application: What the user is trying to accomplish, for example, email, file transfer, Web browsing,
etc.
Today, TCP/IP (Transmission Control Protocol/Internet Protocol) is by far the most dominant suite of
networking protocols since it is the basic communication protocol of the Internet.
TCP/IP is a two-layer program. The higher layer, Transmission Control Protocol, manages the assembling of
each message or file into smaller packets that are transmitted over the Internet and received by a TCP layer
that reassembles the packets into the original message. The lower layer, Internet Protocol, handles the
address part of each packet so that it gets to the right destination. Each device communicating over the
network checks this address to see where to forward the message. Even though some packets from the
same message are routed differently than others, they'll be reassembled at the destination.
The Internet uses additional higher layer application protocols that utilize TCP/IP. These include the W orld
Wide Web's (www) Hypertext Transfer Protocol (http), the File Transfer Protocol (ftp), Telnet, which lets you
logon to remote computers, and the Simple Mail Transfer Protocol (smtp) that handles eMail. These and
other protocols are often packaged together with TCP/IP as a "suite."
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HVAC Design Criteria and Guidelines
For wired networks, Ethernet is a popular standard that defines a number of wiring and signaling standards
for the physical layer of a network. Ethernet has been standardized as IEEE 802.3. Its twisted pair wiring
form has became the most widespread LAN technology in use.
In recent years, WiFi, the wireless LAN standardized by IEEE 802.11, has been used instead of (or in
addition to) Ethernet in many installations.
Three types of networks are typcially used in DDC systems. At the highest level of communication, a clientserver network is usually utilized. This may be a stand-alone network utilized solely by the DDC system or
may be a site network (LAN or WAN) that serves as an "Intranet". In the second case, the control
applications share use of the network with the normal IT functions. This type of network is typically used as
the primary LAN in most DDC systems.
A client-server network consists of one or more computers that act as servers, managing the network and
the data traffic over it. Each device connected to this network, then, is a client that makes use of the
network for communications with other clients. In the control environment, the client may be a controller or a
communications interface device.
For smaller DDC systems, a peer-to-peer network may be used as the primary network. This type may also
be used as a "sub-network" on larger systems. On this type of network each device can share information
with any other device on the LAN without going through a server or communications manager, as indicated
in the following figure:
A peer-to-peer network can be enhanced to utilized the Internet protocol (IP), allowing any device on the
network to be connected to and communicate over the Internet.
At the lowest level, some systems use a polling controller LAN in which the individual controllers can not
pass information directly to each other. Instead, data flows from one controller to the interface and then from
the interface to the other controller, as indicated by the following:
The interface device manages communication between the polling LAN controllers and the higher levels in
the system architecture. It may also supplement the capability of polling LAN controllers by providing the
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HVAC Design Criteria and Guidelines
following functions: clock functions; buffer for trend data, alarms, messages; and higher order software
support.
Different DDC systems combine the communications of a server-client network, a peer-to-peer network, or a
polling network to create communication "levels". In the figure above, the interface communicates in a peerto-peer fashion with the devices on the peer-to-peer LAN. The polling LAN-based devices can receive data
from the peer-to-peer devices, but the data must flow through the interface.
In some DDC systems the network is "flat" (i.e., there are no sub-networks) and all devices communicate
over a single network.
The next critical element in the DDC system is an operator interface device. Operator interfaces are required
to:





View and interrogate data
Program the system
Exercise manual control
Store long term data
Provide a dynamic graphical interface.
There are five basic types of operator interface devices:

Desktop computers which act as operator workstations. Desktop computers are centralized
operator workstations where the main function is programming, building and visualizing system
graphics; long term data collection; and alarm and message filtering.

Notebook computers which act as portable operator workstations. Notebook computers may
connect to the LAN through a communication interface that stands alone or is built into another
device.

Keypad type liquid crystal displays. Keypad liquid crystal displays typically are limited to point
monitoring and control. They may have some limited programming capability, such as
changing a set point or time schedule.

Handheld consoles/ palmtops/ service tools. Handheld consoles, palmtops and service tools
are proprietary devices that connect to primary controllers or secondary controllers. Typically
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HVAC Design Criteria and Guidelines
they allow point monitoring and control, controller configurations (addressing and
communication set-up), and calibration of inputs and outputs.

Smart thermostats. Smart thermostats are sensors with additional capabilities. They connect to
secondary controllers and have a service mode to allow for point monitoring, control and
calibration. They also have a user mode that allows point information to be displayed, setpoint
adjustment and an override mode.
Operator interface devices can be connected to the primary LAN through a communications interface as
illustrated below. But, a Web server and appropriate software can be used to allow each interface device to
communicate the Internet, using a conventional Web browser, such as Internet Explorer or Firefox. This
type of connect allows the DDC system to broadcast alarms or other information as email or text messages
to cell phones, PDA's, etc.
When systems become larger than the capacity of a single sub-network, such as at a university campus,
medical center, or even a larger secondary school district, a higher level of architecture is added to allow the
use of multiple sub-networks, as shown in by the following figure:
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HVAC Design Criteria and Guidelines
The site LAN or WAN may be an existing network used as an Intranet or it may be a dedicated controls
network. The site LAN/WAN is used to connect multiple sub-networks and site computers. Multiple subnetworks can be connected to a single site LAN/WAN that allows information sharing between devices on
different sub-networks, which could be individual buildings on a common site or different sites.
Multiple site computers can be added to the site LAN/WAN. They can connect the site LAN/WAN via a
communications interface, which may be a router. Site LAN/WAN computers can send and receive
information from the entire system. Information can be received by each of the site computers, but can not
be subsequently shared from one computer to another. Sub-network computers may only be able to see
their own sub-network.
Site LANs allow multiple computers to communicate with each other. They may use commercially available
computer network software and hardware. Messages, alarms and other data can be re-routed to other
computers on the primary site LAN. Information stored in other computers can be remotely accessed. This
includes graphics, programming and stored trend and operational data.
Some vendors combine multiple functions into a single device. In the following system architecture, the
communication interface is built into each primary controller. A peer-to-peer LAN or sub-network is
connected directly to the controller/interface.
Alternatively, as shown by the following figure, the key component in the system can consist of a
communication interface, a combined primary controller and interface to the secondary polling network.
For the HVAC designer, the details of networks and vendor arrangements of controllers and other devices
on them is relatively immaterial. Only two aspects are really important: (1) all networks and the
communications over them must meet open standards so that any device meeting that standard can be
connected to it and (2) the network must be reasonably "robust" to support the level of data traffic required
for HVAC control. As discussed in Section 6, the two major protocols used in the industry for DDC systems
meet both of these criteria.
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HVAC Design Criteria and Guidelines
Section 6: DDC COMMUNICATION AND PROTOCOLS
Communication between two different devices on a common network, requires a common protocol, a
common communication speed and method of data formatting. Vendors build their devices around these
criteria, so communication between devices by the same manufacturer is routine. The ideal condition for
"interconnectivity" is for any third party to provide a component controller that is "native" to the overall
building control system.
For a building DDC system to communicate with other DDC systems using a different communications
protocol, there is a need for an interface or gateway to translate between the two systems. The proper
operation of the gateway is dependent on the continued use of the specific revised levels of software on
both systems. It typically requires the support of the manufacturer at the corporate level to implement and
cooperation between manufacturers, a highly improbable state of affairs!
In the past, it has been a particular irritant to designers and owners that no DDC system provided by one
controls manufacturer could be modified or extended by any other controls manufacturer because the
protocols used by each manufacturer was both unique and proprietary, what is called a closed protocol.
Today, designers require the use of an open system that uses a "standard" protocol and allows components
from different manufacturers to co-exist on the same network. These components would not need a gateway
to communicate with one another and would not require a manufacturer specific workstation to visualize
data. This would allow more than one vendor’s product to meet a specific application requirement.
Unfortunately, the sole use of an open or standard protocol does not guarantee that a DDC system will be
an open system. A manufacturer has the ability to use open or standard protocols, yet create a closed
system, thus continuing a building owner’s dependence on a single manufacturer. This can be accomplished
by using unique communication speeds, unique data formatting and by not adopting the full range of an
open protocol. The job of policing compliance with a standard protocol by the manufacturer falls to the
designer.
Today, the most widely accepted and applied standard protocol used for commercial controls is BACnet, an
open protocol published by the American Society of Heating, Refrigerating and Air-conditioning Engineers
(ASHRAE), defined by ASHRAE Standard 135, BACnet - A Data Communication Protocol for Building
Automation and Control Networks.
BACnet has seen rapid acceptance because it can be "scaled" to any size application. BACnet can be easily
applied at the device level to create an interoperable sub-network of intelligent systems. Another key factor
to BACnet's growth is the implementation of reliable compatibility testing to ensure that BACnet devices are
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HVAC Design Criteria and Guidelines
truly fully compliant with the standard. The BACnet Manufacturers Association (BMS) operates BACnet
Testing Laboratories (BTL), which tests the BACnet functionality of any product submitted by a manufacturer
in accordance with ASHRAE Standard 135.1, Method of Test for Conformance to BACnet. Products that
pass these tests are "listed" by BTL.
The primary communication protocol for BACnet is BACnet/IP with operates over an Ethernet TCP/IP
physical network and allows BACnet devices to communicate directly with each other, over the Internet, or
over an owner's Intranet. Web servers, connected to the Internet and the BACnet/IP LAN, are configured to
receive information from BACnet devices and sub-networks and present them in a form that can be viewed
and altered from standard Web browsers.
Section 7: THE DDC DESIGN PROCESS
Most design firms have controls standards in place…standard sequences, standard control diagrams and
details, standard controls specifications, and, perhaps, even standard control point definitions. But, too
often, these firms do not have a standardized controls design "process" as part of these design standards.
This lack of a process generally is justified due to controls design’s perceived status as "art" rather than
science.
However, good controls design is a relatively straightforward five step process, as follows:
1. Review the HVAC design to initially judge if it can meet the building’s/owner’s requirements for
controllability and function, e.g., zoning, part-load and low-load operation, areas of varying
operating schedules, etc. Most commonly, poor control results not from poor controls, but from
poor HVAC system design. This process starts during the conceptual or schematic design phase
and is simply developed in more detail as the design proceeds to completion of contract
documents.
2. Review the HVAC equipment specifications to determine what controls/safeties are specified
with each piece of HVAC equipment (AHUs, packaged units, chillers, VFD's, packaged pumping
sets, etc.) It may be obvious that some of these controls or safeties should be eliminated and
instead provided by the DDC system. Other OEM or vendor controls may require integration into
the DDC system.
3. Develop the control point definitions and operational sequence in a top-down fashion:
a. Determine all HVAC systems, subsystems, and components to be controlled.
Determine the owner's specific intent for systems operation, setpoints, etc. Keep the
input and output points to the minimum number required for HVAC control and efficient
building operation…follow the KISS principle, "keep it simple, stupid."
b. Using the office standards, create a diagram of each system, including the all
components to be controlled (e.g., fan/pump motors, valves/dampers, etc.).
c. Define the modes of operation for each component (e.g., occupied vs. unoccupied,
heating vs. cooling, etc.). Alternatively, if an operation mode affects multiple components
directly, then these modes may be defined as separate items.
d. Define failure modes. Failure can occur from a global aspect such as building power
loss, control system failure, etc. or can be more localized due to the failure of as specific
component such as a fan, pump, chiller, etc.
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HVAC Design Criteria and Guidelines
For global failures, there is very little to be done except to ensure that the HVAC systems
re-start automatically when power or control is restored.
For localized failures, failure mode definition can be more complex. For example, if the
chiller fails and cooling is not available, what should a VAV fan speed controller do? In
this same case, should VAV terminal units fail open, fail closed, or simply remain where
they are? What should happen if the campus steam supply fails during the winter? Are
there specific control modes required for smoke or fume venting to be initiated by an alarm
condition?
e. Finally, define the sequence of operation for each of the HVAC systems' operating
modes. Each of these sequence elements should be short. If they aren’t, this is an
indication that not all of the operation modes were listed. The idea here is to create a
sequence that looks more like an outline and less like a narrative. This outline approach
makes the sequence easier to write (each sequence element is a small and easy to
comprehend piece of the whole), and easier to implement by the contractor (the sequence
essentially should be one step removed from the software code they develop). Further, be
very deliberate with the use of such words as “and” and “or” since they can (and will) be
directly translated into the Boolean logic statements within a DDC program.
4. Perform another review of the HVAC system design and specifications following Steps 1 and 2.
This may provide greater insight into whether the HVAC system can be controlled as intended.
(Determining whether to modify the sequence of operation or the HVAC design will require
judgment.) This step also may reveal that further refinement to the equipment specifications is
needed.
5. Keep the design simple. Complex HVAC systems (or system functions) will result in complex
sequences of operation, increasing the potential for error and failure. Make sure the sequences are
as simple as possible since HVAC control and building operations do not generally perform well
with overly complex operation sequences.
Two important points about this control design process are (1) it reflects the strong interdependence
between an HVAC system’s mechanical and controls designs and (2) it is a process that may dictate a
simplifying change or refinement of the mechanical design.
HVAC designers must include the following elements in their contract documents (preferably on the
drawings rather than buried in the specifications…everyone on the jobsite has drawings, but no one on the
jobsite can find the specifications) to adequately define these requirements:
1. System Schematics: For each HVAC system…air-handling system, chilled water system, steam
system, hot water system, etc…there must be a system schematic or diagram showing the HVAC
components and the location of each control sensor and actuator that is required.
2. Points Definition and Sequences of Operation: Regardless of the details of construction, every
DDC controller provides the same functionality…based on defined inputs and defined sequences,
defined outputs occur. But, the HVAC designer must define the input points, the output points, and
the required control sequence(s) based on them.
Some designers use "points lists". But, points lists often are incomplete or too vague and can
result in problems when applied. A better approach is to use a generic "controller diagram". With
this, a DDC controller is shown as simply a "box", but each required input and output point
connection is show, complete with input devices, output to final control elements, and required
interlock wiring…the details of the actual DDC controller are unimportant.
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HVAC Design Criteria and Guidelines
Sequences should be incorporated into the controller diagrams and should be as complete as
possible without being too "wordy"...write the sequence in outline form, not narrative form. And,
since control contractors are not very HVAC savvy, "design narratives" or design intent descriptions
are also needed so that the control contractor has a clear understanding of the goals for overall
HVAC systems performance (is a major consideration during the commissioning process).
Where hardwire interlock is required, provide an elementary electrical ladder diagram for each
electrical circuit controlled.
To reduce the learning curve on each project and to help ensure consistency of design, the control
diagrams and sequences should be "standard details" that are incorporated and edited as
necessary for each project. (Other standard details for valve installation, damper installation, etc.
must also be included.)
3. Floor Plans: On the HVAC floor plans show the location of key control system elements such as
panels, workstations, major cable and raceway routes, and the sources of power. Certain input
points must also be located…duct static pressure sensors, piping differential pressure sensors, flow
sensors, flow monitors, etc.
DDC system specifications should address the overall control system performance and ignore the
unimportant details that most vendor "guide" specifications typically include:
1. Overall system functionality must be specified along with required control modes, end-to-end
sensor accuracies, and the requirements for final control elements such as valves and dampers.
The requirements for wiring and raceways must be defined.
Finally, there must be a
"commissioning" requirement that involves testing the HVAC systems and their controls through
every mode of operation.
Don't list "allowable" DDC manufacturers. This type of list generally overrules (and makes moot)
specifications.
2. Open/standard communications protocols and Internet-interface must be addressed. The use of
proprietary DDC systems and communications protocols is not acceptable to most owners unless
there is already a system in place and a new project must be connected to the existing system.
3. Designer "check-out" and commissioning of the controls, along with the entire HVAC system,
must be required. It is axiomatic that a DDC system will not function as well as expected without
some form of commissioning. However, the specification for commissioning requirements should
not be treated as a separate task in the overall mechanical design process rather than an effort
specific to the control system design (see Section 8).
Specific DDC Design Concerns
When applying DDC systems, the designer must consider the following specific design concerns:
1. Although DDC technology brings us better accuracy, repeatability, and reliability, the high-precision
capability of current-technology sensors can be difficult to assess in the field without some fairly exotic test
equipment. In addition, absolute accuracy may not be as important as relative accuracy when you are up
and running.
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HVAC Design Criteria and Guidelines
Control system accuracy and stability can be improved with only a modest increase in price by going from a
"commercial grade" sensor to a "process grade" device.
Frequently, absolute accuracy requirements of key sensors in a system are critical if demands of the
process the system serves are to be met. These requirements
vary from process to process. For example, a discharge sensor with an accuracy of ±1.5F may be sufficient
to ensure that a system hits a particular psychrometric state point and comfort envelope for an office
application. However, a system serving a cleanroom with lower temperatures and tighter tolerances may
require more-precise control of discharge temperature and, thus, a better sensor.
Once accuracy issues of sensors associated with critical process parameters are addressed, the accuracy
of other sensors in the system relative to the key sensor can be far more important in the day-to-day
operating arena. For example, consider a simple make-up air unit with steam preheat and chilled water
cooling, operating at 55F outdoor air temperature:
This system requires a 55F cooling coil discharge temperature and uses independent control loops for each
heat-transfer element. All of the sensors serving the system meet a typical ±1.5F accuracy requirement for
averaging type sensors. But because the sensor serving the preheat coil is operating at the bottom limit of
that range, it detects the outdoor-air condition as being lower than desired and adds heat unnecessarily. The
cooling coil’s controller, operating at the upper limit of its accuracy window, detects the cooling-coil leaving
condition as being warmer than it actually is and then overcools the air. As a result, the air-handling unit
uses heating and cooling energy in an unsuccessful attempt to achieve a leaving-air temperature that could
have been obtained simply by bringing outdoor air into the system at the current condition.
It is important to consider the requirements of the overall system relative to a sensor’s
capabilities. Some of the newer sensing technologies are sensitive enough to detect information you may
not need. An operating problem can result if the control system tries to do something with this excess
information
2. The computing power of modern electronics allows complex programming strategies to be implemented
in compact packages Complex building blocks such as the proportional-integral-derivative (PID) algorithm
are easily invoked and replicated. But digital technology controlling analog processes can be much more
difficult to describe in programming code than discrete analog components assembled into an analog control
system.
A significant number of instability problems with DDC systems may be a result of the technology being so
precise. For example, with a pneumatic thermostat, a setpoint of 75F typically means that, as a load
changes over the course of a day, the temperature in the space gradually floats inside the throttling range of
the device. In contrast, a digital-control algorithm may see 74.9999F as vastly different from 75F and 75F
as vastly different from 75.0001F and change to a different operating mode. Operating-mode changes can
introduce step changes that trigger problems in the local process; these problems, in turn, can ripple out to
all other processes in the HVAC system.
In some cases, the hysteresis and slower response times of mechanically based sensing technologies may
have worked in our favor by damping and filtering input signals to our
control processes.
With pneumatic control technology, when you create a VAV zone, above the ceiling you install a pneumatic
flow controller that was reset based on space temperature from a pneumatic thermostat. The controller
modulates a pneumatic damper actuator and perhaps a pneumatic reheat valve sequenced by proper
selection of its spring ranges. With DDC, to create the same zone control, you install above the ceiling the
equivalent of a small desktop computer that uses nonlinear transducers as inputs, provides electronic output
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HVAC Design Criteria and Guidelines
to gear-train type damper and valve actuators that sequence based on software control, and talks to other
computers on the network using a communication protocol.
On the plus side, the communication capabilities make detecting and troubleshooting a problem much
easier. However, the complexity and non-mechanical nature of the technology can be intimidating and make
fixing a problem difficult, even if the controllers and actuators are of the plug-and-play variety. With
pneumatic technology, operators have to go looking for problems to some extent. But, once they found
them, the solutions often were more intuitive.
3. How does the HVAC designer deal with integrating OEM equipment controls ("third party" interfaces) into
a DDC system?
Today, most major HVAC equipment manufacturers are also in the control business and incorporate OEM
(original equipment manufacturer) controls with their larger commercial packaged equipment…packaged
units, chillers, variable frequency drives, etc.
In a stand-alone environment, this is wonderful since it eliminates the need to install additional operating
controls…everything is already in place, programmed, and ready to run. However, when this equipment is
installed within the scope of a building-wide DDC system, it makes more sense to integrate the OEM
controls into the DDC system rather than replace them with new, field-installed controls.
OEM controls can be interfaced with a DDC system in one of two ways: (1) the designer can specify a
specific open technology required for the OEM controls (LonWorks or BACnet) or (2) the designer can allow
the OEM or the DDC vendor to install a "gateway" to provide the required interface. Option 1 is preferred
simply because it represents the best integration of the OEM controls into the DDC system. This is the
method required by the SW master specifications.
But, often the OEM controls are not compatible with the DDC system and a gateway is the only answer short
of wholesale replacement. In the case where gateways are required, the designer must clearly understand
the design requirements.
Typical HVAC equipment control gateways consist of three elements:
1. A communication link that connects the two (or more) systems.
2. One or more translators (or "drivers") that understand the communication rules of each system
being connected so that data streams can be converted into useful information for the system or
systems that require the interconnection.
3.
A simple operating system and section of memory (or buffer) where required
intercommunication data is mapped so that relevant data is captured and stored until it can be
communicated to the other system(s).
Where a gateway is the only option, the designer must specify the following:
1. Describe exactly what data needs to be transferred across the interface and how that data is to
be employed by each system involved. For example, if a chiller must be controlled on and off and
have its maximum capacity limit set across a gateway interface, it is necessary to call for the chiller
start/stop and demand-control points to be networked across the gateway. It also is necessary to
explain how and where in the network the commands are to be made. Finally, it is useful to detail
how the information will be employed on each side of the interface, how often data must be sent
and/or received, and any other information that helps ensure that the system operates effectively.
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HVAC Design Criteria and Guidelines
2. Assign a single source of responsibility and authority in making each interface work. Normally,
the DDC system vendor should be responsible for each interface and be given the authority to
determine if the interfaces provided by the equipment vendor meet the requirements of the
specified interface standard and other specification items. If the controls vendor concludes that an
interface does not meet specified requirements, then the provider of that interface must make the
changes mandated by the controls vendor or clearly show where the controls vendor erred. This
approach greatly reduces the finger pointing when a multi-vendor network is brought on-line.
Section 8: COMMISSIONING HVAC SYSTEMS CONTROLS
Once HVAC systems and the DDC system are designed and installed, they must be properly commissioned
to ensure that they function in accordance with the design intent and provide the intended level temperature,
humidity, air cleaning, ventilation, air distribution.
Before placing the control system in operation, the vendor must perform the following start-up checks and
procedures:
a. Verify proper voltages and amperages and that all circuits are free from grounds or faults.
b. Verify integrity/safety of all electrical connections.
c. Verify proper interface with fire alarm system and all other safety and interlock circuits.
d. Operate all valves and dampers, manual and automatic, through their full stoke. Ensure smooth
operation through full stroke and appropriate sealing or shutoff. Verify actuators are properly
installed with adequate clearance for maintenance/replacement.
e. Check and set zero and span adjustments for all actuators.
The controls vendor must verify calibration of sensors, sensor/transmitters, and sensor/controllers, including
temperature, humidity, pressure, flow, and electricity consumption for compliance with the requirements of
this Section. Select at least 10% of the installed sensors, including at least one of each sensor type, for
testing. If calibration of 10% or more of this sample is found to be incorrect, select an additional 10% of the
installed sensors for testing. If calibration of 10% or more of this second sample is found to be incorrect, all
sensors must be tested and calibrated.
Sensor calibrating instruments, using a standard traceable to the National Institute of Standards and
Technology (NIST), must be used in checkout of the overall performance. The sensors of these instruments
must be placed at the proximity of DDC system sensors to indicate the conditions of the controlled media
(air, water, etc.). A preliminary evaluation must be made as to the suitability of having the DDC system
sensors checked in-place or they may be placed in simulated environment. If the response times of the two
sensors (DDC system sensor and calibration sensor) are similar, testing may be performed with the sensors
in place. If the conditions of the controlled media change slowly, testing may also be performed with the
sensors in place. However, if the conditions of the controlled media change rapidly and the time responses
of the two sensors vary considerably, testing must be done with the sensors placed in a known environment
such as a temperature bath.
Checkout procedures for air, steam, and water temperature, air humidity, air static pressure, and steam
pressure with sensors in place must be as follows:
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HVAC Design Criteria and Guidelines
a. Place calibration sensor in the controlled medium (air, water, or steam). Caution must be
exercised that DDC system and calibration sensors do not interfere with each other (such as for
temperature or pressure sensing) and that the calibration sensor does sense the true conditions.
b. Wait sufficient time for the calibration sensor to stabilize. The time required depends on the time
constant and the initial condition of the sensor.
Coordinate time between the site and central control room for taking readings.
c. Simultaneously take readings of the calibration instrument and the DDC system readout
terminal.
d. Take at least one more reading for the same sensor. The range of the two readings taken must
be as wide as the HVAC operation allows, but not more than that called for in the contract
specifications. In order to have the wide range desired the second test may need to be conducted
at a different time from the first test when the HVAC equipment experiences different loads.
e. Compare readings from the calibration instrument and the DDC system. They must be within the
accuracy requirements of the specifications. The values of certain data need to be calculated
before comparison. These are specifically noted in the following paragraphs.
The designer must verify existence and operation of all specified points and software:
a. Verify backup system operations and switchovers including redundant processors, backup
power supplies, battery backed memories, etc.
b. Verify DDC system command software. This may be checked by issuing commands observing
display, printer output, or HVAC equipment responses.
c. Test the software's ability to check input commands and issue error messages by entering
various correct and incorrect commands.
d. Check system and point addressing. Enter command to display I/O data. Verify existence of all
data points defined on the drawings and/or required by the specifications.
e. Test start-stop or enable-disable of HVAC equipment or DDC system control points. Enter
commands to start/stop selected HVAC equipment, and to disable and enable selected points.
f. Test the operator override/automatic mode capability. Enter commands to change selected
automatic control under DDC system to manual and vice versa.
g. Test the display format. Enter commands to display data and graphics on terminal and graphic
display. Check display content for adequacy and clarity as specified.
h. Verity the ability to modify, cancel and confirm operator’s commands by entering commands and
then manipulating them to verify system response.
i. Test setpoint adjustment and setting of limits. Enter commands to adjust setpoints of controllers
and range limits of the controlled media. Verify by display. Also enter commands to adjust setpoints
outside their range limits. DDC system must display error messages.
j. Test system access and access level control. Try to log on to system with both incorrect and
correct ID codes. Try to enter different commands with different access level of the operators. The
responses of the DDC system must be as specified.
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HVAC Design Criteria and Guidelines
k. Test the ability to change parameters of points. Enter commands to change parameters of
selected points such as high and low limit alarms, scale factor, etc. to test the adequacy of
software.
l. Verify graphic display of each HVAC system and component. Confirm that the graphic is in
accordance with the design data and reviewed submittals, includes all data points required,
displayed data is correct and in the correct format and units, and changes in point conditions or
status are accurately updated. Evaluate the refresh rate of data display.
m. Verify report generation (status, profile, energy, etc.) by entering commands to generate reports
such as all points, trend, total display of a system, timed display, and other specified reports.
Examine the report content for general format, system/point code, time interval of reporting, point
status/value/unit, energy amount/rate/unit, status of control and set time (manual or automatic), and
other specification required information.
n. Check for proper operation of system status reports, including point status reviews which would
include information such as points currently in alarm, points removed from alarm checking, points
off of scan, etc.
o. Test alarm reporting by initiating alarm conditions of different points at different alarm levels in
sequence to examine alarm reports. The reports must show alarm location and device, alarm time,
cause of alarm, current status of the point, etc. as required in the specifications. When alarm
conditions are removed the printer must print updated status report. Also verify audible alarm
operations in accordance with specification requirements. Then initiate alarm conditions at different
levels at the same time to check alarm priority.
The controls vendor, in the presence of the designer, must demonstrate the proper performance of all
control system application software. The test procedures described below do not check the details of the
software, rather, they try to verify the final output as indicated by the field equipment. Modify the test
procedures to suit particular projects as necessary. Before testing each program, the required input and
output of the program and those listed in the contract specifications must be compared to make sure that the
program covers the specified operations. Verification of HVAC equipment operation (such as equipment
status or temperature of space air) may be done by either (1) actual observation of equipment status and
test instruments, or (2) obtaining control system reports if the accuracy of these reports has been verified
previously.
a. Scheduled start-stop control. Verify that input includes start/stop time and days for specified
equipment. Verify input for time delays of specified equipment. Check holiday effect. For each air
handler, log operation for at least three days to confirm that unit starts and stops in accordance with
the schedule.
b. Optimum start-stop control. Verify that the required input includes space temperature, outdoor
temperature, occupied time and days of the week, and the response time of the air handling
equipment, on an individual system by system basis. For each air handler, log space temperature
for at least three days to confirm that space is at required temperature by the start of the occupied
period.
c. Electrical demand limiting control. Verify required input data, as follows: electrical loads under
control, load priority, demand metering interval, demand limit setpoint, delay time, minimum offtime, minimum on-time, and maximum off-time. Test control by the following process:
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HVAC Design Criteria and Guidelines
Override demand sensor and input demand limit setpoint 10% lower than current electrical
consumption rate (kW)..
Confirm equipment shutdown by observation of equipment and/or display.
d. Day-night/occupied-unoccupied setback control. Confirm that required input data is provided.
To test control, change the setback time from occupied to unoccupied time and confirm that HVAC
systems respond to the setback mode. If system is an air-handling system, the outside air damper
should close and the fan should cycle to maintain the setback temperature setpoint. Change the
setback temperature setpoint to 5F higher than the actual space temperature. The system should
operate to increase the space temperature to the new setpoint condition. For air-handlers, the
outside air damper should remain closed.
e. Economizer cycle control. If the outdoor air temperature is above the discharge air temperature
setpoint, but below the defined 100% outdoor air changeover temperature setpoint, verify that the
outside air damper is fully open, the return air damper is fully closed, and that the air-handler
discharge air temperature is maintained by modulation of the chilled water control valve.
f. Heating/cooling coil discharge temperature control. Verify that the required input points are
provided and that the coil discharge air temperature setpoint is in accordance with the input
conditions and the sequence of operation defined on the control drawings.
g. Hot water temperature control. Verify that the required input points are provided and that the
heat exchanger supply hot water temperature setpoint is in accordance with the input conditions
and the sequence of operation defined on the control drawings.
h. Chilled water temperature control. Verify that the required input points are provided and that the
chilled water supply temperature setpoint is in accordance with the input conditions and the
sequence of operation defined on the control drawings.
i. Condenser water temperature control. Verify that the required input points are provided and that
the condenser water supply temperature setpoint is in accordance with the input conditions and the
sequence of operation defined on the control drawings.
During these performance tests, the controls vendor must tune all control loops to obtain the fastest, stable
response without hunting or overshoot.
Appendix: GLOSSARY OF DDC TERMS
Accuracy: The term accuracy describes the total of all deviations between a measured value and the actual
value. Accuracy is usually expressed as the sum of non-linearity, repeatability and hysteresis. Accuracy may
be expressed as the percent of a full-scale range or output, or in engineering units.
Address: A unique numeric or alphanumeric data (point) identifier.
Algorithm: A logical procedure for solving a recurrent mathematical problem. A prescribed set of
well-defined rules or processes for the solution of a problem in a finite number of steps.
Analog/Modulating/Continuous: These synonymous terms are used to describe data that has a value that is
continuous between set limits represented by a range or span of voltage, current or resistance. The value is
non-integer (real) with a resolution (number of significant digits) limited only by the measurement and
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HVAC Design Criteria and Guidelines
analog-to-digital signal conversion technology. In typical DDC systems, analog data from an input device is
converted into a value for processing within the controller. Likewise, values are converted into analog output
signals for use by a controlled device, such as an actuator.
BACnet: The ASHRAE building automation and control protocol defined by ASHRAE Standard 135.
Baud: A signal change in a communication link. One signal change can represent one or more bits of
information depending on type of transmission scheme. Simple peripheral communication is normally one
bit per Baud. (e.g., Baud Rate = 1200 Baud/sec is 1200 bits/sec if one signal change = 1 bit).
Binary: A two-state system where an "on" condition is represented by a high signal level and an "off"
condition is represented by a low signal level.
Bridge: A device that routes messages or isolates message traffic to a particular segment subnet or domain
of the same physical communication media.
Building Level Controller (BLC): Supervisory control panel and the primary means of communication outside
the building. May also act as a global controller, implementing building wide global strategies and energy
management routines.
Communication Interface: A method by which an operator is capable of communicating with a DDC system,
allowing an operator to command, monitor, and program the system.
Control Wiring: Includes conduit, wire and wiring devices to install complete HVAC control systems including
motor control circuits, interlocks, thermostats, etc., including wiring from a DDC controllers to all sensors and
operators required to execute the required sequence of operation.
Controlled Medium: A process medium of which one or more properties are made to conform to desired
conditions by means of a control loop.
DDC (Direct Digital Control): A control loop in which a digital controller periodically updates the process as a
function of a set of measured control variables and a given set of control algorithms.
Deadband: A temperature range over which no heating or cooling is supplied (i.e., 72-78 degrees F, as
opposed to single point changeover or overlap).
Diagnostic Program: Machine-executable instructions used to detect and isolate system and component
malfunctions.
Digital/Binary/Discrete: These synonymous terms are used to describe data that has a value representing
one state or another. Typical values are "on/off", alarm or normal, 0 or 1, high or low, etc. In the hardware
side of the DDC world, these values most commonly relate to the state of a set of switch or relay contacts
(open or closed).
External/Virtual Point: Data that is received by a controller from an external source, or sent by a controller to
an external source, is an external point. The terms hardware, input or output may be used to describe an
external point.
Fat client: A network computer with a hard disk drive.
Gateway: A device that contains an I/O software driver to translate data from a non-standard format to a
format conforming to the network protocol utilized.
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HVAC Design Criteria and Guidelines
Global Point: Global points originate from a controller within a network that is broadcast via the network to
other controllers.
HTML (Hyper Text Markup Language): The document format used on the Web. Web pages are built with
HTML tags (codes) embedded in the text. HTML defines the page layout, fonts and graphic elements as well
as the hypertext links to other documents on the Web. Each link contains the URL, or address, of a Web
page residing on the same server or any server worldwide, hence "World Wide" Web.
Hysteresis: The maximum difference in measured value or output when a set value is approached from
above, and then below the value.
Input: Data flow into a controller or control function.
Intelligent Device: A LonMark product that is configured to provided control over a single control loop or to
monitor a single or multiple control variable(s).
Internal Point: An internal point is one that resides within a digital controller that does not directly originate
from input or output points. Internal points can be constants such as fixed set points created by a
programmer’s or operator’s assignment. Internal points may also be created as defined by the programmer/
operator by applying logic and mathematics to other virtual, input or output points or combinations of points.
The terms virtual, numeric or data may be used to describe an internal point.
Local Area Network (LAN): A communications system that links together electronic equipment, such as
computers and word processors, and forms a network within an office or building.
Network: A data communications system that offers high-speed communications channels organized for
connecting information-processing equipment.
Network Server: A dedicated computer for management and control of the local area network.
Non-linearity: Non-linearity is the maximum difference in measured value or output from a specified straight
line between calibration points.
Output: Data flow out of a controller or control function.
Peer-to-Peer: Mode of communication between controllers in which each device connected to network has
equal status and each shares its database values with all other devices connected to network.
Point: A generic term used to describe a single item of information in a control system. Points may be
further described as input, output, digital, binary, discrete, analog, modulating, internal, external, virtual or
global. Each unique point used by digital controllers, or in digital control systems, is typically identified by an
address.
Process Medium: A material in any phase (solid, liquid or gas) that is being used in a process. The most
common types of process mediums used in commercial and industrial heating ventilating and air
conditioning systems are liquid mediums (i.e., chilled water for cooling) or gaseous mediums (i.e., airflow in
a duct).
Protocol: The term “protocol” and its derivatives when used in this Section shall mean a defined set of rules
and standards governing the on-line exchange of data between control systems of the same or different
manufacturers.
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HVAC Design Criteria and Guidelines
Router: A device which routes messages destined for a node on another segment subnet or domain of the
control network. The device controls message traffic control systems on node address and priority. Routers
shall also serve as communication links between twisted pair and radio frequency media.
Repeatability: The maximum difference in a measured value or output when a set value is approached
multiple times from either above or below the value.
Sensor: A device in primary contact with a process medium. It measures particular properties of the process
medium (i.e., temperature, pressure, etc.) and relates those properties to electrical signals such as voltage,
current, resistance or capacitance.
Software: The term “software” and its derivatives includes all of programmed digital processor software,
preprogrammed firmware and project specific digital process programming and database entries and
definitions as generally understood in the control industry for real-time, on-line, integrated control system
configurations.
Structured Query Language (SQL): a language used to interrogate and process data in a relational
database. Originally developed by IBM for its mainframes, all databases designed for client/server
environments support SQL. SQL commands can be used to interactively work with a database or can be
embedded within a programming language to interface to a database. Programming extensions to SQL have
turned it into a full-blown database programming language, and all major database management systems
support the language. ANSI standardized SQL, but most database management systems have some
proprietary enhancement, which if used, makes SQL non-standard. Moving an application from one SQL
database to another may require tailoring to convert some commands.
TCP/IP (Transmission Control Protocol/Internet Protocol): The communications protocol of the Internet and
the global standard for communications.
Thin Client: A network computer without a hard disk drive.
Tier 1: LAN and/or WAN communication network. Building to building communication or high speed
Ethernet communication level running within a specific building.
Tier 2: Building level communication or low speed tier running under a building level supervisory controller.
Transducer: Transducers accept an input of one character and produce an output of a different character.
(Examples: voltage to current, voltage to pneumatic (pressure) and resistance to current.)
Transmitter: A transmitter is a transducer that is paired with a sensor to produce a higher-level signal
(typically) than is available directly from the sensor. These sensors may be integral or remote and may
include digital or analog signal processing.
Wide Area Network (WAN): A communications network that covers a wide geographic area, such as state or
country. A LAN (local area network) is contained within a building or complex, and a MAN (metropolitan area
network) generally covers a city or suburb.
XIF (eXtended Image File): Gaphics file storage format.
XML (eXtensible Markup Language): An open standard for describing data from the Web. It is used for
defining data elements on a Web page and business-to-business documents. XML uses a similar tag
structure as HTML; however, whereas HTML defines how elements are displayed, XML defines what those
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HVAC Design Criteria and Guidelines
elements contain. While HTML uses predefined tags, XML allows tags to be defined by the developer of the
page. Thus, virtually any data items, such as "product," "sales rep" and "amount due," can be identified,
allowing Web pages to function like database records. By providing a common method for identifying data,
XML supports business-to-business transactions and has become "the" format for electronic data
interchange and Web services.
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