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Transcript
Building Science, Durability &
Energy Efficiency
BCT 206
Lecture 4-5
Jan 29 Feb 5, 2008
Today’s Objectives

Learn basic concepts of Building Science
–
–

Weatherization strategies
–
–

Heat Flow
Energy and Moisture Transport in buildings
Flashing, Drainage Planes
Vapor Barriers vs. Vapor Retarders
Energy Efficiency
–
–
–
Wall Systems
Windows
HVAC
Building
Pollutants
Occupants
Comfort
Moisture
Heat loss
Operating cost
Fire safety
Mechanical
Systems
Environment
Heat/Energy Transfer


Movement of Thermal Energy: Heat Flow
Three methods
–
–
–
Conduction
Convection
Radiant

Most heat transfer has some combination of all three
occurring at the same time
Heat Goes to Cold

Conduction

Affected by:
–
Insulation

Installation practices
–
Windows
–
Framing techniques
Photo © Kevin Kennefick 2001
Conduction
The transfer of thermal energy from one solid
object to another through direct contact.
An object does not have to be warm or hot for
heat transfer through conduction to occur, only
a difference of temperature between the two
solid objects, or across a solid object.
Air Moves Under Pressure

Convection
Temperature
Wind
Exhaust/Mechanicals
Temperature is typically the dominant effect
Convection
The transfer of thermal energy from a fluid flowing over a
solid object- [Air is a fluid!]
 But, air is a relatively poor conductor of heat
–
–
–

A solid object = dense arrangement of molecules
Liquid = less dense arrangement
Gas = least dense arrangement of molecules
Transferring heat using a gas is inefficient
–
Must pass a lot of molecules over an object to equal the carrying
capacity of a denser material.
Positive pressure
(relative to outdoors)
Negative pressure
(relative to outdoors)
Stack
Effect
Hot Surfaces Warm You Up

Radiation
–
–

You feel heat through
space, even if the air is
cold
You feel cold surfaces the
same way-but heat energy
is flowing from you
Function of temperature
difference, area
Radiant Heat Transfer
The transfer of thermal energy or heat that is in direct
line of sight of the object being heated.
–
–
Heat from the sun comes to the earth this way
We feel heat from a camp fire the same way
 Once
the solid materials in a room have heated up to
a desired setpoint temperature, they act as radiant
heating surfaces too
 Comfort is determined by the balance of colder and
warmer surfaces surrounding the body
Heating with Air

Uses a poor conductor (air) to carry heat and
transfer energy to the objects we wish to heat (the
house and contents)
–


Then, the human body manages heat by evaporating liquid
from the skin, which then is carried away by air moving
across its surface.
Using airflow across a human body is effective for
cooling it
To heat a person that way, we have to increase the
temperature of that air to counteract the “chill factor”
The “Chill Factor”


The Chill Factor can be a direct cause of discomfort
A lesser noticed effect of unbalanced forced air
systems is inducing increased infiltration
–

Due to pressure imbalances and duct leaks
Heating the air in a room does a relatively poor job of
heating solid objects
–
Those objects in the room at a temperature lower than
one’s body act to rob the body of heat (through radiation),
requiring higher room temperatures to offset that effect
Mean Radiant Temp
72°F
70°F
25°F
68°F
59°F
64°F
Mean Radiant Temperature: = 67 °F
Cold Surfaces
72°F
70°F
25°F
59°F
59°F
64°F
Mean Radiant Temperature: = 63 °F
How can Energy loss be reduced?

Reduce conductive heat losses
–
–

R-value
Thermal bridging
Reduce air infiltration – convective
heat losses
Air Leakage/Infiltration

Energy movement through convection (air
flow)
–
–
Heat energy enters or escapes a building
enclosure through the process
This happens at transition points, where two
materials meet.
Air Leaks





30% of heat loss in “typical” home
Transport moisture
Reduce comfort
Increase indoor pollution
Largest cause of ice dams
Air leaks move moisture vapor into walls and attics
Air leaks are largest cause of ice dams
…and pipe freezing problems
Where Does Air Leak?
At transitions between building surfaces

–
–
–
Where one material meets another
Where walls/floors/roof lines meet
Where empty spaces are hidden by drywall
Knee Wall
Knee Wall Solution
Wind baffle
Permeable material
to prevent wind
washing
Blocking
R- Value


R-value is a measure of apparent thermal conductivity,
and thus describes the rate that heat energy is
transferred through a material or assembly, regardless of
its original source.
Performance of a material is a function of it’s R-value, but
is also dependent on the temperature difference on either
side of the material.
–
R value is able to be added when layering bulk materials,
example: If the plywood sheathing is R-0.5, the wall cavity is R21, and the drywall is R-0.5, the entire assembly is R-22
Thermal Bridging


Heat Conduction through framing members
Wood is approximately R-1 per inch
–
Framing is a weak link in the thermal boundary of
the building shell.
Advanced Framing
Reduce framing members (24 o.c.
framing)
 Increase insulation space
Wall make up

–
–
–
–
18% Wood (R5.5)
82% Insulation (R21)
R17.7 Perfect installation
R14.2 Probable installation


4% - 6% R-value improvement
Air leakage still an issue
Advanced Framing
Developed by
NAHB
 Saves lumber,
money & energy

Staggered Studs




2 Rows: 2x4 16” OC
offset
Blown-in blanket
Better total wall Rvalue
No thermal bridging
Insulation Types






Batts
Blown-in blanket
Spider
Spray foam
Hybrid
Rigid exterior
Blown Cellulose Insulation
Wet-spray cellulose
Damp spray cellulose
Spray-in cellulose insulation
Recycled content
Cellulose insulation made from
recycled newsprint
Blow-in-Blanket (BIBS)
Fiberglass
Spider
Foam + batts
Some Alternative Wall Systems



SIPS – Structural Insulated Panels
ICF – Insulated Concrete Forms
Composite blocks systems
Structural Insulated Panels
Insulated Concrete Forms –
Alternative Materials
Rastra block
Durisol block
Windows
“Insulating Glass”

This name is commonly applied to double
glazing
–

Approximately R-2
> 3 times the heat loss as the worst wall
anyone is likely to build today (R-11)
Basic Glazing Types



Single Pane – not legal to produce
Double Pane – Common today
Triple Pane – Rare but available
Additional features added to windows



Low-e – coated
Gas fills (argon, krypton)
Heat mirror (extra low-e films)
U- Value
Basic Frame Types

Aluminum

Wood

Vinyl – hollow or insulated

Fiberglass – hollow or
insulated
Window enhancing components

Warm Edge spacers

Inert Gas fills –Argon/Krypton

Metallic coatings (Low E)
Warm Edge Spacers

Thermally improved edge spaces isolate panes of glass
the appropriate distance apart.

Accommodate stress induced by thermal expansion and
pressure differences

Overall U-Value is improved by reduced heat loss at
glass pane edges

Provide gas tight seals
Argon or Krypton Gas Fill

Thermal resistance is increase with gas fills,
reducing winter heat loss and summer heat gain
through conduction

Higher temperatures in winter on interior glass
surface contribute to greater comfort and less
condensation

Visual transmittance is not effected
Low E coatings




Long-wave radiant heat is reflected, giving
an improved U-factor and reduced winter
heat loss
Higher temperatures on interior glass
surface increase comfort and reduce
condensation
Slight affect in visible transmittance
Coatings can allow solar transmittance or
reflection
U-Value and SHGC


U-Value gives the amount of heat that
transmits through a square foot of building
cross section, the lower the value, the less
heat transfer.
SHGC measures the amount of Solar Heat
Gain a material or assembly of materials
allows through.
HVAC
Ventilation - Improving Indoor
Air Quality




Eliminate pollution sources
Minimize unavoidable pollution sources
Separate pollutants from occupants
Ventilate:
–
–
Exhaust known pollutants at their source
Supply fresh (cleaner) air to dilute remaining
pollutants
Mechanical Air
Exchange




Reliable
Occupant control of when, where, and how
much
Outdoor air can be treated as it enters the house
(heat, cool, filter)
Allows houses to be built tighter by providing
fresh air and controlling moisture in winter
Central Exhaust Fan
Bath
Quiet Bath Exhaust Fans
Return Duct Fresh Air
Return
Supply
Supply
Motorized
damper
Fresh
air inlet
hood
Timer/
controller
Furnace/ air
handler
6"
insulated
air inlet
duct
Heating Equipment

Energy Sources
–
–
Natural Gas, Oil #2 or Bio Diesel, Propane
Grid Source electricity – Coal and natural gas
generated electricity, wind or other renewable
Heating and Cooling Systems

Furnace –Heats air that is delivered to
building by an electric blower
Sealed combustion, separate from house air
Supply Air
Exhaust
Gases
Heat Exchanger
Combustion
Air Inlet
Combustion Chamber
Return Air
Exterior
Wall
Heating Cont.

Boiler- Heats fluids, usually water, that are
then circulated to distribution point, can be
circulated as a liquid or a gas (steam)
–
–
–
Radiators
Convectors – Baseboard or convection radiators
Mass (radiant floors)
Hydro-air System
Heating Cont.

Water heaters, used for space heat
–
Same system as boiler but lower water
temperatures, not used for steam.

Preferably high efficiency
Condensing Water
Heater




Provides heat and hot
water
Hydronic heat or hydroair
Sealed combustion
Stainless tank
High Efficiency Tankless
Water Heater
Heating and Cooling

Geo-thermal Heat Pumps
–
–

Use air or hydronic distribution
High efficiency rates
Air Source Heat Pumps
–
–
Also use air distribution
Electrically fueled
BTU’s



Btu is the measurement of heat energyBritish Thermal Unit’s
1 Btu of heat energy is about equivalent to a
kitchen match
The number of Btus a piece of equipment is
rated for is the maximum capacity that
equipment is capable of burning per hour
–
Example: 100,000 btu/h furnace can burn
100,000 btu’s of energy in an hour
BTU and Heat



Heat is measured in Btus
1 Btu is the amount of energy required to
raise a pound of water by 1°F
The number of Btus of heat, a pound of any
material absorbs or releases for each degree
of temperature change is called it’s specific
heat.
Specific Heat



Specific heat is measured in BTUs per pound
per degree Fahrenheit (Btu/lb./°F)
The specific heat of water is 1, because it
requires 1 Btu to raise a pound of water by
1°F
It takes 0.2 Btu to raise a pound of aluminum
by 1°F, so aluminum has a specific heat of
0.2 Btu/lb./°F
Heating of Buildings

Heating Loads in buildings are expressed in
Btus.
The typical method of expressing loads of a
building is in Btu/Hour. This is the number of
Btus need to maintain a particular
temperature in the building at a given
Temperature Difference (Delta T) between
indoors and outdoors during one hour.
Calculating Loads to Size Equipment

Heating load is used to determine the size (power) of
the heating equipment in Btuh or Btu/hour, to
determine this we use a formula:
Btuh = U x A x ∆T (Corrected! Incorrectly this was originally
posted as UA x A x ∆T the extra A was a typo)
U = heat transmission
A = Area
∆T = Temperature Change between Indoors/Outdoors
U – Value and Area


U- Value is the amount heat transmittance through a square
foot of building cross section (A combination of multiple
materials)
U is expressed as
Btus .
ft2 ∙ hr ∙ °F


U is the inverse or R-Value U=1/R and R=1/U
When R=1 then U=1
When R=2 then U=0.50
The Area of the building is simply the shell area expressed in
square feet.
Finding the ∆T



Design Temperature. For heating, this is the temperature
equaled or exceeded 97.5% of the time, during the three
coldest months, Dec. Jan. Feb.
To standardize the sizing of loads we use a standard desired
interior temperature during heating season of 68°, combining
the desired interior temp and our design temp gives us the
Delta T of the indoors and outdoors
In Portland, OR the design for heating temperature is 27°F
Interior Temp 68° - Design Temp 27° = ∆T 41°
Calculating Heat Load


We do this to select enough power (heating
equipment size)
Too small a system will not provide enough
heat during the coldest days, too big a
system will waste substantial energy (not
sustainable)
BTU’s & Efficiency




Although a heating system may be capable of burning 100,000
btu/hr, the amount of that heat energy available for distribution
to the space depends on the efficiency of the system.
We usually see this expressed as a percentage.
The input of the equipment is the btu/hr of energy entering the
equipment (potential energy of fuel), the output of the system is
the heat energy available to distribute once the losses are
accounted for (what goes up the chimney)
A 100,000 btu/hr furnace has an input of 100,000 btu/hr and if
it is 80% efficient has an output of 80,000 btu/hr of heat energy
available for distribution
Common Equipment Efficiency’s

AFUE -Furnaces and Boilers

COP, HSPF, SEER –Air Source Heat Pumps
& Geo-thermal Heat Pumps

EF -Water Heaters
AFUE

Annual Fuel Utilization Efficiency- tells us
how much of the potential energy in the fuel
makes it into the heating distribution system.
–
–
AFUE accounts for fuel burning losses, chimney
losses, cycling loss, and heat losses through the
heaters cabinet.
If a Furnace is rated at 100,000 btus, and has an
AFUE of 80%, this means on 80% of the total
rating is available to heat the space- in this case,
80,000 btus
COP



The oldest and most common way to measure heat
pump efficiency is the COP (Coefficient of
Performance) This tells us how many times more
efficient a particular heat pump is compared to
electric resistance heat (100% efficient). COP 1.6
means for every 1 kW hours of electric input, 1.6 kW
hours of heat is delivered.
Geo-thermal heat pumps are rated this way as well
as air source heat pumps
COP range from 1.6 to as high as 5 (That’s 500%
more efficient than standard electric heaters)
HSPF

Heat Seasonal Performance Factor
–
–
This account for the addition of electric resistance
heat during heat pump operation.
In some climates when the air temperature is too
cold to extract usable heat, back-up electric heat
engages. This less efficient heat is factored into
the overall performance and a seasonal efficiency
is assigned.
SEER

Seasonal Energy Efficiency Ratio measures
the ratio of btu of cooling provided, divided
by the electrical energy input. SEER ratings
from 9 to about 19 currently exist although
the standard for all newly manufactured air
conditioning equipment is now 13 SEER
13 SEER is 30% more efficient than 9 SEER
1-(9/13)=30
EF


Energy Factor describes the fraction of the water
heaters energy that actually remains in the water
leaving the unit. Factors range from .50 - .93, current
federal minimum is .59EF
Energy Factor takes into account:
–
–
–
–
Use- assumes 64 gallons a day is used
Energy loss during heating
Pilot light loss (if gas)
Thermal loss through the tank
Ducts-Distribution



We worry about ducts,
because they leak!
With Hydronic systems
leaks are very apparent, air
systems tend to leak
unnoticed.
Avoid locating ducts in the
attic or crawlspace for this
reason
Why Seal Duct Leaks?

Leaks reduce heating system efficiency
–

Increase or magnify house air leakage
–

When they leak into unconditioned spaces
30-300% while blower is running
Reduce comfort for occupants
–
–
drafts
unbalanced air delivery
Impact of Duct Leakage
Duct Leakage in typical homes
12%
% of Sample
10%
8%
6%
4%
2%
0%
5%
15%
25%
35%
45%
55%
65%
75% >80%
CFM 25 / Nominal Airflow
1210 homes, Duct Blaster® at 25 pa.
(0.10”WC)
Source: CheckMe!® database, PEG
Courtesy of Proctor Engineering Group
Worst duct leakage areas







Swivel elbows
Branch takeoffs from trunk ducts
Other finger jointed connections
Folded corners of boots and other fittings
Filter racks, other plenum connections
Sealing only the connections between duct
sections will result in a leaky system!
Missing pieces!
Duct testing

Testing for duct leakage
–
Total Leakage Test- Measures the total system
leakage and does not differentiate from indoor or
outdoor leakage

–
Most common in New Construction – Will be an optional
code requirement in OR after April 2008
Leakage to Outside Test
Measures only the outdoor leakage by counter
pressurizing indoor leakage with a blower door
Duct Sealing Essentials

Use Mastic not tape
–



UL181A or UL 181B type mastic
Use mastic over mesh tape for larger gaps
Seal the inner duct material, not the vinyl or insulation
wraps
No tapes (including butyl tape or “mastic tape”) and
never use duct tape on ducts!