Download 40in Gas Furnace Ser..

Table of Contents
Safety Circuits
Vent Pressure Safety Switches (3-11)
High Limit Controls (12-14)
Flame Roll-out Limit Switches (15-17)
Induced Draft Blower Housing Limit
Switches (18-19)
Measuring Flame Current (20-22)
Gas Valves
Measuring Inlet Supply Gas Pressure (23)
Induced Draft Motors
Checking Shaded Pole Induced Draft Motor
Windings (35-36)
Checking PSC Induced Draft Motor Windings
(37-38)
Checking Earlier Variable Speed Induced Draft
(39-40)
Motor Variable Speed 3 Phase Induced Draft
Motors (41-42)
ECM Blower Motors
ECM Blower Motor Quick Test (43-45)
Manifold Pressure Adjustment (24-30)
LitePort Furnace Control Information
Hot Surface Ignitors
Checking Silicon Carbide Hot Surface
Ignitors (31-32)
Checking Silicon Nitride Hot Surface
Ignitors (33-34)
(46-48)
Vent Pressure Safety Switches
The vent pressure safety switch on 80% efficient furnaces
is connected to the induced draft blower housing. The
vent pressure safety switch monitors the negative
pressure developed by the induced draft blower
assembly. If the induced draft blower pressure falls
below the vent pressure safety switch minimum closing
pressure, the switch will open.
On 80% efficient gas furnaces, this switch (fig. 1) is used to
prove the presence of adequate combustion air. It does
not prove any operation of the vent system, chimney,
because 80% furnaces do not use the induced draft
blower to vent products of combustion. (THRU
CHIMNEY) These furnaces are Category 1 appliances.
Category 1 means these furnaces vent the products of
combustion by natural draft.
The vent pressure safety switches on 90 % efficient gas
furnaces (fig. 2) are connected to the secondary heat
exchanger area. The vent pressure safety switch on
these models monitor the secondary heat exchanger
area to ensure there is adequate negative pressure
present.
The induced draft blower assembly draws in combustion air
and also vents the products of combustion.
Fig. 1
Fig. 2
Vent Pressure Safety Switches
These furnaces are Category 4 appliances. Category 4
appliances use the induced draft blower to create a
positive pressure on the vent system.
Fig. 3
Vent pressure safety switches on Category 4 appliances
protect the furnace from operating with a lack of
combustion air and from blockages or restrictions in
the flue system.
The vent pressure safety switch circuit is powered by 24
volts. The switch is a normally open device that closes
on an increase in negative pressure. Modern furnace
integrated furnace controls (IFC) monitor the position of
this switch during a call for heat via a connection to
either a 9 pin or 12 pin plug located on the IFC board.
The 9 pin plugs (fig. 3) are for single stage furnaces
(fig. 4) and the 12 pin plugs are for 2 stage furnaces
(fig. 5).
Fig. 5
Fig. 4
Continued on Next Slide…
Vent Pressure Safety Switches
Continued…
The plugs have two pins for each vent pressure safety
switch. One pin sends 24 volts potential to the switch
and the other pin receives the 24 volt potential back to
the IFC. If the furnace is a two stage model, there will
be one pin for voltage out to both switches and then
two pins to receive the 24 volt potential back at the IFC
board (fig. 6).
When a call for heat is received by the IFC, the IFC sends 24
volts out to the vent pressure safety switch. If the
switch is open, the IFC will not receive 24 volts back.
However, if the switch is shorted, or stuck in the closed
position, the IFC will receive 24 volts back. The IFC will
then lock out and flash an appropriate fault code on its
LED. If the IFC determines the vent pressure switch is
open, it will start the induced draft motor for a prepurge cycle if all other safety devices are in their proper
position.
Fig. 6
Notice: Prior production variable speed
vent motor models had the pressure
switches report to the microprocessor
on the vent motor assembly.
The
status of the pressure switches was
then
communicated
by
the
microprocessor to the IFC board.
Vent Pressure Safety Switches
Notice there are two vent pressure safety switches shown
(fig. 7), they are installed on a two stage gas furnace.
The front switch is for second stage induced draft
motor speed and the back one is for first stage induced
draft motor speed. The first stage switch will have a
closing pressure that is lower than the second stage
switch (fig. 8). The switch has a lower close pressure to
match the slower speed of the induced draft blower
when on low fire.
During low speed operation the high speed switch should
be open and the low speed switch closed.
Fig. 7
Fig. 8
During a call for second stage heat the induced draft motor
goes to high speed and generates an increased amount
of negative pressure. Both the low speed switch and
high speed switch should be closed during both low
and high speed induced draft motor operation.
The vent pressure switch must be in the open position prior
to induced draft motor operation (fig. 9). If the furnace
control senses a closed vent pressure switch when the
induced draft motor is off, the furnace control will
lockout. The control will not allow heat operation until
it senses the switch has opened. If a technician tries to
bypass the switch with a jumper, the furnace will
lockout on the next call for heat cycle.
Fig. 9
Vent Pressure Safety Switches
This
safety feature prevents furnace operation when an
insufficient level of combustion air is being drawn into
the burners. The switch also prevents operation of the
burners on 90% efficient models when the flue is
restricted. If the furnace was allowed to operate with
inadequate combustion air, the furnace would
malfunction and produce carbon monoxide. If the
burners were allowed to operate with insufficient vent,
products of combustion would spill into the home.
Fig. 10
How to Check for a Shorted Vent
Pressure Safety Switch
1. Remove power to the furnace.
2. Disconnect the wires from the vent
pressure switch terminals.
3. Using an ohmmeter, check for
continuity across both vent pressure
switch terminals (fig. 10) .
4. A shorted vent pressure switch will
measure resistance. Replace the switch
if it is shorted.
5. An open vent pressure switch will
measure infinite resistance. The switch
is not shorted.
6. When finished, replace the wires and
remember to restore power to the
furnace.
Vent Pressure Safety Switches
How to Check for Proper Pressure Switch Close Operation
To determine if a vent pressure safety switch is closing at
the proper pressure you will need to measure the
negative pressure being sensed by the switch. A good
instrument for measuring induced draft negative
pressure is a Magnehelic gauge.
Fig. 11
For 80% efficient gas furnace induced draft pressure, the
gauge scale should be capable of reading accurately
down to as low as .05 inches of water column and as
high as 3 inch of water column. For 90% efficient gas
furnace induced draft pressure, the scale should be
capable of reading accurately down to as low as .05
inches of water column and as high as 2 to 3 inches of
water column.
To sense the pressure at the back of the pressure switch, a
tee is placed in the tubing between the switch and
inducer housing (fig. 11). One opening in the tee is
connected to the induced draft blower housing, one
opening to the LOW PORT of the Magnehelic gauge,
and the other tee port is connected to the vent pressure
switch. The HIGH PORT of the Magnehelic is left open
to atmosphere. If the furnace is a sealed combustion
model, the high port of the Magnehelic is connected to
the tube leading to the burner enclosure.
Continued on Next Slide…
Vent Pressure Safety Switches
Continued…
When the induced draft motor runs, the pressure being
sensed by the vent pressure safety switch is displayed
on the Magnehelic gauge fig. 12).
Fig. 12
If the pressure is greater than the closing set point of the
switch, yet the switch does not close, replace the
switch. If the pressure is not high enough to close the
switch, there is a problem with the induced draft blower
assembly or the vent/combustion air system.
Switching Function Check
Fig. 13
1. Connect a Magnehelic gauge between the induced
draft motor housing and the pressure switch (fig. 13).
2. With the induced draft motor running, note the
negative pressure reading on the Magnehelic gauge.
The pressure must be greater than the close pressure
of the pressure switch being tested. (continued)
Continued on Next Slide…
Vent Pressure Safety Switches
Continued…
Fig. 14
3. Use a voltmeter to check for 24 volts at the pressure
switch VOLTAGE IN terminal. Check by placing one
voltmeter lead to ground and the other to the
VOLTAGE IN terminal (fig. 14). (MOTOR RUNNING)
You should measure 24 volts. If the furnace is a two
stage model, check both the low and high fire
pressure switches to ensure you have 24 volts at the
VOLTAGE IN terminals. (MOTOR RUNNING)
Next, check for 24 volts at the VOLTAGE OUT terminal.
Place one voltmeter lead to ground and the other to
the VOLTAGE OUT terminal (fig. 15). You should
measure 24 volts. If you do not measure 24 volts, the
pressure switch is open. Replace the switch. If you
measure 24 volts, the switch is in the closed position.
(MOTOR RUNNING)
Fig. 15
Vent Pressure Safety Switches
Causes of Low Induced Draft Blower Housing Pressure 80% Efficient Models
· Seized induced draft blower wheel.
· Failed induced draft motor, or capacitor.
· A lack of combustion air supply.
· Kinked or plugged tube from induced draft blower to pressure switch.
Causes of Low Vent Pressure on 90% Efficient Models.
· Plugged vent or induced draft assembly.
· Failed vent motor or capacitor.
· Plugged heat exchanger.
· Improper sized vent piping.
· Blocked condensate drain.
LED Flash Code Response (40” Furnace Models)
If
a White Rodgers IFC board has detected a vent pressure safety switch fault, the
diagnostic LED will flash 3 times and lock the furnace out until the pressure switch fault
has been corrected.
High Limit Controls
High limit controls protect against abnormally high air temperature in
the heat exchanger airstream area. These limit switches (fig. 1) are
24 volt operated automatic reset devices that open when the air
temperature they sense exceeds the switches open setting. The
open setting of the limit switch is usually not found on the switch
itself, but rather on the furnace nameplate.
Fig. 1
The high limit is wired in series with other furnace limit switches (fig. 2).
Modern furnace integrated furnace controls (IFC) monitor the
position of limit switches during a call for heat via a connection to
either a 9 pin or 12 pin plug located on the IFC board.
The 9 pin plugs (fig. 3) are for single stage furnaces and the 12 pin plugs
(fig. 4) are for 2 stage furnaces. The plugs have two pins that
connect to the furnace limit devices.
Fig. 2
Fig. 3
Fig. 4
High Limit Controls
One pin sends 24 volts potential to
the limit switches and the other pin receives the 24 volt
potential back to the IFC.
When a call for heat is received by the IFC, the IFC sends
24 volts out to the high limit circuit. If the high limit
switch is open, the IFC will not receive 24 volts back. If
the high limit or any of the other limits open, the
furnace will shut down burner operation and run the
furnace blower and induced draft blower in an attempt
to cool down the furnace.
Check Procedure with Voltmeter
1. Call for heat
2. Place your voltmeter leads across the high limit
switch terminals (fig 5).
3. You should measure 0 volts if the limit is closed. If
the limit is open you will measure 24 volts.
4. If you measure 0 volts across the high limit, yet the
furnace is indicating an open limit fault, check for 24
volts to ground at each side of the high limit switch. If
you do not measure voltage, another limit switch is
open.
Fig. 5
High Limit Controls
Check Procedure with Ohmmeter
1. If the furnace is hot, allow it to cool down, and then
remove power to the furnace.
2. Remove the two wires from the high limit switch.
3. Using an ohmmeter, check for continuity across the
switch (fig. 6).
4. You should measure 0 ohms if the switch is closed.
If the high limit is open, replace it.
Causes of High Limit Trips
· Dirty filters
· Dirty A/C Coil
· Undersized ducting
· Blower motor failure
· Dirty blower assembly
· Over-firing of the furnace burners
Fig. 6
Flame Roll-out Limit Switches
Flame
roll-out limit controls (fig. 1) protect against
abnormally high air temperature in the furnace burner
area. These limit switches are 24 volt operated manual
reset or one time failure devices that open when the air
temperature they sense exceeds the switches open
setting. The switches can be either snap disc or fuse
link devices. The open temperature of the switch is
normally not listed on the part.
Fig. 1
The
flame roll-out switch circuit is powered by 24 volts.
The flame roll-out switch is wired in series with other
furnace limit switches (fig. 2). (TCO)
Modern furnace integrated furnace controls (IFC) monitor
the position of limit switches during a call for heat via a
connection to either a 9 pin or 12 pin plug located on
the IFC board.
Fig. 2
Continued on Next Slide…
Flame Roll-out Limit Switches
Continued…
Check Procedure with Ohmmeter
The 9 pin plugs (fig. 3) are for single stage furnaces and
the 12 pin plugs (fig. 4) are for 2 stage furnaces.
The plugs have two pins that connect to the furnace
limit devices. One pin sends 24 volts potential to
the limit switches and the other pin receives the 24
volt potential back to the IFC.
If the limit opens, the furnace will shut down burner
operation and run the furnace blower and induced
draft blower in an attempt to cool down the furnace.
Fig. 3
Fig. 4
1. If the furnace is hot, allow it to cool
down, and then remove power to the
furnace.
2. Remove the two wires from the flame rollout switch.
3. Using an ohmmeter, check for continuity
across the switch (fig. 5).
4. You should measure 0 ohms if the switch
is closed. If the switch is open, you will
measure infinite ohms. If you measure
infinite ohms reset the switch if it is a reset
type, if it cannot be reset, replace the
switch.
Fig. 5
Flame Roll-out Limit Switches
Check Procedure with Voltmeter
1. Call for heat.
2. Place your voltmeter leads across the limit switch
terminals (fig. 6).
3. You should measure 0 volts if the limit is closed. If the
limit is open you will measure 24 volts.
4. If you measure 0 volts across
· Low gas pressure
· A lack of combustion air
· Plugged burner orifice
the high limit, yet the furnace is indicating an open limit fault,
check for 24 volts to ground at each side of the switch (fig. 7
) (fig. 8). If you do not measure voltage, another limit switch
is open.
Fig. 6
Causes of Flame Roll-out Trips
Fig. 7
· Plugged heat exchanger
Fig. 8
Induced Draft Blower Housing Limit Switches
The induced draft blower housing limit control (fig.
Fig. 1
1) protects against abnormally high flue
temperature in the induced draft blower housing.
This limit is only present on 90% efficient
models.
These switches are 24 volt or 115 volt operated auto
reset devices. The switches are mounted to the
plastic induced draft blower housing. The open
temperature of the switch is normally not listed
on the part.
Fig. 2
Furnaces wired to break 115 volt power to the
induced draft blower, report a pressure switch
error when this switch opens because the vent
motor never comes on during a call for induced
draft blower operation. (See fig. 2)
Furnace models may use a 24 volt switch that is
wired directly to the furnace IFC board 9 or 12
pin plug. Furnaces wired in this manner have the
capability to report an open induced draft blower
limit switch on the diagnostic LED. (See fig. 3)
Fig. 3
Continued on Next Slide…
Induced Draft Blower Housing Limit Switches
Continued…
Fig. 4
The IFC board plugs have two pins that connect to the
furnace limit devices. One pin sends 24 volts potential
to the limit switches and the other pin receives the 24
volt potential back to the IFC.
When a call for heat is received by the IFC, the IFC sends 24
volts out to the induced draft blower limit circuit. If the
limit switch is open, the IFC will not receive 24 volts
back. If this limit is open, the IFC will flash a fault code.
The induced draft blower and the indoor blower motor
will run until the switch re-closes.
Fig. 5
Check Procedure with Ohmmeter
1. If the furnace is hot, allow it to cool down, and then remove
power to the furnace.
2. Separate the plug assembly located near the limit switch (fig. 5).
3. Using an ohmmeter, check for continuity across the two plug
pins (fig. 6).
Causes of Flame Roll-out Trips
· Overfiring of the furnace.
· Excessive supply air temperature due to a lack of indoor air
volume (Plugged secondary.)
Fig. 6
Measuring Flame Current
The
flame sensing circuit (fig. 1) proves the presence of
flame. Without this circuit the furnace would not know
when flame is present. The circuit consists of a flame
sensing rod (fig. 2) located at the opposite end of the
burner assembly from the furnace gas manifold
connection. The flame sensing rod is positioned in front
of the last burner.
The flame rod has an electrical terminal connection with one
wire (fig. 3).
Fig. 1
Fig. 2
Fig. 3
Fig. 4
The wire connects the flame sensing rod to the furnace 9 or
12 pin plug located on the IFC board (fig. 4). On some
models of furnaces, the flame sensing wire is connected to
a terminal on the IFC board. The voltage to the flame
sensing rod is typically around 80 volts AC when measured
with a digital voltmeter.
Measuring Flame Current
When
a call for heat takes place and the burners
ignite, flame envelopes the flame rod. The flame
is a conductor and can conduct a very small
amount of current. The voltage potential at the
flame rod now has a conductor path through the
flame. Since the flame rod is much smaller than
the burner ground area, more current flows in
one direction than in the other. This is called
flame rectification. The current flowing in one
direction is now greater than in the other
direction so it is DC current. The amount of
current that actually flows is in the microamp
range. Typically from 2 to 4 DC microamps. The
minimum flame current necessary to maintain
ignition on White Rodgers ignition controls is 1
DC microamp.
Fig. 5
How to Measure Flame Current with a
Digital Multimeter Capable of Reading
DC Microamp Range
1. Disconnect power to the furnace.
2. Set the multimeter to the DC microamp
scale.
3. Remove the wire from the spade terminal on
the flame sensing rod.
4. Connect one end of a jumper wire to the
flame sensing rod spade terminal (fig. 5).
5. Connect the other end of the jumper wire to
one lead of the multimeter (fig. 6).
Fig. 6
Continued on Next Slide…
Measuring Flame Current
Continued…
Fig. 7
6. Connect the other lead of the multimeter to the wire
removed from the flame sensing rod (fig. 7).
7. The meter is now wired in series between the flame
sensing rod and the IFC board (fig. 8).
8. Restore power to the furnace and call for heat.
9. When flame is established the actual flame current
will be displayed on the multimeter.
10. When you are finished with your measurements,
remove the call for heat. Allow the furnace to run
through the complete sequence of operation and then
remove power to the furnace. Reconnect the wire to the
flame sensing rod and restore power.
Causes of Low or No Flame Current
· Dirty flame sensing rod
· Cracked ceramic insulator on flame sensing rod
· Cracked or broken flame sense rod wire
· Poor furnace ground
· Reverse polarity of 115 volt power
Fig. 8
Measuring Inlet Supply Gas Pressure
Inlet supply gas pressure requirements for single
stage natural gas models is 5 to 7 inches water
column. LP models require an inlet supply
pressure of 11 to 13 inches water column. The
gas valve has a pressure port tap from which
to measure the inlet gas pressure. Inlet supply
gas pressure is measured with a manometer or
Magnehelic gauge.
Fig. 1
Fig. 2
Procedure for Reading Inlet Gas Pressure
1. Turn off power to the furnace and shut off the gas
supply to the furnace.
2. Remove the pressure port plug on the inlet side of
the gas valve body (fig. 1).
3. Insert a hose barb fitting into the port and tighten
till snug (fig. 2).
4. Connect a manometer to the hose barb (fig. 3).
5. Restore power to the furnace and call for heat. If
the furnace is two stage, make sure both stages of
heat are calling. HAVE ALL GAS APPLIANCES ON
WHEN CHECKING INLET GAS PRESSURE.
6. Once the gas valve has energized, the inlet
pressure will be displayed in inches of water column
on the manometer.
Fig. 3
Fig. 4
If the inlet pressure falls above or below the inlet
requirement, contact the gas company for
correction.
7. When finished replace screw cover. End call for
heat. Remove power. Disconnect hose barb and
replace the pressure port plug (fig. 4).
8. Restore power.
Measuring Manifold Gas Pressure
Manifold
pressure requirements for single stage
natural gas models is 3 to 3.5 inches water
column. LP models require a manifold pressure
of 10.5 to 11 inches water column. The gas valve
has a pressure port tap from which to measure
the manifold pressure (fig. 1) and has an
adjustment screw to raise or lower the manifold
pressure (fig. 2). Manifold pressure is measured
with a manometer or Magnehelic gauge.
Manifold Pressure Adjustment
Before making an adjustment to the
manifold pressure, make sure the inlet gas
pressure is within acceptable limits.
Procedure for Reading Inlet Gas
Pressure
1. Turn off power to the furnace.
2. Remove the pressure port plug on
the outlet side of the gas valve body
(fig. 3).
Fig. 1
Fig. 2
Fig. 3
Continued on Next Slide…
Manifold Pressure Adjustment
Continued…
3. Insert a hose barb fitting into the port and
tighten till snug (fig. 4).
Fig. 4
Fig. 5
Fig. 6
Fig. 7
4. Connect a manometer to the hose barb (fig. 5).
5. Restore power to the furnace and call for heat.
6. Once the gas valve has energized, the manifold
pressure will be displayed in inches of water
column on the manometer.
7. If the manifold pressure must be adjusted,
remove the slotted cover to gain access to the
manifold pressure adjustment screw (fig. 6).
Turn the screw clockwise to increase pressure and
counterclockwise to decrease pressure (fig. 7).
8. When finished replace screw cover. End call for
heat. Remove power. Disconnect hose barb and
replace the pressure port plug.
9. Restore power.
Manifold Pressure Adjustment
Gas Pressure Manifold Adjustment Two Stage Furnaces
Before
making an adjustment to the manifold pressure, make sure the inlet gas pressure is within
acceptable limits.
Manifold pressure requirements for two stage natural gas models is 1.4 to 1.7 inches water column first
stage and 3 to 3.5 inches water column second stage. LP models require a manifold pressure of 6 to
6.2 inches water column first stage and 10.5 to 11 inches water column second stage.
The gas valve has a pressure port tap to measure the manifold pressure. There are two adjustment
screws to raise and lower manifold pressure for first and second stage fire rates. The first stage
adjustment screw is on top of the gas valve (fig. 8) . The second stage adjustment screw is next to the
pressure port plug on the outlet side of the gas valve (fig. 9). Manifold pressure is measured with a
manometer or Magnehelic gauge.
Fig. 7
Fig. 8
Fig. 9
Continued on Next Slide…
Manifold Pressure Adjustment
Continued…
1. Turn off power to the furnace.
2. Remove the pressure port plug on the outlet side of the gas valve body (fig. 10).
3. Insert a hose barb fitting into the port and tighten till snug (fig. 11).
4. Connect a manometer to the hose barb (fig. 12).
5. Restore power to the furnace and call for first and second stage heat.
6. Once the gas valve has energized, the second stage high fire manifold pressure will be displayed in
inches of water column on the manometer.
Fig. 10
Fig. 11
Fig. 12
Continued on Next Slide…
Manifold Pressure Adjustment
Continued…
7. If the manifold pressure must be adjusted, remove the slotted cover on the outlet side of the gas
valve to gain access to the manifold pressure adjustment screw (fig. 13).
Turn the screw clockwise to increase pressure and counterclockwise to decrease pressure.
8. Once the second stage manifold pressure has been set, replace the high fire slotted cover.
9. Remove the low fire slotted cover (fig. 14) and reduce the call for heat to first stage only.
Note the drop in gas pressure on the Magnehelic gauge. If the pressure needs adjustment, turn the
adjustment screw to get the proper pressure.
10. When finished replace screw cover. End call for heat. Remove power. Disconnect hose barb and
replace the pressure port plug.
11. Restore power.
Fig. 13
Fig. 14
NOTE about LP gas pressure adjustment:
When adjusting the gas pressure on LP
equipped furnaces, first turn the high fire
adjustment screw clockwise until it bottoms
out before setting the low fire propane fire
rate. If you do not do this, you will not be
able to set the low fire rate to its proper level.
Manifold Pressure Adjustment
Gas Pressure Manifold Adjustment Sealed Combustion
Furnace
Fig. 15
Sealed
combustion burners (fig. 15) are not at atmospheric
pressure. The burners are actually at a negative pressure
compared to atmosphere. Therefore, you must reference
the burner box pressure when measuring manifold
pressure.
To measure manifold pressure on a system that has sealed
combustion you must have a manometer or magnehelic
gauge with a high and low pressure connection.
To measure gas pressure with sealed combustion (fig. 16), first,
connect a tee into the hose connecting the gas valve to the
burner box. Next, connect one end of the tee to the tube
leading to the burner enclosure. Next, connect the last tee
opening to the low port of the magnehelic gauge or
manometer. Connect high port of the magnehelic gauge or
manometer to the pressure port tap on the gas valve. The
pressure you read will be the manifold pressure referenced
to the burner box pressure.
Fig. 16
Manifold Pressure Adjustment
Pressure
Valves
Measurement:
New
36G
Gas
Fig. 17
Fig. 18
Fig. 19
Fig. 20
The new White Rodgers 36G gas valves (fig. 17) require a
special kit to measure the inlet and outlet gas pressure.
The kit number is KIT07611. The kit consists of a 5/16
tube, a connector, and a 3/32 hex wrench (fig. 18). The
kit is used to connect a gas pressure manometer to
pressure bosses located on the gas valve body.
The kit’s 5/16 inch tubing fits over the inlet and outlet
pressure bosses of the gas valve. The kit’s connector
connects the manometer to the 5/16 inch tubing.
The hex wrench opens the port inside of the pressure
bosses. Rotating the screw inside of the boss counter
clockwise one turn will open the pressure port. To
close the port, rotate the screw clockwise one turn (fig.
19).
Always follow the safety instructions included with the
pressure tap adaptor kit.
New production units will have new easy to service White
Rodgers 36 series gas valves. The valves feature easy
to access pressure adjustment screws and easy
ON/OFF pressure access bosses on the gas valve body
(fig. 20 and 21). The small size of the valve makes
service a breeze.
LP conversion kits are available. The two stage valve
shares the same spring as single stage models.
Fig. 21
Checking Silicon Carbide Hot Surface Ignitors
Fig. 1
Silicon
carbide hot surface ignitors (fig. 1) are used as the
ignition source on some furnace models. The igniters are
120 volt operated devices that are energized by the IFC
board during a trial for ignition period. The igniters are
connected to the IFC board at the igniter HOT and igniter
neutral terminals (fig. 2).
When a call for heat occurs, the furnace goes through a prepurge cycle. After the pre-purge cycle is completed, the
trial for ignition cycle is started. The igniter goes through
a warm-up period and the gas valve is energized. If the
igniter fails to glow, the trial for ignition will fail.
Fig. 2
Checking the Igniter
1. Turn off power to the furnace.
2. Let the furnace go through the pre-purge
cycle and enter the trial for ignition period.
Fig. 3
Checking Silicon Carbide Hot Surface Ignitors
Measure the voltage at the plug (fig. 4). There should be 120 volts. If 120 volts is present and the
igniter does not glow, replace the igniter.
If there is no voltage, check for 120 volts at the IGNITER HOT and IGNITER NEUTRAL terminals on
the IFC (fig. 5). If voltage is present, repair the wiring between the IFC and the plug. If there is no
voltage at the IFC board, replace the IFC board.
Fig. 4
Fig. 5
Checking Silicon Nitride Hot Surface Ignitors
Silicon nitride igniters (fig.1) are used as the ignition source on some furnace models.
The igniters
operate at a voltages below line level. Typically in the 70 to 80 volt range. These igniters cannot
operate at 120 volts. If 120 volts is applied to the igniter over an extended period of time, the
igniter will fail.
Furnaces that have silicon nitride igniters have IFC boards that monitor furnace line voltage levels.
The IFC will regulate the voltage to the igniter based upon the line voltage level (fig. 2). By
reducing voltage to the igniter, the life-span of the igniter is extended.
When a call for heat occurs, the furnace goes through a pre-purge cycle.
After the pre-purge cycle is
completed, the trial for ignition cycle is started. The igniter goes through a warm-up period and
the gas valve is energized. If the igniter fails to glow, the trial for ignition will fail.
Fig. 1
Fig. 2
Checking Silicon Nitride Hot Surface Ignitors
Check Procedure
1. If the igniter fails to glow, disconnect power to the furnace.
2. Unplug the igniter wiring harness and access the pins in the plug assembly leading to the igniter (fig. 3).
3. Using an ohmmeter, measure the resistance through the igniter (fig. 4). At room temperature you should
measure between 11 and 17 ohms. If the resistance is infinite or zero, replace the igniter.
4. If the igniter ohms out OK, replace the IFC board.
Fig. 3
Fig. 4
Ignitor Malfunction
Check voltage from ground to neutral. It should never measure more than 10 volts.
Cause: Loose neutral wire connection.
Checking Induced Draft Blower Motor
Checking Shaded Pole Induced Draft Motor Winding
80% efficient gas furnace models use shaded pole motors
to turn the induced draft blower assembly (fig. 1).
These motors are 115 volt operated motors that have an
internal overload switch.
Fig. 1
When a call for heat occurs, the motor is energized by the
IFC board. The IFC has a HOT INDUCER terminal that
has 115 volts present when motor operation is called
for.
The IFC also has a terminal connection for
NEUTRAL.
Check Procedure
Fig. 2
If the motor does not run when it should, the motor
winding may be open, the motor may be seized, the
internal overload may be open, or the IFC board may
not be switching 115 volts to the motor.
1. Disconnect power to the furnace.
2. Unplug the motor from the wiring harness (fig. 2).
Continued on Next Slide…
Checking Induced Draft Blower Motor
Continued…
3. Using an ohmmeter, check for continuity between
the two pins on the plug (fig. 3). You should measure a
small resistance. If you measure infinite resistance, the
Internal overload may be open, or the motor winding
may be open. If the motor is cool, replace the motor. If
the motor is hot, allow time for it to cool and recheck to
see if the internal overload resets. Check for a seized
motor.
Fig. 3
4. If the motor windings are OK, connect the motor plug
back into the wiring harness.
5. Restore power to the furnace and call for heat.
Fig. 4
6. Check for 115 volts at the IFC board terminals that
power the induced draft blower (fig. 4). There should be
115 volts present. If there is no voltage, replace the IFC.
7. If voltage is present, check the wiring harness
between the IFC and the induced draft blower motor.
Checking PSC Induced Draft Motor Windings
90% efficient gas furnace models use PSC motors to
turn the induced draft blower assembly. These
motors are 115 volt operated motors that have an
internal overload switch. The motors have start
and run windings along with a run capacitor. If
the furnace is a single stage model, the motor
will have one run winding speed tap if the
furnace is a two stage model, there will be two
run winding speed taps.
When a call for heat occurs, the motor is energized
by the IFC board (fig. 1). The IFC has a HOT
INDUCER terminal that has 115 volts present
when motor operation is called for. The IFC also
has a terminal connection for NEUTRAL. If the
furnace is a two stage model, there will be a low
and a high speed terminal on the IFC board.
Check Procedure
If the motor does not run when it should,
the motor winding may be open, the motor
may be seized, the internal overload may
be open, or the IFC board may not be
switching 115 volts to the motor, or the run
capacitor may be bad.
1. Disconnect power to the furnace.
2. Unplug the motor from the wiring
harness.
Fig. 1
Continued on Next Slide…
Checking PSC Induced Draft Motor Windings
Continued…
Fig. 2
3. Using an ohmmeter, check for continuity between
the white wire pin on the plug and both black high
speed pin and the red low speed pin (fig. 2). You
should measure a small resistance between white and
black and white and red. If you measure infinite
resistance, the Internal overload may be open, or the
motor winding may be open. If the motor is cool,
replace the motor. If the motor is hot, allow time for it
to cool and recheck to see if the internal overload
resets. Check for a seized motor. If the motor is not
seized, make sure the run capacitor is not bad.
4. If the motor windings are OK, connect the motor plug
back into the wiring harness.
5. Restore power to the furnace and call for heat.
6. Check for 115 volts at the IFC board terminals that
power the induced draft blower (fig. 3). Make sure you
check the proper speed tap. There should be 115 volts
present. If there is no voltage, replace the IFC. If
voltage is present, check the wiring harness between
the IFC and the induced draft blower motor.
Remember to check the run capacitor!
Fig. 3
Checking Earlier Variable Speed Induced Draft Motor
Some variable speed gas furnaces use a DC drive induced draft motor
Fig. 1
that has an on board microprocessor control (fig. 1). The vent
pressure switches report directly to the vent motor assembly.
Pressure switch status is reported to the IFC board on a
communication bus between the IFC board and the microprocessor
board on the vent motor. There are status LED indicator lights on
the IFC that communicate diagnostic information received from the
vent motor microprocessor.
The vent motor has a 12 pin plug that has a line voltage connection
Fig. 2
(fig. 2). The 115 volt power must be present for the vent motor to
run. If the power is not present at the vent motor, the induced draft
blower housing limit switch may be open.
The 12 pin plug also has a 13 volt DC pin that must have power present
for the vent motor microprocessor to function. The 13 volt DC
power is received from the IFC board’s 5 pin plug.
Fig. 3
The IFC tells the vent motor whether to operate at first stage speed or
second stage speed. The stage signal is sent out on the 5 pin IFC
plug (fig. 3). The voltage levels are 1 volt DC on or 12 volts DC off.
For example, the first stage speed pin would have 1 Volt DC if the
IFC is calling for first stage induced draft motor operation.
Checking Earlier Variable Speed Induced Draft Motor
Checking Vent Motor Quick Test
1. If the induced draft blower does not run, make sure there is 115 volts to the Vent motor 12 pin plug.
Check the voltage between NEUTRAL and pin 12 of the 12 pin plug on the vent motor (fig. 4). If voltage is
not present, check to make sure the induced draft blower limit is closed. If it is open, replace it.
2. If voltage is present, the IFC board green LED labeled RPM should be flashing slow. If it is not, check
for 13 volts DC on the IFC board. Measure at the 5 pin plug between pins 2 and 3 (fig. 5). If voltage is not
present, replace the IFC board.
3. If 13 volts DC is present at the 5 pin plug, jumper pin 4 on the 5 pin plug to ground (fig. 6). The motor
should start. If the motor starts, replace the IFC board.
4. If the motor does not start, replace the vent motor.
NOTE: To see if the motor will change speeds, start the motor on low speed by jumpering pin 4 to
ground. Once the motor is running remove the jumper from pin 4 and rapidly jumper pin 5 to ground (fig.
7). If the motor does not change speed, replace the motor.
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Variable Speed 3 Phase Induced Draft Motors
Some variable speed furnace models feature variable speed 3
phase induced draft motors (fig. 1). The
motor speed is adjusted to optimize venting performance. The motor speed is controlled by the
furnace IFC board.
A 4 pin plug connects the motor to the IFC board (fig. 2).
The IFC board sends out three phase voltage
to change the speed of the motor. If the motor fails to run, the problem is either with the motor or
the IFC board.
Fig. 1
Fig. 2
Variable Speed 3 Phase Induced Draft Motors
Check Procedure
Fig. 3
1. If the motor does not run, make sure there is
power to the furnace. Ensure that a shorted vent
pressure switch is not present.
2. Disconnect power to the furnace and unplug the
vent motor from the wiring harness (fig. 3).
3. Using an ohmmeter check the resistance between
all three motor leads (fig. 4). L1 to L2, L1 to L3, L2 to
L3. You should measure equal resistance through all
winding combinations. If any combination reads
infinite, the winding is open. Replace the motor. If all
combinations read infinite resistance, the internal
overload is open. If the motor is cool, replace the
motor. If the internal overload resets, check for a
seized motor.
4. If the motor windings are OK, make sure the
motor is not seized.
5. If the motor is not seized, replace the IFC board.
Fig. 4
ECM Blower Motor Quick Test
ECM blower motors are used on some furnace models.
These motors are variable speed. They will
adjust their RPM in an attempt to deliver the CFM that is programmed by the installing technician.
The motors operate on 115 volts if installed on furnaces and 230 volts if installed on air handlers.
The motor has two plugs (fig. 1). One plug
is a five pin plug that connects the line voltage to motor. The
line voltage must be present for the motor to operate.
The second plug is a 16 pin plug that connects to the furnace IFC board (fig. 2). This plug carries control
signals between the IFC and the ECM Motor.
Fig. 1
Fig. 2
ECM Blower Motor Quick Test
Check Procedure
Fig. 3
If the motor does not run, make sure you have power to
the IFC board and then check the voltage between pins
4 and 5 of the 5 pin plug on the motor. You should read
115 volts.
If voltage is not present, the motor will not run. Check
for a break in the wiring between the IFC board and the
motor. If the motor is equipped with a choke coil, the
choke coil may be open.
If line voltage is present to the motor and the motor
does not run, jumper R to G on the thermostat terminal
strip (fig. 3). If the motor runs the problem is in the low
voltage thermostat wire.
Fig. 4
If the motor does not run, remove the 16 pin plug from
the IFC board. Locate pins 12 and 15. Connect 24
volts from R to pin 12 and pin 15. Connect pins 1 and 3
to the common side of the 24 volt transformer (fig. 4).
The motor should run.
Continued on Next Slide…
ECM Blower Motor Quick Test
Continued…
If the motor does not run, unplug the 16 pin wiring harness from the motor (fig. 5). Put 24 volts to
pins 12 and 15 and 24 volt common to pins 1 and 3 at the motor (fig. 6). If the motor starts, the
fault is in the harness. If the motor does not run, replace the motor module.
Fig. 5
Fig. 6
LitePort Furnace Control Information
Control Response
RED LED Fault Display
Past IFC boards lost all fault information once
power was removed to the furnace. New
production LitePort equipped boards will
retain fault information after power has
been removed.
If power is interrupted to a furnace that has
detected faults, the faults will be retained
in memory. When power is restored to the
furnace, the last four faults in the IFC
board memory will be displayed on the
Red LED. The faults are displayed in order
last in first out. The LED will not display
these faults again, until power is removed
and then restored. If a fault is present, the
LED will display that fault only.
The new LitePort series IFC boards store event
history in memory. The event history can
be recovered by using the LitePort optical
coupler and related software.
Some of the events stored in IFC board memory
include:
•
Number of recycles.
•
Low flame current events.
•
Line voltage irregularities.
•
Pressure switch faults.
•
Limit circuit faults.
•
Calls for heat over a period of time.
•
Number of calls for cooling over a period of
time.
•
Safety trips.
LitePort Furnace Control Information
Why the RED diagnostic LED flashes every so often.
The furnace control board communicates diagnostic information at least every 20 seconds. The
information is contained in the light emitted by the RED LED. This information can be read with a
LitePort optical coupler system.
If information is being read by the optical coupler, the information could be delayed by as much as 60
seconds from the actual time the event occurred. If the IFC gathers information during the
transmit period, the information you read may be delayed for a very short period of time.
How to clear out the fault history
1.
2.
3.
4.
5.
Remove power to the furnace.
Turn heat and cool demand off at the thermostat.
Turn fan mode to ON mode. Not auto. (At thermostat) (OR JUMPER R to G at Furnace Board.)
Turn the power back on.
Jumper the following low voltage connections at the IFC thermostat terminal strip within 8
seconds of power up:
W1 on and then OFF three consecutive times. ( R to W1 at the low voltage terminal on the furnace IFC.)
Control will blink the red and green LED for 2 seconds to confirm fault history cleared.
LitePort Furnace Control Information
To clear event history.
1.
2.
3.
4.
5.
Turn power off.
Fan to ON position at the thermostat. (OR JUMPER R to G at the furnace control board.)
At the thermostat turn heat and cool demands off.
Turn the power back on.
Jumper the following low voltage connections at the IFC thermostat terminal strip within 8
seconds of power up:
R to W1 On and then off.
R to Y On then off.
R to W1 On then off.
Red and Green LED’s will blink for 2 seconds to confirm erase of history.