Comfortaire Century GUH92C Series User manual

508107-01 Issue 2041 Page 1 of 55
SERVICE MANUAL
GUH92C
(P) 508107-01
*P508107-01*
Table of Contents
Technical Specifications - GUH92C.............................2
Parts Arrangement.......................................................5
Electrical......................................................................6
Placement & Installation............................................20
Joint Cementing Procedure.......................................21
Venting Practices.......................................................22
Vent Piping Guidelines ..............................................23
Condensate Piping ....................................................37
Start-Up .....................................................................42
Heating System Service Checks ...............................43
Typical Operating Characteristics..............................47
Maintenance..............................................................48
Wiring Diagram..........................................................51
Troubleshooting: Heating Sequence of Operation ....52
Troubleshooting: Heating Sequence of Operation
(Continued)................................................................53
Troubleshooting: Cooling Sequence of Operation.....54
Troubleshooting: Continuous Fan / Accessories
Sequence of Operation..............................................55
This is a safety alert symbol and should never be ignored. When you see this symbol on labels or in manuals, be alert to
the potential for personal injury or death.
Electric shock hazard.
Can cause injury or death. Before attempting
to perform any service or maintenance,
turn the electrical power to unit OFF at
disconnect switch(es). Unit may have
multiple power supplies.
WARNING
As with any mechanical equipment, contact with sharp
sheet metal edges can result in personal injury. Take
care while handling this equipment and wear gloves
and protective clothing.
CAUTION
Improper installation, adjustment, alteration, service
or maintenance can cause property damage, personal
injury or loss of life. Installation and service must be
performed by a licensed professional HVAC installer (or
equivalent), service agency or the gas supplier.
WARNING

508107-01 Issue 2041 Page 2 of 55
Technical Speci ications - GUH92C
REV CODE
REVISION
NOM. CFM X 100
2 TON ADD ON COOLING
3 TON ADD ON COOLING
4 TON ADD ON COOLING
5 TON ADD ON COOLING
CABINET WIDTH
A = 14.5” WIDTH
B = 17.5” WIDTH
C = 21.0” WIDTH
D = 24.5” WIDTH
BTUH INPUT
HEATING INPUT X 1000
GAS
G = GAS FURNACE
CONFIGURATION
UH = UPFLOW/HORIZONTAL
AFUE
80 = 80% EFFICENCY
92 = 92% EFFICENCY
95 = 95% EFFICENCY
96 = 96% EFFICENCY
G UH 92 CG UH 92 C 004455BB33MM
STAGES/MOTOR
A =SINGLE STAGE PSC
C =SINGLE STAGE CONSTANT TORQUE
T = TWO STAGE VARIABLE SPEED ECM
MODEL NUMBER GUIDE
PHYSICAL AND ELECTRICAL DATA
Model
Input
(Btuh)
Output *
(Btuh)
AFUE
(ICS)
Nominal*
Cooling
Capacity
(tons)
Gas
Inlet
(in.)
Volts/
Hertz/
Phase
Max.
Time
Delay
Breaker
or Fuse
Nominal
F.L.A.
Trans.
(V.A.)
Approx.
Shipping
Weight
(lbs.)
UPFLOW / HORIZONTAL
GUH92C045B3M 44,000 42,000 92.1% 1.5-3 1/2 120-60-1 15 6.8 40 121.5
GUH92C070B3M 66,000 63,000 92.1% 1.5-3 1/2 120-60-1 15 6.8 40 128.5
GUH92C090C3M 88,000 84,000 92.1% 1.5-3 1/2 120-60-1 15 6.8 40 145.0
GUH92C090C4M 88,000 84,000 92.1% 2.5-4 1/2 120-60-1 15 8.4 40 145.0
GUH92C110C4M 110,000 104,000 92.1% 2.5-4 1/2 120-60-1 15 8.4 40 157.0
GUH92C110C5M 110,000 105,000 92.1% 3-5 1/2 120-60-1 15 10.9 40 159.0

508107-01Issue 2041
Page 3 of 55
BLOWER PERFORMANCE DATA
Model
Motor
Size
(hp)
Blower
Size
Heating
Temp Rise
(F°)
Blower
Speed
Heating
CFM @ .10" - .80" w.c.
0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80
UPFLOW / HORIZONTAL
GUH92C045B3M 1/2 10 x 8 25 - 55
High 1390 1355 1330 1295 1265 1240 1210 1160
Med/High 1270 1230 1205 1180 1140 1100 1065 1030
Med 1135 1100 1060 1025 995 955 920 880
Med/Low 885 850 795 755 720 670 615 580
Low 735 680 625 570 515 475 425 355
GUH92C070B3M 1/2 10 x 8 40 - 70
High 1395 1365 1340 1305 1270 1250 1210 1185
Med/High 1270 1245 1215 1180 1145 1110 1080 1045
Med 1145 1100 1080 1040 1005 970 930 885
Med/Low 910 865 815 760 720 685 635 600
Low 705 655 600 550 505 455 405 355
GUH92C090C3M 1/2 10 x 8 50 - 80
High 1375 1340 1310 1275 1450 1210 1170 1135
Med/High 1315 1285 1260 1225 1190 1160 1130 1105
Med 1125 1090 1055 1015 965 920 885 855
Med/Low 885 840 780 730 675 620 580 535
Low 730 685 645 590 535 495 455 405
GUH92C090C4M 3/4 10 x 10 45 - 75
High 1675 1655 1615 1595 1560 1525 1490 1445
Med/High 1475 1460 1410 1365 1335 1295 1260 1230
Med 1365 1315 1290 1245 1200 1160 1105 1085
Med/Low 1275 1230 1195 1155 1105 1060 1015 980
Low 1160 1110 1065 1010 965 915 875 830
GUH92C110C4M 3/4 10 x 10 50 - 80
High 1715 1680 1640 1615 1580 1540 1515 1470
Med/High 1495 1470 1425 1395 1345 1315 1285 1230
Med 1380 1355 1300 1260 1225 1180 1135 1095
Med/Low 1295 1255 1220 1170 1130 1080 1035 995
Low 1175 1135 1085 1025 975 935 895 850
GUH92C110C5M 111 1/2 x 10 45 - 75
High 2120 2085 2040 2005 1965 1935 1885 1855
Med/High 1880 1830 1795 1760 1710 1680 1635 1600
Med 1715 1665 1630 1585 1540 1510 1485 1440
Med/Low 1605 1560 1515 1470 1425 1390 1345 1305
Low 1435 1360 1325 1270 1230 1180 1140 1085

508107-01 Issue 2041 Page 4 of 55
ACCESSORY LIST
CATALOG NUMBER DESCRIPTION
External Filter Rack Kits
1.841018 1 pack (16 x 25)
1.841039 10 pack (16 x 25)
Natural to LP Kits
11K50 1-Stage - 90%
11K45 High Altitude 2-Stage >7500’
17H62 1-Stage - 90% - For 030 Capacity Only
17H90 High Altitude (>7500’) - For 030 Capacity Only
Return Air Base
68W62 17.5" B Width
68W63 21.0" C Width
Night Service Kits
18F15 Single Stage
Horizontal Suspension Kit
51W10 80% & 90% Kit
Flush Mount Termination (90% Furnaces only) US Only
51W11 2" & 3" Vent Version
Concentric Vent Kit (90% Furnaces only) US Only
71M80 1-1/2" Vent Version
69M29 2" Vent Version
60L46 3" Vent Version
2-Stage Compressor Kit
85W66 Blower Relay Kit
Twinning Kit
16W72 Constant Torque Gas Furnace Twinning Kit
For vent length and clearances to combustibles, please reference installation instructions.

508107-01Issue 2041
Page 5 of 55
Parts Arrangement
Figure 1.
Burner Box
Assembly
Gas Valve
Combustion
Air Inducer
Blower
Access Door
Combustion
Air Pressure
Prove Switch
Cold End
Header Box
Control Box
(includes integrated ignition control,
transformer and interlock switch)
Blower Assembly
Bag Assemblies
Cabinet
Top Cap
Heat Exchanger
Assembly
Flexible No-HJB
Exhaust Collar

508107-01 Issue 2041 Page 6 of 55
Electrical
ELECTROSTATIC DISCHARGE (ESD)
Precautions and Procedures
Electrostatic discharge can a󰀨ect electronic
components. Take precautions to
neutralize electrostatic charge by touching
your hand and tools to metal prior to
handling the control.
CAUTION
GUH92C unit components are shown in Figure 1. The
combustion air inducer, gas valve and burners can be
accessed by removing the outer access panel. The
blower and control box can be accessed by removing the
blower access panel.
Control Box Components (Figure 2)
Unit transformer (T1) and integrated ignition
control (A92) are located in the control box. In addition,
a door interlockswitch (S51) is located in the control box.
Integrated Control
Door Interlock
Switch
Transformer
Circuit Breaker
Figure 2. Control Box
Transformer (T1)
A transformer located in the control box provides power
to the low voltage section of the unit. The transformers on
all models are rated at 40VA with a 120V primary and 24V
secondary.
Door Interlock Switch (S51)
A door interlock switch rated 14A at 120VAC is located
on the control box. The switch is wired in series with line
voltage. When the blower door is removed the unit will shut
down.
Circuit Breaker (CB8)
A 24V circuit breaker is also located in the control box. The
switch provides overcurrent protection to the transformer
(T1). The breaker is rated at 3A at 32V. If the current
exceeds this limit the breaker will trip and all unit operation
will shutdown. The breaker can be manually reset by
pressing the button on the face.
Integrated Ignition Control (A92)
Shock hazard.
Disconnect power before servicing.
Control is not eld repairable. If control is
inoperable, simply replace entire control.
Can cause injury or death. Unsafe
operation will result if repair is attempted.
WARNING
The hot surface ignition control system consisting of
an integrated control (Figure 3 with control terminal
designations in Table 1 through Table 3), sensor and
ignitor (Figure 5). The integrated control and ignitor work
in combination to ensure furnace ignition and ignitor
durability. The integrated control, controls all major furnace
operations. The integrated control also features a RED
LED for troubleshooting and two accessory terminals rated
at (1) one amp. See Table 4 or Table 5 for troubleshooting
diagnostic codes. The nitride ignitor is made from a non-
porous, high strength proprietary ceramic material that
provides long life and trouble free maintenance.
Pin # Function
1Combustion Air Inducer Line
2Ignitor Line
3 Combustion Air Inducer Neutral
4Ignitor Neutral
Table 1. 4-Pin Terminal Designations
Pin # Function
1High Limit Output
2Not Used
324V Line
4Not Used
5Rollout Switch Out
624V Neutral
7High Limit Input
8Ground
9 Gas Valve Common
10 Pressure Switch In
11 Rollout Switch In
12 Gas Valve Out
Table 2. 12-Pin Terminal Designations

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1/4” Quick Connect Terminals
120HUM Humidier 120VAC
LINE 120VAC
XFMR Transformer 120VAC
CIRC Indoor Blower 120VAC
EAC Indoor Air Quality Accessory 120VAC
NEUTRALS Common 120VAC
HUM24 Humidier 24VAC
3/16” Quick Connect Terminals
COOL Cooling Tap 24VAC
HEAT Heating Tap 24VAC
FAN Continuous Blower 24VAC
PARK (no power) Park terminal for unused speed taps
FS Flame Sense
24 COM Common 24VAC
Table 3.
The integrated control is equipped with an LED light for troubleshooting. The diagnostic codes are listed in Table 4 or Table
5.
Red LED Flash Code2Diagnostic Codes / Status of Furnace
O󰀨 No power to control or board fault detected
Heartbeat1Control powered - displayed during all modes of operation if no errors are detected
1Reverse Line Voltage Polarity
2Improper Earth Ground
3 Burner failed to light, or lost ame during heat demand
4Low Flame Signal - check ame sensor
5Watchguard - burner failed to light, exceeded maximum number of retries or recycles
6Not Used
7Primary or Secondary Limit Open or Watchguard Mode - Limit Switch Open longer than 3 minutes
8Rollout Switch Open
9 Pressure Switch failed to close or opened during heat demand
10 Watchguard - Pressure Switch opened 5 times during one heat demand
11 Pressure Switch stuck closed prior to activation of combustion air inducer
12 Flame Sensed without gas valve energized
13 Low Line Voltage
1 A ”Heartbeat” is indicated by a ”Slow Flash” - 1 sec on 1 sec o󰀨, repeating
2 Error codes are indicated by a ”Rapid Flash” - the LED ashes X times at 1/2 sec on 1/2 sec o󰀨, remains o󰀨 for 3 sec, then repeats
NOTE: Last 10 error codes are stored in memory including when power is shut o󰀨 to the unit. - To recall, press and release button,
most recent will be displayed rst, LED o󰀨 for 3 sec, then next error code is displayed, etc. To clear error codes, depress and
hold button longer than 5 seconds.
Table 4. -02 Control Diagnostic Codes

508107-01 Issue 2041 Page 8 of 55
Red LED Flash Code2Diagnostic Codes / Status of Furnace
O󰀨 No power to control or board fault detected
Heartbeat1Normal Operation - Idle, Continuous Fan, Cool
Continuous Rapid Flash Call for Heat / Burner Operation
1Reverse Line Voltage Polarity
2Improper Earth Ground
3 Burner failed to light, or lost ame during heat demand
4Low Flame Signal - check ame sensor
5Watchguard - burner failed to light, exceeded maximum number of retries or recycles
6Not Used
7Primary or Secondary Limit Open or Watchguard Mode - Limit Switch Open longer than 3 minutes
8Rollout Switch Open
9 Pressure Switch failed to close or opened during heat demand
10 Watchguard - Pressure Switch opened 5 times during one heat demand
11 Pressure Switch stuck closed prior to activation of combustion air inducer
12 Flame Sensed without gas valve energized
13 Low Line Voltage
1 A ”Heartbeat” is indicated by a ”Slow Flash” - 1 sec on 1 sec o󰀨, repeating
2 Error codes are indicated by a ”Rapid Flash” - the LED ashes X times at 1/2 sec on 1/2 sec o󰀨, remains o󰀨 for 3 sec, then repeats
NOTE: Last 10 error codes are stored in memory including when power is shut o󰀨 to the unit. - To recall, press and release button,
most recent will be displayed rst, LED o󰀨 for 3 sec, then next error code is displayed, etc. To clear error codes, depress and
hold button longer than 5 seconds.
Table 5. -03 Control Diagnostic Codes

508107-01Issue 2041
Page 9 of 55
Figure 3. Integrated Control
(Automatic Hot Surface Ignition System)
BLOWER OFF DELAY
RED LED RECALL BUTTON
Electronic Ignition
On a call for heat the integrated control monitors the
combustion air inducer pressure switch. The control board
will not begin the heating cycle if the pressure switch is
closed (by-passed). Once the pressure switch is determined
to be open, the combustion air inducer is energized. When
the di󰀨erential in the pressure switch is great enough, the
pressure switch closes and a 15-second pre-purge begins.
If the pressure switch is not proven within 2-1/2 minutes,
the integrated control goes into Watchguard-Pressure
Switch mode for a 5-minute re-set period.
After the 15-second pre-purge period, the ignitor warms
up for 20 seconds during which the gas valve opens at
19 seconds for a 4-second trial for ignition. The ignitor
remains energized for the rst 3 seconds during the 4
second trial. If ignition is not proved during the 4-second
period, the integrated control will try four more times with
an inter purge and warm-up time between trials of 35
seconds. After a total of ve trials for ignition (including the
initial trial), the integrated control goes into Watchguard-
Flame Failure mode. After a 60-minute reset period, the
integrated control will begin the ignition sequence again.
Fan Time Control
Heating Fan On Time
The fan on time of 30 seconds is not adjustable.
Heating Fan O󰀨 Time
Fan o󰀨 time (time that the blower operates after the heat
demand has been satised) can be adjusted by moving
the jumper to a di󰀨erent setting. The unit is shipped with
a factory fan o󰀨 setting of 90 seconds. For customized
comfort, monitor the supply air temperature once the heat
demand is satised. Note the supply air temperature at the
instant the blower is de-energized.
Adjust the fan-o󰀨 delay to achieve a supply air temperature
between 90° - 110° at the instant the blower is de-energized.
(Longer delay times allow for lower air temperature, shorter
delay times allow for higher air temperature). See Figure 4.
Cooling Fan On Time
The fan on time is 2 seconds and is not adjustable.
Cooling Fan O󰀨 Time
The control has a 45 second fan o󰀨 delay after cooling
demand has been met. This delay is factory set and not
adjustable.
Figure 4.
Heating Components
Combustion air inducer (B6), primary limit control (S10),
ignitor, burners, ame rollout switch (S47), gas valve
(GV1), combustion air pressure switch (S18), and heat
exchangers are located in the heating compartment. The
heating compartment can be accessed by removing the
outer access panel.
GUH92C units use a nitride ignitor made from a
proprietary ceramic material. To check ignitor, measure
its resistance and voltage. A value of 39 to 70 ohms
indicates a good ignitor. Voltage to the ignitor should be
102 - 132VAC. See Figure 9 for resistance and voltage
checks.

508107-01 Issue 2041 Page 10 of 55
Figure 5. Burner Box Assembly
Gas Valve
Burner Box Front
Rollout Switch(s)
Flame Sensor
Ignitor
Burner Assembly
Flame Rollout Switches (Figure 5)
Flame rollout switches S47 are SPST N.C. high temperature
limits located on the left and right of the front buner box
plate. S47 is wired to the burner ignition control A92. When
either of the switches sense ame rollout (indicating a
blockage in the combustion passages), the ame rollout
switch trips, and the ignition control immediately closes
the gas valve. Switch S47 in all GUH92C units is
factory preset to open at 210 F + 12 F (99 C + 6.7 C) on a
temperature rise. All flame rollout switches are manual
reset. See flash code 8 in Table 4 or Table 5 for
troubleshooting.
Flame Sensor (Figure 5)
A flame sensor is located on the left side of the burner
support. The sensor is mounted on the front burner box
plate and the tip protrudes into the flame envelope of the
leftmost burner. The sensor can be removed for
service (use steel wool only to clean) without removing
any part of the burners. During operation, flame is sensed
by current passed through the flame and sensing
electrode. The ignition control allows the gas valve to
remain open as long as flame signal is sensed.
NOTE: The GUH92C is polarity sensitive. Make sure that
the furnace is wired correctly and properly grounded.
A microamp DC meter is needed to check the ame signal
on the integrated control.
Flame (microamp) signal is an electrical current which
passes from the integrated control to the sensor during
unit operation. Current passes from the sensor through the
ame to ground to complete a safety circuit.
Heat Exchanger (Figure 6)
GUH92C units use an aluminized steel primary and
stainless steel secondary heat exchanger assembly. Heat
is transferred to the air stream from all surfaces of the
heat exchanger. The shape of the heat exchanger
ensures maximum efficiency.
The combustion air inducer pulls fresh air through the
burner box. This air is mixed with gas in the burners. The
gas / air mixture is then burned at the entrance of each
clamshell. Combustion gases are then pulled through the
primary and secondary heat exchangers and exhausted
out the exhaust vent pipe.

508107-01Issue 2041
Page 11 of 55
Primary Limit Control (Figure 6)
Primary limit (S10) used on GUH92C units is located
in the heating vestibule panel. When excess heat is
sensed in the heat exchanger, the limit will open. Once
the limit opens, the furnace control energizes the supply
air blower and de-energizes the gas valve. The limit
automatically resets when unit temperature returns to
normal. The switch is factory set and cannot be
adjusted. For limit replacement remove wires from limit
terminals, remove mounting screws, rotate limit
switch 90 degrees and slowly remove from the
vestibule panel. Install replacement limit with same
care. See flash code 7 in Table 4 or Table 5 for
troubleshooting if limit switch opens during operation.
Install limit face down
Figure 6. Primary Limit Location and Heat Exchanger
Gas Valve (Figure 7)
The GUH92C uses an internally redundant valve to
assure safety shut-off. If the gas valve must be replaced,
the same type valve must be used.
24VAC terminals and gas control switch are located on
top of the valve. All terminals on the gas valve are
connected to wires from the ignition control. 24V applied
to the terminals opens the valve.
Inlet and outlet pressure taps are located on the valve. A
manifold adjustment screw is also located on the valve.
An LP/Propane changeover kit is available.
Figure 7. Gas Valve
GAS VALVE SHOWN IN ON POSITION
MANIFOLD
PRESSURE
OUTLET
PORT
INLET
PRESSURE
PORT
MANIFOLD PRESSURE
ADJUSTMENT SCREW
(under barbed fitting)
To Measure Flame Signal - Integrated Control:
Use a digital readout meter capable of reading DC
microamps. See Figure 8 for ame signal check.
1. Set the meter to the DC amps scale.
2. Turn o󰀨 supply voltage to control.
3. Remove sensor wire from integrated control.
4. Connect (-) lead to ame sensor wire.
5. Connect (+) lead to Terminal FS on integrated control.
6. Turn supply voltage on and close thermostat contacts
to cycle system.
7. When main burners are in operation for two minutes,
take reading.

508107-01 Issue 2041 Page 12 of 55
Figure 8. Primary Limit Location and Heat Exchanger
Flame Signal In Microamps
NormalLow Drop Out
1.50.5 - 1.40.4
Set Dial to DC MicroAmps
(+)
Multi−Meter
(+)
(+) To Control
Sensor Terminal
(−) To Flame
Terminal
Flame Sensor
Terminal
Flame Sensor
Wire
Intergrated
Control
Remove sensor from integrated
control and connect alligator clip
(−) to flame sensor lead.
Remove sensor from integrated
control and connect alligator clip
(+) to terminal on control.

508107-01Issue 2041
Page 13 of 55
Figure 9.
Test 1
Check ignitor circuit for correct resistance.
Remove 4-pin plug from control.
Check ohms reading across terminals 2 and 4.
Reading should be between 39 and 70 ohms. If
value is correct, this is the only test needed.
If the reading on the meter is not correct, (0 or
infinity) then a second test is needed.
Test 2
Check ignitor for correct resistance.
Seperate the 2-pin jack-plug near the manifold and check
resistance of ignitor at the plug. Reading should be
between 39 and 70 ohms. If the reading is correct, then
the problem is with the wiring between the jack-plug and
the control. If reading is not correct, the issue is the ignitor.
Test 3
Check ignitor for correct voltage
Insert meter probes into terminals 2 and 4 (use small
diameter probes in order not to damage plug).
Check voltage during 20 second ignitor warm up period.
Voltage should read 120 volts +10%. If voltage reads below
these values, check for correct supply voltage to furnace.
Multi−Meter
(set to ohms)
Multi−Meter
(set to ohms)
Multi−Meter
(set to VAC)

508107-01 Issue 2041 Page 14 of 55
Combustion Air Inducer (B6) & Cold End Header Box
All GUH92C units use a combustion air inducer to move air
through the burners and heat exchanger during heating
operation. The blower uses a shaded pole 120VAC motor.
The motor operates during all heating operation and is
controlled by integrated control A92. Blower operates
continuously while there is a call for heat. The integrated
control will not proceed with the ignition sequence until
combustion air inducer operation is sensed by the proving
switches.
The combustion air inducer is installed on the cold end
header box. The cold end header box is a single piece made
of hard plastic. The box has an internal channel where the
combustion air inducer creates negative pressure at unit
start up. The channel contains an orice used to regulate
ow created by the combustion air inducer. The box has
pressure taps for the combustion air inducer pressure
switch hoses. The pressure switch measures the pressure
across the combustion air inducer orice or di󰀨erence in
the channel and the box. If replacement is necessary the
gaskets used to seal the box to the vestibule panel and the
combustion air inducer to the box, must also be replaced.
GUH92C series units are equipped with a differential
pressure switch located on the cold end header box. The
switch monitors across the combustion air inducer orifice
to insure proper flow through the heat exchanger.
The switch is a SPST N.O. pressure switch
electrically connected to the integrated control. The
purpose of the switch is to prevent burner operation if the
combustion air inducer is not moving enough air for
proper combustion.
Figure 10. Pressure Switch
On start-up, the switch monitors whether the combustion
air inducer is operating. It closes a circuit to the integrated
control when the di󰀨erence in pressure across the
combustion air inducer orice exceeds a non-adjustable
factory setting. If the switch does not successfully sense
the required di󰀨erential, the switch cannot close and the
furnace cannot operate. If the ue or air inlet become
obstructed during operation, the switch senses a loss of
pressure di󰀨erential and opens the circuit to the integrated
control. If the condensate line is blocked, water will back
up into the header box and reduce the pressure di󰀨erential
across the switch. The pressure switch opens if the
di󰀨erential drops below the set point. See Table 6.
Checks of pressure di󰀨erential can aid in troubleshooting.
When measuring the pressure di󰀨erential, readings should
be taken at the pressure switch. See Figure 11 and Table 7.
Lack of di󰀨erential usually indicates problems in the intake
or exhaust piping, but may indicate problems in the heat
exchanger, condensing coil, header boxes, combustion
inducer or other components.
Unit
Altitude (ft.)
0 - 4500
Set Point “w.c.
045 -0.65”
070 -0.65”
090 -0.65”
110 -0.65”
NOTE: Set point is factory set and non-adjustable
Table 6.

508107-01Issue 2041
Page 15 of 55
Figure 11. Measuring Pressure Di󰀨erential
To Cold End Header Box
Field Provided Tubing
To Pressure Switch
To Cold End Header Box
1 -Remove thermostat demand and allow unit to
cycle off.
2 -Install a tee in the negative (-) line (red and black tubing
or red tubing) and a tee in the positive (+) line (black
tubing) running from the pressure switch to the cold
end header box.
3 - Install a manometer with hose from the negative (-)
side of the manometer to the tee installed in the
negative (-) line and with hose from the positive (+)
side of the manometer to the tee in the positive (+)
line.
NOTE -
tive. However the (+) port reads less negative pressure
than the (-) port.
4 -Operate unit and observe manometer reading.
Readings will change as heat exchanger warms.
a. Take one reading immediately after start‐up.
b. Take a second reading after unit has reached
steady state (approximately 5 minutes). This will be
the pressure differential.
The pressure differential should be greater
than those listed in table 7.
5 -Remove thermostat demand and allow to cycle off.
6 -Remove manometer and tee's. Reinstall combustion
air sensing hoses to the pressure switch.
Black Tubing
(positive +)
Red and Black Tubing
or Red Tubing
(negative -)
“+”
High
“-”
Low

508107-01 Issue 2041 Page 16 of 55
Problem Corrective Action
Pressure switch stuck closed Check that the pressure switch is open without the combustion air inducer
operating. Replace if defective.
Pressure switch does not close due to obstruction in
vent pipe
Check for restricted vent. Remove all blockage.
Check for proper vent sizing. See Table 11 through Table 12.
Pressure switch does not close due to incorrect
routing of the pressure switch tubing
Check that the pressure switch tubing is correctly routed. Correctly route
pressure switch line.
Pressure switch does not close due to obstructions in
the pressure switch line Remove any obstructions from the the pressure switch line and/or taps
Pressure switch tubing damaged Check pressure switch tubing for leaks. Replace damaged tubing if
necessary.
Condensate in pressure switch tubing Check pressure switch tubing for condensate. Remove condensate from
tubing.
Pressure switch does not close due to a low
di󰀨erential pressure across the pressure switch
Check the di󰀨erential pressure across the pressure switch.
Check for restricted inlet vent. Remove all blockage.
Check for proper vent sizing and run length. See Table 11 through Table 12.
Wrong pressure switch installed in the unit, or
pressure switch is out of calibration
Check that the correct pressure switch is installed in the unit. Replace
pressure switch if necessary.
Miswiring of furnace or improper connections at
pressure switch
Check for correct wiring and loose connections. Correct wiring and/or replace
any loose connections.
Pressure switch failure If all the above modes of failure have been checked, the pressure switch
may have failed. Replace pressure switch and determine if unit will operate.
Damaged condensate trap Check trap for any cracks or damage and replace if necessary.
Cold end header box does not drain properly Check that the furnace is set properly with a slight tilt (0 - 1/2”) towards the
front if necessary. See furnace installation instruction.
Air leakage around the combustion air inducer gasket Check gasket and replace if necessary.
Air leakage around the cold end header box gasket Check gasket and replace if necessary.
Damaged cold end header box tubing Check tubing and replace if necessary.
Table 7. Pressure Switch Troubleshooting Guide
Figure 12. Electronic Ignition
DEMAND
CAI
GAS VALVE
15
ON
OFF
38
IGNITOR
34
1
Pre -Purge Ignitor Warm-up Trial for
Ignition Post
Purge
5 SEC80
*Blower on time will be 30 seconds after flame is sensed. Blower off time will depend on “OFF TIME” Setting.
INDOOR BLOWER
Blower “On”*
Delay

508107-01Issue 2041
Page 17 of 55
Blower Compartment
Each blower is statically and dynamically balanced as
an assembly before installation in the unit.
IMPORTANT
GUH92C units are equipped with a constant torque ECM
motor. It has a DC motor coupled to an electronic
control module both contained in the same motor housing.
The motor is programmed to provide constant torque at
each of the five selectable speed taps. Each tap requires
24 volts to energize.
Input Voltage Requirements
The circuit is designed to be operated with AC voltage. To
enable a tap requires 12 to 33VAC. Expected current draw
will be less than 20mA.
Troubleshooting the Motor
Troubleshooting the motor is an easy process. Follow
steps below.
1. Shut o󰀨 power to unit.
2. Remove input plugs P48 and P49 from motor. See
Figure 16 for troubleshooting procedure.
If correct voltage is present in tests 1 and 2 and motor is
not operating properly, replace motor. The motor is not eld
repairable.
If replacing the indoor blower motor or blower wheel is
necessary, placement is critical. The blower wheel must
be centered in the blower housing as shown in Figure 13.
When replacing the indoor blower motor the set screw
must be aligned and tightened with the motor shaft as
shown in Figure 14.
Secondary Limit Controls
The secondary limit is located in the blower compartment
on the back side of the blower housing. See Figure 15.
When excess heat is sensed in the blower compartment,
the limit will open. If the limit is open, the furnace control
energizes the supply air blower and closes the gas valve.
The limit automatically resets when unit temperature
returns to normal. The secondary limit cannot be adjusted.
Figure 13. Blower Wheel Replacement
Center Blower Wheel
in Blower Housing
Figure 14.
Set Screw
Housing Hub
ALIGN AND TIGHTEN SET SCREW WITH
FLAT SIDE OF MOTOR SHAFT
Motor
Shaft
Figure 15. Secondary Limit Control
Secondary Limits

508107-01 Issue 2041 Page 18 of 55
Figure 16.
1
2
3
4
5
C
L
G
N
Multi−Meter
(set to VAC)
P48
P49
120 120
Turn on power to unit. Check for 120 volts across terminals
“L” and “N” on input plug P48. If voltage is present continue
upstream of plug P48. Proceed to test 3.
1
2
3
4
5
C
L
G
N
Multi−Meter
(set to VAC)
P48
P49 24
Switch thermostat to CONTINUOUS FAN MODE. Check for
24 volts across terminal “C” on input plug P48and speed tap
used for continuous fan. (1, 2, 3, 4 or 5) on input plug P49. If
24 volts is not present problem may be up stream of plug P49.
Proceed to test 4.
Multi−Meter
(set to VAC)
120 120
24
Multi−Meter
(set to VAC)
problem is with the harness. If voltage is not present,
problem may be may be with the integrated control.
Check for 120 volts across terminals “CIRC” and “Neutrals”
on the integrated control. If voltage is present, problem is
with the harness. If voltage is not present problem may be
may be with the integrated control.
Test 1
Test 2
Test 3 (if necessary)
Test 4 (if necessary)
to test 2. If voltage is not present, problem may be
Check for 24 volts across terminals “24 COM” and “FAN”
terminals on the integrated control. If voltage is present,

508107-01Issue 2041
Page 19 of 55
Testing the Motor (Figure 17)
If any motor fails the below tests, do not install the new
control module. The motor is defective and it also must be
replaced. The new control can fail if placed on a defective
motor.
1. Using an ohmmeter check the resistance from any
one of the motor connector pins to the aluminum end
plate of the motor. This resistance should be greater
than 100k ohms.
2. Check the resistances between each of the three motor
connector pins. These should all read approximately
the same resistance within an ohm.
3. Check to see if the blower wheel spins freely.
Figure 17. Motor Test
Scale Measurement Range in Words ohms
2 M two megohms - two million ohms 0 - 2,000,000
200 K two hundred kil-ohms -
two hundred thousand ohms 0 - 200,000
20 K twenty kilo-ohms -
twenty thousand ohms 0 - 20,000
2 K two kilo-ohms -
two-thousand ohms 0 - 2,000
200 two hundred ohms 0 - 200
Motor Module Installation
All replacement motor control modules look similar;
however, each module is designed for a specic motor
size. It is very important to make sure that you are using the
correct replacement motor control module. USE OF THE
WRONG MOTOR CONTROL MODULE MAY RESULT IN
UNEXPECTED UNIT OPERATION.
1. Verify electrical power to unit is disconnected.
2. Connect three-wire harness from motor to control
module.
3. Mount new motor
control module to
motor using two hex
head bolts. Torque
bolts to 22 inch pounds
or 1/16th clock turn as
exampled to the right.
12
3
4
5
6
7
8
9
10
11 12
1/16
TURN
4. Reconnect the two
harnesses to the motor
control module.
5. The electrical connectors of the motor should be
facing down to form a drip loop (Figure 18). This will
directs moisture away from the motor and its electric
connections on the motor.
Figure 18. Drip Loop
CONNECTOR
ORIENTATION
BETWEEN 4 AND 8
O'CLOCK
BACK OF CONTROL
MODULE
DRIP LOOP

508107-01 Issue 2041 Page 20 of 55
Placement & Installation
Pipe & Fittings Specications
All pipe, ttings, primer and solvent cement must conform
with American National Standard Institute and the American
Society for Testing and Materials (ANSI/ASTM) standards.
The solvent shall be free owing and contain no lumps,
undissolved particles or any foreign matter that adversely
a󰀨ects the joint strength or chemical resistance of the
cement. The cement shall show no gelation, stratication,
or separation that cannot be removed by stirring. Refer to
Table 8 for approved piping and tting materials.
Solvent cements for plastic pipe are ammable liquids
and should be kept away from all sources of ignition.
Do not use excessive amounts of solvent cement when
making joints. Good ventilation should be maintained to
reduce re hazard and to minimize breathing of solvent
vapors. Avoid contact of cement with skin and eyes.
CAUTION
GUH92C exhaust and intake connections are made of
PVC. Use PVC primer and solvent cement when using
PVC vent pipe. When using ABS vent pipe, use
transitional solvent cement to make connections to the
PVC fittings in the unit.
IMPORTANT
Use PVC primer and solvent cement or ABS solvent
cement meeting ASTM specications, refer to Table 8.
As an alternate, use all purpose cement, to bond ABS,
PVC, or CPVC pipe when using ttings and pipe made of
the same materials. Use transition solvent cement when
bonding ABS to either PVC or CPVC.
Low temperature solvent cement is recommended during
cooler weather. Metal or plastic strapping may be used for
vent pipe hangers. Uniformly apply a liberal coat of PVC
primer for PVC or use a clean dry cloth for ABS to clean
inside socket surface of tting and male end of pipe to
depth of tting socket.
NOTE: Canadian Applications Only - Pipe, ttings,
primer and solvent cement used to vent (exhaust) this
appliance must be certied to ULC S636 and supplied by a
single manufacturer as part of an approved vent (exhaust)
system. In addition, the rst three feet of vent pipe from the
furnace ue collar must be accessible for inspection.
Piping and Fittings Specications
Schedule 40 PVC (Pipe) D1785
Schedule 40 PVC (Cellular Core Pipe) F891
Schedule 40 PVC (Fittings) D2466
Schedule 40 CPVC (Pipe) F441
Schedule 40 CPVC (Fittings) F438
SDR-21 PVC or SDR-26 PVC (Pipe) D2241
SDR-21 CPVC or SDR-26 CPVC (Pipe) F442
Schedule 40 ABS Cellular Core DWV
(Pipe) F628
Schedule 40 ABS (Pipe) D1527
Schedule 40 ABS (Fittings) D2468
ABS-DWV (Drain Waste & Vent)
(Pipe & Fittings) D2661
PVC-DWV (Drain Waste & Vent)
Pipe & Fittings) D2665
PRIMER & SOLVENT CEMENT ASTM
SPECIFICATION
PVC & CPVC Primer F656
PVC Solvent Cement D2564
CPVC Solvent Cement F493
ABS Solvent Cement D2235
PVC/CPVC/ABS All Purpose Cement For
Fittings & Pipe of the same material
D2564, D2235,
F493
ABS to PVC or CPVC Transition Solvent
Cement D3138
CANADA PIPE & FITTING & SOLVENT
CEMENT MARKING
PVC & CPVC Pipe and Fittings
ULCS636PVC & CPVC Solvent Cement
ABS to PVC or CPVC Transition Cement
POLYPROPYLENE VENTING SYSTEM
ULC-S636
PolyPro® by Duravent
InnoFlue® by Centrotherm ULC-S636
ECCO Polypropylene VentTM ULC-S636
Table 8.