Comfortaire GUH80C User manual

508096-01 Issue 2037 Page 1 of 31
SERVICE MANUAL
GUH80C
(P) 508096-01
*P508096-01*
Table of Contents
Technical Specifications - GUH80C.............................2
Parts Arrangement.......................................................5
Unit Components.........................................................6
Placement & Installation............................................19
Start-Up .....................................................................19
Heating System Service Checks ...............................19
Typical Operating Characteristics..............................23
Maintenance..............................................................24
Wiring Diagram..........................................................27
Troubleshooting: Heating Sequence of Operation ....28
Troubleshooting: Heating Sequence of Operation
(Continued)................................................................29
Troubleshooting: Cooling Sequence of Operation.....30
Troubleshooting: Continuous Fan / Accessories
Sequence of Operation..............................................31
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

508096-01 Issue 2037 Page 2of 31
Technical Speci ications - GUH80C
MODEL NUMBER GUIDE
PHYSICAL AND ELECTRICAL DATA
Model
Input
(Btuh)
Output
(Btuh)
AFUE
(ICS)
Nom.
Cooling
Capacity
Gas
Inlet
(in.)
Volts/
Hz/
Phase
Max. Time
Delay
Breakeror
Fuse
Nominal
F.L.A.
Trans.
(V.A.)
Approx.
Shipping
Weight (lbs.)
UPFLOW / HORIZONTAL
GUH80C045A3M 44,000 35,200 80.0% 1-1/2 — 3 1/2 120-60-1 15 6.8 40 111
GUH80C070A3M 66,000 52,800 80.0% 1-1/2 — 3 1/2 120-60-1 15 6.8 40 111
GUH80C070B3M 66,000 52,800 80.0% 1-1/2 — 3 1/2 120-60-1 15 6.8 40 127
GUH80C090B4M 88,000 70,400 80.0% 2-1/2 — 4 1/2 120-60-1 15 8.4 40 142
GUH80C090C5M 88,000 70,400 80.0% 3 — 5 1/2 120-60-1 15 10.9 40 152
GUH80C110C5M 110,000 88,000 80.0% 4 — 5 1/2 120-60-1 15 10.9 40 160
GUH80C135D5M 132,000 105,600 80.0% 4 — 5 1/2 120-60-1 15 10.9 40 178
Note: For vent sizing and clearances to combustibles, please reference installation instructions.
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
G UH 80 CG UH 80 C 004455BB3 M3 M
STAGES/MOTOR
C = SINGLE STAGE CONSTANT TORQUE

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Model
Motor
Size
(hp)
Blower
Size
Temp
Rise
(°F)
Blower
Speed
CFM @ External Static Pressure - " w.c.
0.20 0.30 0.40 0.50 0.60 0.70 0.80
UPFLOW / HORIZONTAL
GUH80C045A3M 1/2 10 x 8 15 - 45
High 1320 1290 1265 1230 1190 1165 1130
Med/High 1080 1055 1010 990 945 915 880
Med 900 875 825 790 750 705 670
Med/Low 805 750 710 660 630 570 535
Low 700 640 595 535 500 435 380
GUH80C070A3M 1/2 10 x 10 40 - 70
High 1400 1366 1335 1310 1285 1252 1217
Med/High 1171 1145 1106 1075 1042 1012 975
Med 1088 1055 1015 949 950 904 858
Med/Low 1050 1023 986 945 903 867 829
Low 877 844 795 761 704 663 623
GUH80C070B3M 1/2 10 x 10 40 - 70
High 1355 1330 1290 1245 1225 1190 1160
Med/High 1225 1190 1155 1115 1045 1000 925
Med 1110 1060 1015 980 920 855 790
Med/Low 1080 1035 970 930 865 790 735
Low 885 825 770 695 625 540 445
GUH80C090B4M 3/4 10 x 10 35 - 65
High 1730 1690 1645 1615 1590 1545 N/A
Med/High 1505 1460 1435 1395 1350 1300 1270
Med 1380 1345 1310 1265 1210 1175 1140
Med/Low 1310 1275 1240 1180 1150 1095 1040
Low 1110 1065 1010 955 915 860 820
GUH80C090C5M 1 11-1/2 x 10 30 - 60
High 2150 2125 2090 2060 2020 1980 1935
Med/High 1865 1835 1800 1760 1725 1680 1635
Med 1655 1635 1585 1545 1515 1465 1420
Med/Low 1490 1450 1405 1370 1320 1265 1225
Low 1340 1285 1235 1200 1145 1105 1055
GUH80C110C5M 1 11-1/2 x 10 35 - 65
High 2135 2090 2050 2025 2010 1965 1905
Med/High 1865 1830 1785 1740 1710 1670 1635
Med 1630 1595 1550 1500 1470 1420 1380
Med/Low 1470 1440 1385 1340 1305 1255 1215
Low 1280 1235 1175 1130 1080 1015 975
GUH80C135D5M 1 11 x 11 30 - 60
High 2390 2360 2285 2240 2225 2160 2105
Med/High 2175 2140 2125 2060 2015 1955 1925
Med 1935 1895 1850 1815 1785 1755 1715
Med/Low 1735 1720 1660 1610 1535 1500 1435
Low 1550 1485 1455 1415 1330 1265 1215
BLOWER PERFORMANCE DATA

508096-01 Issue 2037 Page 4of 31
ACCESSORY LIST
Catalog Number Description
External Filter Rack kits
1.841018 1 Pack (16x25)
1.841039 10 Pack (16x25)
Natural to LP Kits
11K50 1-Stage - 80
11K45 High Altitude 1-Stage ( <4500ft. )
Return Air Base
68W61 14.5", A Width
68W62 17.5”, B Width
68W63 21.0”, C Width
86W57 24.5", D Width
Downow Combustible Flooring Base
11M59 14.5", A Width
11M60 17.5”, B Width
11M61 21.0”, C Width
Night Service Kits
86W57 Single Stage
Horizontal Suspension Kits
51W10 80 & 90% Kit
2-Stage Compressor Relay Kit
85W66 Blower Relay Kit
Twinning kit
16W72 Constant Torque
Gas Furnace Twinning Kit
Note: For vent sizing and clearances to combustibles, please reference
installation instructions.
2-Stage Compressor Relay Kit 2-Stage Compressor Relay Kit

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Figure 1.
Control Box
(includes integrated control,
interlock switch and transformer)
Gas Valve
Burner Box
(includes sensor, ignitor
and rollout switches)
Combustion Air Inducer
Combustion Air Inducer
Pressure Switch
Heat Exchanger
Upper Access Panel
Blower Access Panel
Primary Limit
(under
combustion
air inducer)
Make-Up Box
Blower Assembly
Power Choke
(4 and 5 Ton Only)
Parts Arrangement

508096-01 Issue 2037 Page 6of 31
Unit Components
GUH80-C unit components are shown in Figure
1. The gas valve, combustion air inducer and
burners can be accessed by removing the upper
access panel. Electrical components are in the control
box (Figure 2) found in the blower section.
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
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.
DOOR INTERLOCK
SWITCH (S51)
INTEGRATED IGNITION
CONTROL
(A92)
TRANSFORMER
(T1)
CIRCUIT
BREAKER
(CB8)
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.
NOTE: The door interlock switch is a safety switch. Do not
by-pass or jumper switch.
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
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 6). 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
3Combustion Air Inducer Neutral
4Ignitor Neutral
Table 1. 4-Pin Terminal Designations
Pin # Function
1High Limit Output
2Not Used
3 24V Line
4Not Used
5Rollout Switch Out
624V Neutral
7High Limit Input
8Ground
9Gas 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
3Burner 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
9Pressure 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

508096-01 Issue 2037 Page 8of 31
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
3Burner 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
9Pressure 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

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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.
Figure 4.
Figure 5. 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

508096-01 Issue 2037 Page 10 of 31
Figure 6. Heating Components
Gaskets
Collector Box
Orifice Plate
Flue Transition
Pressure Switch
Combustion Air Inducer
Manifold And Gas Valve
Ignitor
Sensor
Rollout Switches
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.
Ignitor (Figure 6)
GUH80C 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.
NOTE: The GUH80C furnace contains electronic
components that are polarity sensitive. Make sure that
the furnace is wired correctly and is properly
grounded.
Flame Rollout Switches (Figure 6)
Flame rollout switch (S47) is a high temperature limit. Each
furnace is equipped with two identical switches. The limit
is a N.C. SPST manual-reset limit connected in series
with the integrated control A92. When S47 senses rollout,
the integrated control immediately stops ignition and
closes the gas valve. If unit is running and ame rollout is
detected, the gas valve will close and integrated control
will be disabled. Rollout can be caused by a blocked
heat exchanger, blocked ue or lack of combustion air.
The switch has a factory setpoint of 210°F and cannot
be adjusted. To manually reset a tripped switch, push the
reset button located on the control.
Flame Sensor (Figure 6)
A ame sensor is located on the left side of the burner
support. The sensor is mounted on the ame rollout plate
and the tip protrudes into the ame envelope of the left-
most burner. The sensor can be removed for service (clean
with steel wool) without removing any part of the burners.
During operation, ame is sensed by current passed
through the ame and sensing electrode. The integrated
control allows the gas valve to remain open as long as
ame signal is sensed.

508096-01Issue 2037
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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.
Primary Limit Control (Figure 7)
The primary limit on GUH80C units is located in the
heating vestibule panel under the combustion air
inducer. See Figure 7. When excess heat is sensed
in the heat exchanger, the limit will open. If the limit is
open, the integrated control energizes the supply air
blower and closes the gas valve. The limit
automatically resets when unit temperature returns to
normal. The switch must reset within three minutes or
SureLight® control will go into Watchguard for one
hour. The switch is factory set and cannot be adjusted.
The switch may have a different setpoint for each unit
model number.
Figure 7. Primary Limit Under Combustion Air Inducer
Primary
Limit
Gas Valve (Figure 6)
The GUH80C 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 valve switch are located on the
valve. All terminals on the gas valve are connected to wires
from the integrated control. 24V applied to the terminals
energizes the valve.
Inlet and outlet pressure taps are located on the valve. A
regulator adjustment screw is located on the valve.
LPG changeover kits are available from Comfort-Aire/Century.
Kits include burner orifices and a gas valve regulator spring.
To Measure Flame Signal - Integrated Control:
Use a digital readout meter capable of reading DC
microamps. See Figure 8 for flame 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.
Flame Signal in Microamps
Normal Low Drop Out
≥ 1.5 0.5 - 1.4 ≤ 0.4
Table 6.

508096-01 Issue 2037 Page 12of 31
Figure 8. Measuring Flame Signal
Flame
Sensor
Flame Sensor
Wire
Integrated
Control
Remove Sensor Wire from
Integrated Control and
Connect Alligator Clip (−)
to Flame Sensor Lead
Flame Sensor
Terminal
Remove Sensor Wire from
Integrated Control and
Connect Alligator Clip (+)
to Terminal on Control
(+)
DIGITAL METER
(+) To Control
Sensor
Terminal
(-) To
flame
sensor
Set dial to measure dc microamps
(+)
Red Collar
Indicates
Positive
Lead

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Figure 9. Check Ignitor
Test 2
Check ignitor for correct resistance.
Separate 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.
Meter
(set to ohms)
Meter
(set to ohms)
Integrated Control Board
Detail
Meter
(set to ohms)
Test 1
Check ignitor circuit for correct resistance.
Remove 4-pin plug from control.
Check ohms reading across terminals 2 and 4.
The 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.

508096-01 Issue 2037 Page 14of 31
Combustion Air Inducer (B6)
All GUH80C units use a combustion air inducer to move air
through the burners and heat exchanger during heating
operation. The blower uses a 120VAC motor. The
motor operates during all heating operation and is
controlled by integrated control A92. The inducer also
operates for 15 seconds before burner ignition (pre-purge)
and for 5 seconds after the gas valve closes (post-purge).
A pressure switch mounted on the combustion air inducer
orice plate is used to prove inducer operation. The
combustion air inducer orice will be di󰀨erent for each
model. The switch monitors air pressure in the inducer
housing. During normal operation, the pressure in the
housing is negative. If pressure becomes less negative
(signifying any obstruction in the ue) the pressure switch
opens. When the pressure switch opens, the integrated
control (A92) immediately de-energizes the gas valve to
prevent burner operation.
Combustion Air Inducer Pressure Switch (S18)
GUH80C series units are equipped with a combustion
air pressure switch located on the combustion air inducer
orifice bracket. The switch is connected to the combustion
air inducer housing by means of a flexible silicone
hose. It monitors negative air pressure in the
combustion air inducer housing.
The switch is a single-pole single-throw proving switch
electrically connected to the furnace control. The
purpose of the switch is to prevent burner operation if
the combustion air inducer is not operating or if the ue
becomes obstructed.
On start-up, the switch senses that the combustion air
inducer is operating. It closes a circuit to the integrated
control when pressure inside the combustion air inducer
decreases to a certain set point. Set points vary depending
on unit size. See Table 7. The pressure sensed by the
switch is negative relative to atmospheric pressure. If
the ue becomes obstructed during operation, the switch
senses a loss of negative pressure (pressure becomes
more equal with atmospheric pressure) and opens the
circuit to the integrated control and gas valve. A bleed port
on the switch allows relatively dry air in the vestibule to
purge switch tubing, to prevent condensate build up.
The switch is factory set and is not eld adjustable. It is a
safety shut-down control in the furnace and must not be
bypassed for any reason. If switch is closed or by-passed,
the integrated control will not initiate ignition at start up.
Unit Break ± 0.05
(in. w.c.)
045 -0.65
070 -0.68
090 -0.65
110-01,02,03 -0.68
110-04 -0.70
135 -0.65
Table 7.
Troubleshooting
See Figure 10 for measuring operating pressure and
checking resistance in the pressure switch.
Figure 10.
MULTI−METER
SET TO MEASURE OHMS
+-
or
Field Provided
Tubing To CAI Port
To Pressure Switch
Remove tubing from CAI and
insert “Tee” and additional tubing.
High
Low

508096-01Issue 2037
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Multiple Venting
The GUH80C furnace can vent in multiple positions.
See Figure 11.
The make up box may be removed and the combustion air
inducer may be rotated clockwise or counterclockwise 90°
to allow for vertical or horizontal vent discharge in a vertical
or horizontal cabinet position. Remove the four mounting
screws, rotate the assembly (assembly consists of orice
plate, proving switch, gasket and combustion air inducer),
then reinstall the mounting screws. See unit Installation
Instructions for more detail.
The combustion air pressure switch must be moved for
horizontal discharge air left position.
IMPORTANT
Figure 11.
vent pipe
vent pipe
flue
transition
vent pipe
FLOW
AIR
flue transition
vent pipe
pressure switch
vent pipe
In all positions route the wires away from moving parts and the heat of the inducer
motor to prevent damage to the wires.The pressure switch must be installed above
the CAI to prevent moisture from collecting in the hoses or switch.
FLOW
AIR
Collector Box
Vent Pipe
Pressure
Switch
vent pipe
Collector Box
FLOW
AIR
Pressure
Switch

508096-01 Issue 2037 Page 16of 31
Blower Compartment
Each blower is statically and dynamically balanced as
an assembly before installation in the unit.
IMPORTANT
GUH80C 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 14 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 12.
When replacing the indoor blower motor the set screw
must be aligned and tightened with the motor shaft as
shown in Figure 13.
Figure 12. Blower Wheel Replacement
Center Blower Wheel
in Blower Housing
Figure 13.
Set Screw
Housing Hub
ALIGN AND TIGHTEN SET SCREW WITH
FLAT SIDE OF MOTOR SHAFT
Motor
Shaft

508096-01Issue 2037
Page 17 of 31
Figure 14.
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,

508096-01 Issue 2037 Page 18 of 31
Testing the Motor (Figure 15)
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 15. 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 16). This will
directs moisture away from the motor and its electric
connections on the motor.
Figure 16. Drip Loop
CONNECTOR
ORIENTATION
BETWEEN 4 AND 8
O'CLOCK
BACK OF CONTROL
MODULE
DRIP LOOP

508096-01Issue 2037
Page 19 of 31
Placement & Installation
Make sure unit is installed in accordance with installation
instructions and applicable codes.
Start-Up
Heating Start-Up
Shock and burn hazard.
GUH80C units are equipped with a hot surface
ignition system. Do not attempt to light manually.
WARNING
Gas Valve Operation (Figure 17)
1. STOP! Read the safety information at the beginning
of this section.
2. Set the thermostat to the lowest setting.
3. Turn o󰀨 all electrical power to the unit.
4. This furnace is equipped with an ignition device which
automatically lights the burners. Do not try to light the
burners by hand.
5. Remove the access panel.
6. Move gas valve switch to OFF. See Figure 17.
7. Wait ve minutes to clear out any gas. If you then
smell gas, STOP! Immediately call your gas supplier
from a neighbor’s phone. Follow the gas supplier’s
instructions. If you do not smell gas go to next step.
8. Move gas valve switch to ON. See Figure 17.
Figure 17. Gas Valve
GAS VALVE SHOWN IN ON POSITION
MANIFOLD
PRESSURE
OUTLET
PORT
INLET
PRESSURE
PORT
MANIFOLD PRESSURE
ADJUSTMENT SCREW
(under barbed fitting)
9. Replace the access panel.
10. Turn on all electrical power to to the unit.
11. Set the thermostat to desired setting.
NOTE: When unit is initially started, steps 1 through
11 may need to be repeated to purge air from gas line.
12. If the appliance will not operate, follow the instructions
“Turning O󰀨 Gas to Unit” and call your service
technician or gas supplier.
Turning O󰀨 Gas to Unit
1. Set the thermostat to the lowest setting.
2. Turn o󰀨 all electrical power to the unit if service is to
be performed.
3. Remove the upper access panel.
4. Move gas valve switch to OFF position. Do not force.
See Figure 17.
5. Replace the upper access panel.
Safety or Emergency Shutdown
Disconnect main power to unit. Close manual and main
gas valves.
Extended Period Shutdown
Turn o󰀨 thermostat or set to “UNOCCUPIED” mode.
Close all gas valves (both internal and external to unit) to
guarantee no gas leaks into combustion chamber. Turn
o󰀨 power to unit. All access panels and covers must be in
place and secured.
Heating System Service Checks
C.S.A. Certication
All units are C.S.A. design certified without modifications.
Refer to the GUH80C Operation and Installation
Instruction.
Gas Piping
Gas supply piping should not allow more than 0.5”
W.C. drop in pressure between gas meter and unit.
Supply gas pipe must not be smaller than unit gas
connection.
Compounds used on gas piping threaded joints should
be resistant to action of liquefied petroleum gases.
Testing Gas Piping
If a exible gas connector is required or allowed by
the authority that has jurisdiction, black iron pipe shall
be installed at the gas valve and extend outside the
furnace cabinet.
CAUTION

508096-01 Issue 2037 Page 20of 31
In case emergency shutdown is required, turn o󰀨 the
main shut-o󰀨 valve and disconnect the main power to
unit. These controls should be properly labeled by the
installer.
IMPORTANT
Do not over torque (800 in-lbs) or under torque (350
in-lbs) when attaching the gas piping to the gas valve.
WARNING
When pressure testing gas lines, the gas valve must be
disconnected and isolated. Gas valves can be damaged if
subjected to more than 0.5psig (14” W.C.). See Figure 18.
If the pressure is equal to or less than 0.5psig (14”W.C.),
close the manual shut-o󰀨 valve before pressure testing to
isolate furnace from gas supply.
Figure 18. Gas Piping Test Procedure
MANUAL MAIN SHUT-OFF VALVE
WILL NOT HOLD TEST PRESSURE
IN EXCESS OF 0.5 PSIG (14”W.C.)
GAS VALVE
CAP
FIELD PROVIDED
LINE PRESSURE TAP
When checking piping connections for gas leaks, use
preferred means. Kitchen detergents can cause harmful
corrosion on various metals used in gas piping. Use of a
specialty Gas Leak Detector is strongly recommended.
Do not use matches, candles, ame or any other source
of ignition to check for gas leaks.
WARNING
Gas Pressure Adjustment
Gas Flow (Approximate)
Furnace should operate at least 5 minutes before checking
gas ow. Determine time in seconds for two revolutions of
gas through the meter. (Two revolutions assures a more
accurate time.) Divide by two and compare to time in
Table 8. If manifold pressure matches Table 10 and rate is
incorrect, check gas orices for proper size and restriction.
Remove temporary gas meter if installed.
NOTE: To obtain accurate reading, shut o all other gas
appliances connected to meter.
Capacity
Seconds for One Revolution
Natural LP
1 cu ft
Dial
2 cu ft
Dial
1 cu ft
Dial
2 cu ft
Dial
-45 80 160 200 400
-70 55 110 136 272
-90 41 82 102 204
-110 33 66 82 164
-135 27 54 68 136
Natural-1000 btu/cu ft LP-2500 btu/cu ft
Table 8. Gas Meter Clocking Chart
Supply and Manifold Pressure
Supply Pressure Measurement
1. Remove the threaded plug from the inlet side of the
gas valve and install a eld-provided barbed tting.
Connect to a test gauge to measure supply pressure.
2. Start unit and allow 5 minutes for unit to reach steady
state.
3. After allowing unit to stabilize for 5 minutes, record
supply pressure and compare to value given in Table
10.
Manifold Pressure Measurement
1. Remove the threaded plug from the outlet side of the
gas valve and install a eld-provided barbed tting.
Connect to a test gauge to measure manifold pressure.
2. Start unit and allow 5 minutes for unit to reach steady
state.
3. While waiting for the unit to stabilize, observe the
ame. Flame should be stable and should not lift from
burner. Natural gas should burn blue.
4. After allowing unit to stabilize for 5 minutes, record
manifold pressure and compare to value given in
Table 10.
NOTE: Shut unit o and remove manometer as soon as an
accurate reading has been obtained. Take care to remove
barbed tting and replace threaded plug.