Comfortaire Century GDD80C User manual

508142-01 Issue 2047 Page 1 of 33
(P) 508142-01
*P508142-01*
Table of Contents
Technical Specifications - GDD80C.............................2
Parts Arrangement.......................................................5
Unit Components.........................................................6
Placement & Installation............................................17
Start-Up .....................................................................17
Heating System Service Checks ...............................19
Typical Operating Characteristics..............................22
Maintenance..............................................................23
Wiring Diagram..........................................................26
Troubleshooting: Heating Sequence of Operation ....27
Troubleshooting: Heating Sequence of Operation
(Continued)................................................................28
Troubleshooting: Cooling Sequence of Operation.....29
Troubleshooting: Continuous Fan / Accessories
Sequence of Operation..............................................30
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
SERVICE MANUAL
GDD80C

508142-01 Issue 2047 Page 2of 30
Technical Specifications - GDD80C
MODEL NUMBER GUIDE
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
DD = DOWNFLOW
AFUE
80 = 80% EFFICENCY
92 = 92% EFFICENCY
95 = 95% EFFICENCY
STAGES/MOTOR
G DD 80 CG DD 80 C 004455BB3 M3 M
C = SINGLE STAGE CONSTANT TORQUE
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.)
DOWNFLOW
GDD80C045A3M 44,000 35,200 80.0% 2 — 3 1/2 120-60-1 15 6.8 40 113
GDD80C070B3M 66,000 52,800 80.0% 2 — 3 1/2 120-60-1 15 6.8 40 128
GDD80C090B4M 88,000 70,400 80.0% 3 — 4 1/2 120-60-1 15 8.4 40 140
GDD80C110C5M 110,000 88,000 80.0% 4 — 5 1/2 120-60-1 15 10.9 40 160

508142-01Issue 2047
Page 3 of 30
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
DOWNFLOW
GDD80C045A3M 1/2 10 x 8 15 - 45
High 1375 1335 1320 1275 1245 1215 1175
Med/High 1145 1120 1075 1045 1000 970 925
Med 950 915 870 835 785 745 695
Med/Low 840 790 745 715 655 605 555
Low 725 665 625 565 520 455 400
GDD80C070B3M 1/2 10 x 10 30 - 60
High 1400 1360 1310 1280 1250 1220 1165
Med/High 1230 1205 1155 1110 1060 1000 935
Med 1125 1085 1050 985 935 865 795
Med/Low 1110 1055 1000 950 865 815 750
Low 900 845 790 705 645 590 480
GDD80C090B4M 3/4 10 x 10 30 - 60
High 1695 1660 1615 1570 1545 N/A N/A
Med/High 1470 1425 1395 1360 1295 1265 1215
Med 1340 1290 1250 1210 1165 1120 1080
Med/Low 1280 1225 1185 1145 1085 1040 1000
Low 1105 1040 1005 945 865 810 755
GDD80C110C5M 1 11-1/2 x 10 30 - 60
High 2325 2280 2235 2165 2125 2090 2035
Med/High 2010 1940 1895 1845 1800 1755 1700
Med 1760 1705 1635 1595 1545 1510 1460
Med/Low 1570 1500 1440 1380 1320 1285 1255
Low 1375 1320 1265 1200 1170 1100 1030
BLOWER PERFORMANCE DATA

508142-01 Issue 2047 Page 4 of 30
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

508142-01Issue 2047
Page 5 of 30
Parts Arrangement
Figure 1.
BLOWER ASSEMBLY
CABINET
CONTROL BOX
INTERNAL FLUE
PIPE ASSEMBLY
FLUE CHASE
HEAT EXCHANGER
GAS VALVE
ACCESS PANEL
BURNER BOX ASSEMBLY
COMBUSTION AIR INDUCER
ROLLOUT LOCATED
INSIDE BURNER BOX

508142-01 Issue 2047 Page 6 of 30
Unit Components
GDD80C 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 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
7 High Limit Input
8Ground
9Gas Valve Common
10 Pressure Switch In
11 Rollout Switch In
12 Gas Valve Out
Table 2. 12-Pin Terminal Designations

508142-01Issue 2047
Page 7 of 30
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
7 Primary 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

508142-01 Issue 2047
Page 8 of 30
Red LED Flash Code2Diagnostic Codes / Status of Furnace
Off No power to control or board fault detected
Heartbeat1Normal Operation - Idle, Continuous Fan, Cool
Continuous Rapid Flash Call for Heat / Burner Operation
1 Reverse Line Voltage Polarity
2 Improper Earth Ground
3 Burner failed to light, or lost flame during heat demand
4 Low Flame Signal - check flame sensor
5 Watchguard - burner failed to light, exceeded maximum number of retries or recycles
6 Not Used
7 Primary or Secondary Limit Open or Watchguard Mode - Limit Switch Open longer than 3 minutes
8 Rollout 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 off, repeating
2 Error codes are indicated by a ”Rapid Flash” - the LED flashes X times at 1/2 sec on 1/2 sec off, remains off for 3 sec, then repeats
NOTE: Last 10 error codes are stored in memory including when power is shut off to the unit. - To recall, press and release button,
most recent will be displayed first, LED off 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

508142-01Issue 2047
Page 9 of 30
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 differential 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 first 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 five 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 Off Time
Fan off time (time that the blower operates after the heat
demand has been satisfied) can be adjusted by moving
the jumper to a different setting. The unit is shipped with
a factory fan off setting of 90 seconds. For customized
comfort, monitor the supply air temperature once the heat
demand is satisfied. Note the supply air temperature at the
instant the blower is de-energized.
Adjust the fan-off 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

508142-01 Issue 2047 Page 10 of 30
Figure 6. Heating Components
Gaskets
Collector Box
Orifice Plate
Pressure Switch
Combustion Air Inducer
Manifold And Gas Valve
Ignitor
Sensor
Rollout Switches
Internal Flue Pipe
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)
GDD80C 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 8 for resistance and voltage
checks.
NOTE: The GDD80C 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.

508142-01Issue 2047
Page11 of 30
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
The primary limit on GDD80C units is located in the
heating vestibule panel under the combustion air inducer.
See Figure 1. 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.
Gas Valve (Figure 6)
The GDD80C 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 7 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.

508142-01 Issue 2047 Page 12 of 33
Figure 7. 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

508142-01Issue 2047
Page 13 of 30
Figure 8. Check Ignitor
Test 1
Check ignitor circuit for correct resistance.
Remove 4-pin plug from control.
Check ohms reading across terminals 2 and 4.
If value is correct (39 to 70 ohms), 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.
Integrated Control B
Detail
Test 1
Integrated Control Board
Detail
Integrated Control Board
Detail

508142-01 Issue 2047 Page 14 of 30
Combustion Air Inducer (B6)
All GDD80 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
orifice plate is used to prove inducer operation. The
combustion air inducer orifice will be different 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 flue) 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)
GDD80C 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 flue
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 -0.60
070 -0.65
090 -0.60
110 -0.65
Table 7.
Troubleshooting
See Figure 9 for measuring operating pressure and
checking resistance in the pressure switch.
Figure 9.
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

508142-01Issue 2047
Page 15 of 30
Blower Compartment
Each blower is statically and dynamically balanced as
an assembly before installation in the unit.
IMPORTANT
GDD80C 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 13 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 10.
When replacing the indoor blower motor the set screw
must be aligned and tightened with the motor shaft as
shown in Figure 11.
Secondary Limit Controls
The secondary limit is located in the blower compartment
on the back side of the blower housing. See Figure 12.
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 10. Blower Wheel Replacement
Center Blower Wheel
in Blower Housing
Figure 11.
Set Screw
Housing Hub
ALIGN AND TIGHTEN SET SCREW WITH
FLAT SIDE OF MOTOR SHAFT
Motor
Shaft
Figure 12. Secondary Limit Control
Blower Assembly
Secondary Limit

508142-01 Issue 2047 Page 16 of 30
Figure 13.
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,

508142-01Issue 2047
Page 17 of 30
Testing the Motor (Figure 14)
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 14. 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 15). This will
directs moisture away from the motor and its electric
connections on the motor.
Figure 15. Drip Loop
CONNECTOR
ORIENTATION
BETWEEN 4 AND 8
O'CLOCK
BACK OF CONTROL
MODULE
DRIP LOOP
Placement & Installation
Make sure unit is installed in accordance with installation
instructions and applicable codes.
Start-Up
Heating Start-Up
Shock and burn hazard.
GDD80C units are equipped with a hot surface
ignition system. Do not attempt to light manually.
WARNING

508142-01 Issue 2047 Page 18 of 30
Gas Valve Operation (Figure 16)
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 16.
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 16.
Figure 16. 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 16.
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 ication
All units are C.S.A. design certified without modifications.
Refer to the GDD80C 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.
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
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 17.
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.
Testing Gas Piping

508142-01Issue 2047
Page 19 of 30
Figure 17. 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
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.
Proper Combustion
Furnace should operate a minimum 15 minutes with correct
manifold pressure and gas ow rate before checking
combustion. Take combustion sample beyond the ue
out let and compare to Table 9. The maximum carbon
monoxide reading should not exceed 100 ppm.
Capacity CO²% For Nat CO²% For LP
-045
7.2 - 7.8 7.5 - 9.0
-070
-090
-110
Table 9.
High Altitude
The manifold pressure may require adjustment and
combustion air pressure switch may need replacing to
ensure proper combustion at higher altitudes. Refer
to Table 10 for manifold pressure and pressure switch
change.
For safety, shut unit o󰀨 and remove manometer as soon
as an accurate reading has been obtained. Take care to
replace pressure tap plug.
IMPORTANT

508142-01 Issue 2047 Page 20 of 30
Input
Size Gas
Line
Pressure
(in. w.c.)
0 - 2,000 ft. 2,000 - 4,500 ft. 4,501 - 7,500 ft. 7,501 - 10,000 ft.
Min. Max. Orice
Size (in)
Manifold
Pressure
(in w.c.)
Pressure
Switch
Orice
Size (in)
Manifold
Pressure
(in w.c.)
Pressure
Switch
Orice
Size (in)
Manifold
Pressure
(in w.c.)
Pressure
Switch
Orice
Size (in)
Manifold
Pressure
(in w.c.)
Pressure
Switch
045
Nat 4.5 10.0
No
Change
0.0625”
3.5
No Change
No
Change
0.0625”
3.5
No
Change
No
Change
0.0625”
3.5
11K45
0.032”
51W01
0.0550” 3.5
74W89
-0.55” w.c.
LP/Propane 11.0 13.0 11K50
0.034” 10.0 11K50
0.034” 10.0 11K50
0.034” 10.0 11K45
0.032” 10.0
070
Nat 4.5 10.0
No
Change
0.0625’
3.5
No
Change
0.0625”
3.5
No
Change
0.0625”
3.3 51W01
0.0550” 3.5
LP/Propane 11.0 13.0 11K50
0.034” 10.0 11K50
0.034” 10.0 11K50
0.034” 10.0 11K45
0.032” 10.0
090
Nat 4.5 10.0
No
Change
0.0625’
3.5
No
Change
0.0625”
3.5
No
Change
0.0625”
3.3 51W01
0.0550” 3.5
LP/Propane 11.0 13.0 11K50
0.034” 10.0 11K50
0.034” 10.0 11K50
0.034” 10.0 11K45
0.032” 10.0
110
Nat 4.5 10.0
No
Change
0.0625’
3.5
No
Change
0.0625”
3.5
No
Change
0.0625”
3.3 74W60
-0.60” w.c.
51W01
0.0550” 3.5
LP/Propane 11.0 13.0 11K50
0.034” 10.0 11K50
0.034” 10.0 11K50
0.034” 10.0 11K45
0.032”.
11K45
0.032” 10.0
NOTE: A natural to L.P. propane gas changeover kit is necessary to convert this unit. Refer to the changeover kit installation instruction for the conversion procedure. NOTE: For Altitudes of 7501’ - 10,000’ call Comfort-Aire/Century Technical Support.
Table 10. Manifold Pressure Settings and Pressure Switch