International comfort products F9MXT0401410A Installation instructions

Printed in U.S.A. 440 04 4321 08 11/25/13
SERVICE AND TECHNICAL
SUPPORT MANUAL
Two−Stage, ECM Blower Motor
35” Tall, High Efficiency Condensing Gas Furnace
(F/G)9MXT
Save this manual for future reference.
Safety Labeling and Signal Words
DANGER, WARNING, CAUTION, and NOTE
The signal words DANGER, WARNING,
CAUTION,and NOTE are used to identify levels of
hazard seriousness. The signal word DANGER is
only used on product labels to signify an immediate
hazard. The signal words WARNING, CAUTION,
and NOTE will be used on product labels and
throughout this manual and other manual that may
apply to the product.
DANGER −Immediate hazards which will result in
severe personal injury or death.
WARNING −Hazards or unsafe practices which
could result in severe personal injury or death.
CAUTION −Hazards or unsafe practices which
may result in minor personal injury or product or
property damage.
NOTE −Used to highlight suggestions which will
result in enhanced installation, reliability, or
operation.
!WARNING
Signal Words in Manuals
The signal word CAUTION is used throughout
this manual in the following manner:
!CAUTION
Signal Words on Product Labeling
Signal words are used in combination with
colors and/or pictures or product labels.
The signal word WARNING is used throughout
this manual in the following manner:
Safety−alert symbol
When you see this symbol on the unit and in
instructions or manuals, be alert to the
potential for personal injury.
TABLE OF CONTENTS
START−UP, ADJUSTMENT, AND SAFETY CHECK 4...........
THERMOSTAT SETUP SWITCH 4...........................
PRIME CONDENSATE TRAP WITH WATER 4.................
PURGE GAS LINES 4......................................
ADJUSTMENTS 5.........................................
ADJUST TEMPERATURE RISE 10............................
ADJUST BLOWER OFF DELAY (HEAT MODE) 11..............
ADJUST COOLING AIRFLOW 11.............................
ADJUST CONTINUOUS FAN AIRFLOW 11....................
ADJUST THERMOSTAT HEAT ANTICIPATOR 11...............
CHECK SAFETY CONTROLS 12.............................
CHECKLIST 12.............................................
SERVICE AND MAINTENANCE PROCEDURES 13.............
ELECTRICAL CONTROLS AND WIRING 13...................
TROUBLESHOOTING 13....................................
CLEANING AND/OR REPLACING AIR FILTER 15..............
BLOWER MOTOR AND WHEEL MAINTENANCE 15............
CLEANING BURNERS AND FLAME SENSOR 17...............
SERVICING HOT SURFACE IGNITER 18......................
FLUSHING COLLECTOR BOX AND DRAINAGE SYSTEM 18....
CLEANING CONDENSATE DRAIN AND TRAP 19..............
CLEANING HEAT EXCHANGERS 19..........................
SERVICE LABEL 21.........................................
WIRING DIAGRAM 22.......................................
SEQUENCE OF OPERATION 25.............................
PARTS REPLACEMENT INFORMATION GUIDE 29.............
PRODUCT NOMENCLATURE 30.............................
MODELS
(F/G)9MXT0401410A
(F/G)9MXT0401712A
(F/G)9MXT0601412A
(F/G)9MXT0601714A
(F/G)9MXT0801716A
(F/G)9MXT0802120A
(F/G)9MXT1002120A
(F/G)9MXT1202422A
Use of the AHRI Certified TM Mark indicates a
manufacturer’s participation in the program.
For verification of certification for individual
products, go to www.ahridirectory.org .

440 04 4321 08
Specifications are subject to change without notice.
2
SAFETY CONSIDERATIONS
Improper installation, adjustment, alteration, service,
maintenance, or use can cause explosion, fire, electrical shock,
or other conditions which may cause death, personal injury, or
property damage. Consult a qualified installer, service agency,
or your distributor or branch for information or assistance. The
qualified installer or agency must use factory−authorized kits or
accessories when modifying this product. Refer to the individual
instructions packaged with the kits or accessories when
installing.
Follow all safety codes. Wear safety glasses, protective clothing,
and work gloves. Use quenching cloth for brazing operations.
Have fire extinguisher available. Read these instructions
thoroughly and follow all warnings or cautions included in
literature and attached to the unit. Consult local building codes,
the current editions of the National Fuel Gas Code (NFCG)
NFPA 54/ANSI Z223.1, and the National Electrical Code (NEC)
NFPA 70.
In Canada refer to the current editions of the National standards
of Canada CAN/CSA−B149.1 and .2 Natural Gas and Propane
Installation Codes, and Canadian Electrical Code CSA C22.1.
Recognize safety information. This is the safety−alert symbol
. When you see this symbol on the unit and in instructions or
manuals, be alert to the potential for personal injury.
Understand these signal words; DANGER, WARNING, and
CAUTION. These words are used with the safety−alert symbol.
DANGER identifies the most serious hazards which will result in
severe personal injury or death. WARNING signifies hazards
which could result in personal injury or death. CAUTION is used
to identify unsafe practices which may result in minor personal
injury or product and property damage. NOTE is used to
highlight suggestions which will result in enhanced installation,
reliability, or operation.
!WARNING
PERSONAL INJURY, AND/OR PROPERTY
DAMAGE HAZARD
Failure to carefully read and follow this warning could
result in equipment malfunction, property damage,
personal injury and/or death.
Installation or repairs made by unqualified persons could
result in equipment malfunction, property damage,
personal injury and/or death.
The information contained in this manual is intended for
use by a qualified service technician familiar with safety
procedures and equipped with proper tools and test
instruments.
Installation must conform with local building codes and
with the Natural Fuel Gas Code (NFCG) NFPA 54/ANSI
Z223.1, and National standards of Canada
CAN/CSA−B149.1 and .2 Natural Gas and Propane
Installation Codes.
!WARNING
ELECTRICAL SHOCK HAZARD
Failure to follow this warning could cause personal
injury or death.
Before performing service or maintenance operations
on unit, always turn off main power switch to unit and
install lockout tag. Unit may have more than one power
switch.
!WARNING
CARBON MONOXIDE POISONING AND FIRE
HAZARD
Failure to follow safety warnings could result in personal
injury, death, and/or property damage.
This furnace is not designed for use in mobile homes,
trailers or recreational vehicles.
!CAUTION
CUT HAZARD
Failure to follow this caution may result in damage
personal injury.
Sheet metal parts may have sharp edges or burrs. Use
care and wear appropriate protective clothing, safety
glasses and gloves when handling parts and servicing
furnaces.

SERVICE AND TECHNICAL MANUAL Gas Furnace: (F/G)9MXT
440 04 4321 08 3
Specifications subject to change without notice.
START−UP CHECK SHEET
For PSC Models (F/G)9MXT
(This sheet is optional. Keep for future reference.)
Date of Start−Up:
Dealer Name:
Address:
City, State(Province), Zip or Postal Code:
Phone:
Owner Name:
Address:
City, State(Province), Zip or Postal Code:
Model Number:
Serial Number:
Setup Checks
Check the box when task is complete.
All Electrical Connections Tight?
Have hoses been relocated for furnace U/D/H application?
Condensate Drain Connected?
Condensate Drain Trapped?
Manual Gas Shut−off Upstream of Furnace/Drip Leg
Gas Valve turned ON?
Type of Gas: Natural: Propane: 
Filter Type and Size:
Shade in Heating Fan “Time OFF” Setting and Thermostat
Type setting:
ON
OFF
1 2 3
TT
OFF
DLY
Calculated Input (BTU) Rate: (See Checks and Adjustments
Section).
Heating Check
Measured Line Pressure During High Heat:
Measured Manifold Pressure: High Heat
Low Heat
Temperature of Supply Air: High Heat
Low Heat
Temperature of Return Air:
Temperature Rise (Supply −Return): High Heat
Low Heat
In Rise Range (see furnace rating plate)?
Static Pressure (Ducts) High Heat: Supply
Return
The Blower Speed Tap used for: High Heat
Low Heat
Optional Check: CO?
CO2?
Cooling Check
Temperature of Supply Air:
Temperature of Return Air:
Temperature Difference:
Static Pressure (Ducts) Cooling: Supply
Return
The Blower Speed Tap used for: Cooling
Dealer Comments:

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
4440 04 4321 08
START−UP, ADJUSTMENT, AND SAFETY
CHECK
NOTICE
IMPORTANT INSTALLATION AND START−UP
PROCEDURES
Failure to follow this procedure may result in a nuisance smoke
or odor complaint.
The manifold pressure, gas rate by meter clocking, temperature
rise and operation must be checked after installation. Minor
smoke and odor may be present temporarily after start−up from
the manufacturing process. Some occupants are more
sensitive to this minor smoke and odor. It is recommended that
doors and windows be open during the first heat cycle.
General
1. Furnace must have a 115-v power supply properly
connected and grounded.
NOTE: Proper polarity must be maintained for 115-v wiring.
Control status indicator light flashes code 10 and furnace does
not operate if polarity is incorrect or if the furnace is not
grounded.
2. Thermostat wire connections at terminals R, W/W1, G,
and Y/Y2 must be made at 24-v terminal block on
furnace control.
3. Natural gas service pressure must not exceed 0.5 psig
(14-in. w.c., 350 Pa), but must be no less than 0.16 psig
(4.5-in. w.c., 1125 Pa).
4. Blower door must be in place to complete 115-v electrical
circuit and supply power to the furnace components.
UNIT OPERATION HAZARD
Failure to follow this caution may result in intermittent unit
operation or performance dissatisfaction.
These furnaces are equipped with a manual reset limit
switch in burner assembly. This switch opens and shuts off
power to the gas valve if an overheat condition (flame
rollout) occurs in the burner assembly/enclosure. Correct
inadequate combustion-air supply, improper gas pressure
setting, improper burner or gas orifice positioning, or
improper venting condition before resetting switch. DO
NOT jumper this switch.
CAUTION
!
Before operating furnace, check flame rollout manual reset
switch for continuity. If necessary, press button to reset switch.
EAC-1 terminal is energized whenever blower operates. HUM
terminal is only energized when the the gas valve is energized
in heating.
Thermostat Setup Switch
This furnace can be installed with either a single−stage heating
or a two−stage heating thermostat. Setup switch SW−1 (TT) is
used to configure the furnace for single or two stage thermostat
operation. (See Figure 4)
For single−stage thermostats, connect thermostat W to W/W1
at furnace control terminal block. (See Figure 7) For
single−stage thermostats, the control will operate for 12
minutes on low heat, then switch to high heat if heat call
remains. Setup switch SW−1 (TT) must be in the
factory−shipped OFF position. See Figure 7 and Figure 15 for
setup switch information.
If a two−stage heating thermostat is to be used, move setup
switch SW−1 (TT) to ON position before starting furnace. This
overrides built−in control process for selecting high and low
heat and allows the two−stage thermostat to select gas heating
modes. The W2 from thermostat must be connected to W2 on
control terminal block.
Prime Condensate Trap with Water
!WARNING
FIRE OR EXPLOSION HAZARD
Failure to follow these warnings could result in personal injury
or death.
Failure to use a properly configured trap or NOT
water-priming trap before operating furnace may allow
positive pressure vent gases to enter the structure through
drain tube. Vent gases contain carbon monoxide which is
tasteless and odorless.
1. Remove upper and middle collector box drain plugs
opposite of the condensate trap. (See Figure 1)
2. Connect field-supplied 5/8-in. (16 mm) ID tube with
attached funnel (see Figure 1) to upper collector box
drain connection.
3. Pour one quart (liter) of water into funnel/tube. Water
should run through collector box, overfill condensate
trap, and flow into open field drain.
4. Remove funnel; replace collector box drain plug.
5. Connect field-supplied 5/8-in. (16 mm) ID tube to middle
collector box drain port.
6. Pour one quart (liter) of water into funnel/tube. Water
should run through collector box, overfill condensate
trap, and flow into open field drain.
7. Remove funnel and tube from collector box and replace
collector box drain plug.
Figure 1 Priming Condensate Drain
L11F065
Representative drawing only, some models may vary in appearance.

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
440 04 4321 08 5
Purge Gas Lines
If not previously done, purge the lines after all connections
have been made and check for leaks.
!WARNING
FIRE OR EXPLOSION HAZARD
Failure to follow this warning could result in personal injury,
death, and/or property damage.
Never purge a gas line into a combustion chamber. Never
test for gas leaks with an open flame. Use a commercially
available soap solution made specifically for the detection of
leaks to check all connections. A fire or explosion may result
causing property damage, personal injury or loss of life.
Adjustments
FIRE HAZARD
Failure to follow this warning could result in personal
injury, death and/or property damage.
DO NOT bottom out gas valve regulator adjusting screw.
This can result in unregulated manifold pressure and
result in excess overfire and heat exchanger failures.
!WARNING
FURNACE DAMAGE HAZARD
Failure to follow this caution may result in reduced furnace
life.
DO NOT redrill orifices. Improper drilling (burrs,
out−of−round holes, etc.) can cause excessive burner
noise and misdirection of burner flames. This can result in
flame impingement of heat exchangers, causing failures.
(See Figure 2)
CAUTION
!
Figure 2 Orifice Hole
A93059
BURNER
ORIFICE
For proper operation and long term reliability the furnace input
rate must be within +/−2 percent of input rate on furnace rating
plate, or as adjusted for altitude.
The gas input rate on rating plate is for installation at altitudes
up to 2000 ft. (610 M).
NOTICE
The NATURAL GAS manifold pressure adjustments in Table 3
compensate for BOTH altitude AND gas heating value. DO
NOT apply an additional de−rate factor to the pressures shown
in Table 3.
The heating content of natural gas at altitude may already
provide for a reduction in capacity or altitude. Refer to Table 3.
No adjustments to the furnace may be necessary at altitude for
certain gas heating values.
Refer to the instructions provided in the factory-specified
LP/Propane conversion kit for instructions for setting gas
manifold pressures for LP/Propane applications.
In the USA, the input rating for altitudes above 2000 ft. (610 M)
must be reduced by 2 percent for each 1000 ft. (305 M) above
sea level refer to Table 1. The natural gas manifold pressures
in Table 3 adjust for BOTH altitude and natural gas heating
value.
In Canada, the input rating must be reduced by 5 percent for
altitudes of 2000 ft. to 4500 ft. (610 to 1372 M) above sea level.
The natural gas manifold pressures in Table 3 adjust for BOTH
altitude and natural gas heating value.
Table 1 Altitude Derate Multiplier for USA
ALTITUDE
FT. (M)
PERCENT
OF DERATE
DERATE MULTIPLIER
FACTOR*
0–2000
(0−610) 0 1.00
2001–3000
(610−914) 4−6 0.95
3001–4000
(914−1219) 6−8 0.93
4001–5000
(1219−1524) 8−10 0.91
5001–6000
(1524−1829) 10−12 0.89
6001–7000
(1829−2134) 12−14 0.87
7001–8000
(2134−2438) 14−16 0.85
8001–9000
(2438−2743) 16−18 0.83
9001–10,000
(2743−3048) 18−20 0.81
* Derate multiplier factors are based on midpoint altitude for altitude range.
NOTE: For Canadian altitudes of 2000 to 4500 ft. (610 to
1372 M), use USA altitudes of 2001 to 3000 ft. (610 to 914
M) in Table 3.
To adjust manifold pressure to obtain the proper input rate, first,
determine if the furnace has the correct orifice installed. At
higher altitudes or different gas heat contents, it may be
necessary to change the factory orifice to a different orifice.
Tables have been provided in the furnace Service and
Technical Manual to match the required orifice to the manifold
pressure to the heat content and specific gravity of the gas. to
do this:
1. Obtain average yearly gas heat value (at installed
altitude) from local gas supplier.
2. Obtain average yearly gas specific gravity from local gas
supplier.
3. Find installation altitude in Table 3.
4. Find closest natural gas heat value and specific gravity in
Table 3. Follow heat value and specific gravity lines to
point of intersection to find orifice size and low-and
high-heat manifold pressure settings for proper
operation.

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
6440 04 4321 08
5. Check and verify burner orifice size in furnace. NEVER
ASSUME ORIFICE SIZE. ALWAYS CHECK AND
VERIFY.
NOTICE
If orifice hole appears damaged or it is suspected to have been
redrilled, check orifice hole with a numbered drill bit of correct
size. Never redrill an orifice. A burr−free and squarely aligned
orifice hole is essential for proper flame characteristics.
6. Replace orifice with correct size, if required by Table 3.
Use only factory−supplied orifices. See EXAMPLE 1.
EXAMPLE 1:
0 - 2000 ft. (0 - 609.6M) altitude
Heating value = 1050 Btu/cu ft.
Specific gravity = 0.62
Therefore: Orifice No. 44
(Furnace is shipped with No. 44 orifices. In this example, all
main burner orifices are the correct size and do not need to be
changed to obtain proper input rate.)
Manifold pressure: 3.4-in. w.c. (847 Pa) for high heat, 1.4-in.
w.c. (349 Pa) for low heat
NOTE: To convert gas manifold Table pressures to Pascals,
multiply the in.w.c. value by 249.1 Pa/in. w.c. (1 in. wc. = 249.1
Pa).
Check Inlet Gas Pressure
The inlet gas pressure must be checked with the furnace
operating in maximum heat. This is necessary to make sure the
inlet gas pressure does not fall below the minimum pressure of
4.5 in. w.c.
1. Make sure the gas supply is turned off to the furnace and
at the electric switch on the gas valve.
2. Remove the 1/8 in. NPT plug from the inlet pressure tap
on the gas valve.
3. Connect a manometer to the inlet pressure tap on gas
valve.
4. Turn on furnace power supply.
5. Turn gas supply manual shutoff valve to ON position.
6. Turn furnace gas valve switch to ON position.
7. Jumper the R to W/W1 and W2 thermostat connections
at the furnace control board.
8. When main burners ignite, confirm inlet gas pressure is
between 4.5 in. w.c. (1125 Pa) and 13.6 in. w.c. (3388
Pa).
9. Remove jumper across thermostat connections to
terminate call for heat. Wait until the blower off delay is
completed.
10. Turn furnace gas valve electric switch to OFF position.
11. Turn gas supply manual shutoff valve to OFF position.
12. Turn off furnace power supply.
13. Remove manometer from the inlet pressure tap of the
gas valve.
FIRE HAZARD
Failure to follow this warning could result in personal injury,
death, and/or property damage.
Re−install manifold pressure tap plug in gas valve to
prevent gas leak.
!WARNING
14. Apply pipe dope sparingly to end of inlet gas pipe plug
and re−install in the gas valve.
Adjust Manifold Pressure
1. Adjust manifold pressure to obtain low fire input rate.
(See Figure 3)
a. Turn gas valve ON/OFF switch to OFF.
b. Remove manifold pressure tap plug from gas valve.
c. Connect a water column manometer or similar device
to manifold pressure tap.
d. Turn gas valve ON/OFF switch to ON.
e. Move setup switch SW−1 (TT) on furnace control to
ON position to lock furnace in low−heat operation.
(See Figure 4 and Figure 15)
f. Manually close blower door switch.
g. Jumper R and W/W1 thermostat connections on
control to start furnace. (See Figure 4)
h. Remove regulator adjustment cap from low heat gas
valve pressure regulator (See Figure 3) and turn
low−heat adjusting screw (3/16 or smaller flat−tipped
screwdriver) counterclockwise (out) to decrease input
rate or clockwise (in) to increase input rate.
NOTICE
DO NOT set low−heat manifold pressure less than 1.3−in. w.c.
(324 Pa) or more than 1.7−in. w.c. (423 Pa) for natural gas. If
required manifold pressure is outside this range, change main
burner orifices.
Figure 3 Redundant Automatic Gas Control
Valve (2−Stage)
ON/OFF Switch
Regulator Seal Cap
RegulatorAdjustment
Regulator Seal Cap under Cap
1/2” NPT Outlet
1/8” NPT Manifold
Pressure Tap
1/8” NPT Inlet
Pressure Tap
1/2” NPT Inlet
A11152
i. Install low−heat regulator adjustment cap.
j. Move setup switch SW−1 (TT) to off position after
completing low−heat adjustment.
k. Leave manometer or similar device connected and
proceed to Step 2.
2. Adjust manifold pressure to obtain high fire input rate.
(See Figure 3)
a. Jumper R to W/W1 and W2 thermostat connections
on furnace control. This keeps furnace locked in
high−heat operation.
b. Remove regulator adjustment cap from high−heat
gas valve pressure regulator (See Figure 3) and turn
high heat adjusting screw (3/16−in. or smaller
flat−tipped screwdriver) counterclockwise (out) to
decrease input rate or clockwise (in) to increase input
rate.

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
440 04 4321 08 7
NOTICE
DO NOT set high−heat manifold pressure less than 3.2−in. w.c.
(797 Pa) or more than 3.8 in. w.c. (947 Pa) for natural gas. If
required manifold pressure is outside this range, change main
burner orifices to obtain manifold pressure in this range.
c. When correct input is obtained, replace caps that
conceal gas valve regulator adjustment screws. Main
burner flame should be clear blue, almost
transparent. (See Figure 13)
d. Reinstall manifold pressure tap plug to gas valve.
e. Remove jumpers R to W/W1 and R to W2.
3. Verify natural gas input rate by clocking meter.
NOTE: Contact your HVAC distributor or gas supplier for metric
gas meter Tables, if required.
a. Turn off all other gas appliances and pilots served by
the meter.
b. Move setup switch SW−1 (TT) to ON position. This
keeps furnace locked in low−heat operation when
only W/W1 is energized.
c. Jumper R to W/W1.
d. Run furnace for 3 minutes in low−heat operation.
e. Measure time (in sec) for gas meter to complete one
revolution and note reading. The 2 or 5 cubic feet dial
provides a more accurate measurement of gas flow.
f. Refer to Table 2 for cubic ft. of gas per hr.
g. Multiply gas rate cu ft./hr by heating value (Btuh/cu
ft.) to obtain input rate.
h. If clocked rate does not match required input from
Step 1, increase manifold pressure to increase input
or decrease manifold pressure to decrease input.
Repeat steps b through e of Step 1 until correct
low−heat input is achieved. Re−install low heat
regulator seal cap on gas valve.
i. Jumper R to W/W1, and W2. This keeps furnace
locked in high--heat operation when both W/W1 and
W2 are energized.
j. Repeat items (d) through (h) for high-heat operation,
repeating Step 2 and adjusting the high-heat
regulator screw, as required.
4. Restore furnace to normal operating condition.
a. Turn gas valve ON/OFF switch to OFF.
b. Remove water column manometer or similar device
from manifold pressure tap.
c. Replace manifold pressure tap plug to gas valve.
d. Turn gas valve ON/OFF switch to ON.
e. Move setup SW-1 switch (TT) on furnace control to
position required for attached thermostat (OFF for
single-stage thermostats, ON for two-stage
thermostats).
f. Check for gas leaks and verify furnace operation.
FIRE HAZARD
Failure to follow this warning could result in personal injury,
death, and/or property damage.
Re−install manifold pressure tap plug in gas valve to
prevent gas leak.
!WARNING
Figure 4 Example of Two−Stage Furnace Control Board
TEST / TWIN
HUM
PLT
PL1
W2 Y1 DHUM G COM W/W1 Y/Y2 R
24V
FUSE 3−AMP
EAC−2
EAC−1
L1 BL−1 XFMR
L2
COM
HI HT
COOL
LO H T
SPARE 1
24V MTR TAPS
BLOWER SPEED
TERMINALS
115−VAC (L2)
NEUTRAL
CONNECTIONS
LED OPERATION
& DIAGNOSTIC LIGHT
3−AMP FUSE
24−V THERMOS TAT
TERMINALS
SET UP SWITCHES
THERMOSTAT TYPE (TT)
AND HEAT OFF−DELAY
TWINNING AND/OR
COMPONENT TEST
TERMINAL
HUMIDIFIER TERMINAL
(24 VAC 0.5 AMPS MAX)
TRANSFORMER
24 VAC CONNECTIONS
P−1 LOW VOLTAGE
P2 −HOT SURFACE
IGNITER/INDUCE R
MOTOR CONNECTION
11 5 VA C
BLOWER POWER (BL1)
CONNECTION
115 VAC LINE (L1)
INPUT
TT
OFF
DLY
ON OFF
1 2 3
IDR HSIR
IDM
IHI/LOR
PL2
1
HSI HI LO
1
EAC TERMINAL
115 VAC 1.0 AMP
MAX
11 5 VA C
TRANSFORMER
PRIMARY
SPARE 2
ÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎ
ÎÎÎÎÎÎÎÎÎÎÎ
HUM
115 VAC HUM
COM/BLUE 24VAC/RED
COM 24V
24VAC
SW1
SW1
L12F037
ON
OFF

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
8440 04 4321 08
Table 2 Gas Rate (CU ft./hr)
SECONDS G
FOR 1 REVOLUTION
SIZE OF TEST DIAL SECONDS
FOR 1 REVOLUTION
SIZE OF TEST DIAL
1 Cu Ft. 2 Cu Ft. 5 Cu Ft. 1 Cu Ft. 2 Cu Ft. 5 Cu Ft.
10 360 720 1800 50 72 144 360
11 327 655 1636 51 71 141 355
12 300 600 1500 52 69 138 346
13 277 555 1385 53 68 136 340
14 257 514 1286 54 67 133 333
15 240 480 1200 55 65 131 327
16 225 450 1125 56 64 129 321
17 212 424 1059 57 63 126 316
18 200 400 1000 58 62 124 310
19 189 379 947 59 61 122 305
20 180 360 900 60 60 120 300
21 171 343 857 62 58 116 290
22 164 327 818 64 56 112 281
23 157 313 783 66 54 109 273
24 150 300 750 68 53 106 265
25 144 288 720 70 51 103 257
26 138 277 692 72 50 100 250
27 133 267 667 74 48 97 243
28 129 257 643 76 47 95 237
29 124 248 621 78 46 92 231
30 120 240 600 80 45 90 225
31 116 232 581 82 44 88 220
32 113 225 563 84 43 86 214
33 109 218 545 86 42 84 209
34 106 212 529 88 41 82 205
35 103 206 514 90 40 80 200
36 100 200 500 92 39 78 196
37 97 195 486 94 38 76 192
38 95 189 474 96 38 75 188
39 92 185 462 98 37 74 184
40 90 180 450 100 36 72 180
41 88 176 439 102 35 71 178
42 86 172 429 104 35 69 173
43 84 167 419 106 34 68 170
44 82 164 409 108 33 67 167
45 80 160 400 110 33 65 164
46 78 157 391 112 32 64 161
47 76 153 383 116 31 62 155
48 75 150 375 120 30 60 150
49 73 147 367

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
440 04 4321 08 9
Table 3 Orifice Size and Manifold Pressure (in. w.c.) for Gas Input Rate
A11252A
SAGLARUTANFOYTIVARGCIFICEPSSAG.GVA
HEATVALUE0.580.600.620.64
AT ALTITUDE Orifice Mnfld Press Orifice Mnfld Press Orifice Mnfld Press Orifice Mnfld Press
(Btu/cu ft) No. High/Low No. High/Low No. High/Low No. High/Low
900 43 3.8 / 1.6 42 3.2 / 1.4 42 3.3 / 1.4 42 3.4 / 1.4
092543 3.6 / 1.5 43 3.7 / 1.6 43 3.8 / 1.6 42 3.2 / 1.4
(0) 950 43 3.4 / 1.4 43 3.5 / 1.5 43 3.6 / 1.5 43 3.7 / 1.6
975 44 3.7 / 1.6 44 3.8 / 1.6 43 3.4 / 1.5 43 3.6 / 1.5
to 1000 44 3.5 / 1.5 44 3.6 / 1.5 44 3.8 / 1.6 43 3.4 / 1.4
1025 44 3.3 / 1.4 44 3.5 / 1.5 44 3.6 / 1.5 44 3.7 / 1.6
2000 1050 44 3.2 / 1.3 44 3.3 / 1.4 44 3.4 / 1.4 44 3.5 / 1.5
(610) 1075 45 3.7 / 1.6 45 3.8 / 1.6 44 3.3 / 1.4 44 3.4 / 1.4
1100 46 3.7 / 1.6 46 3.8 / 1.6 45 3.8 / 1.6 44 3.2 / 1.4
U.S.A. 800 42 3.4 / 1.4 42 3.5 / 1.5 42 3.6 / 1.5 42 3.7 / 1.6
2001 (611) 825 43 3.8 / 1.6 42 3.3 / 1.4 42 3.4 / 1.4 42 3.5 / 1.5
to 850 43 3.6 / 1.5 43 3.7 / 1.6 42 3.2 / 1.3 42 3.3 / 1.4
3000 (914) 875 43 3.4 / 1.4 43 3.5 / 1.5 43 3.7 / 1.5 43 3.8 / 1.6
900 44 3.7 / 1.6 44 3.8 / 1.6 43 3.5 / 1.5 43 3.6 / 1.5
Canada 925 44 3.5 / 1.5 44 3.6 / 1.5 44 3.8 / 1.6 43 3.4 / 1.4
2001 (611) 950 44 3.3 / 1.4 44 3.4 / 1.5 44 3.6 / 1.5 44 3.7 / 1.6
to 975 44 3.2 / 1.3 44 3.3 / 1.4 44 3.4 / 1.4 44 3.5 / 1.5
4500 (1372) 1000 46 3.8 / 1.6 45 3.8 / 1.6 44 3.2 / 1.4 44 3.3 / 1.4
775 42 3.3 / 1.4 42 3.4 / 1.4 42 3.5 / 1.5 42 3.6 / 1.5
3001 800 43 3.8 / 1.6 42 3.2 / 1.4 42 3.3 / 1.4 42 3.4 / 1.4
(915) 825 43 3.6 / 1.5 43 3.7 / 1.6 43 3.8 / 1.6 42 3.2 / 1.4
850 44 3.8 / 1.6 43 3.5 / 1.5 43 3.6 / 1.5 43 3.7 / 1.6
875 44 3.6 / 1.5 44 3.7 / 1.6 43 3.4 / 1.4 43 3.5 / 1.5
4000 900 44 3.4 / 1.4 44 3.5 / 1.5 44 3.7 / 1.5 44 3.8 / 1.6
(1219) 925 44 3.2 / 1.4 44 3.4 / 1.4 44 3.5 / 1.5 44 3.6 / 1.5
950 45 3.7 / 1.6 44 3.2 / 1.3 44 3.3 / 1.4 44 3.4 / 1.4
750 42 3.3 / 1.4 42 3.4 / 1.4 42 3.5 / 1.5 42 3.6 / 1.5
4001 775 43 3.7 / 1.6 43 3.8 / 1.6 42 3.3 / 1.4 42 3.4 / 1.4
(1220) 800 43 3.5 / 1.5 43 3.6 / 1.5 43 3.7 / 1.6 43 3.8 / 1.6
825 44 3.8 / 1.6 43 3.4 / 1.4 43 3.5 / 1.5 43 3.6 / 1.5
850 44 3.5 / 1.5 44 3.7 / 1.5 44 3.8 / 1.6 43 3.4 / 1.4
5000 875 44 3.3 / 1.4 44 3.5 / 1.5 44 3.6 / 1.5 44 3.7 / 1.6
(1524) 900 44 3.2 / 1.3 44 3.3 / 1.4 44 3.4 / 1.4 44 3.5 / 1.5
925 46 3.8 / 1.6 45 3.7 / 1.6 44 3.2 / 1.4 44 3.3 / 1.4
725 42 3.2 / 1.4 42 3.3 / 1.4 42 3.4 / 1.5 42 3.5 / 1.5
5001 750 43 3.7 / 1.5 43 3.8 / 1.6 42 3.2 / 1.4 42 3.3 / 1.4
(1525) 775 43 3.4 / 1.4 43 3.5 / 1.5 43 3.7 / 1.5 43 3.8 / 1.6
800 44 3.7 / 1.6 44 3.8 / 1.6 43 3.4 / 1.5 43 3.5 / 1.5
825 44 3.5 / 1.5 44 3.6 / 1.5 44 3.7 / 1.6 44 3.8 / 1.6
6000 850 44 3.3 / 1.4 44 3.4 / 1.4 44 3.5 / 1.5 44 3.6 / 1.5
(1829) 875 45 3.7 / 1.6 44 3.2 / 1.3 44 3.3 / 1.4 44 3.4 / 1.4
900 46 3.7 / 1.6 46 3.8 / 1.6 45 3.8 / 1.6 44 3.2 / 1.4
675 42 3.4 / 1.4 42 3.5 / 1.5 42 3.6 / 1.5 42 3.8 / 1.6
6001 700 42 3.2 / 1.3 42 3.3 / 1.4 42 3.4 / 1.4 42 3.5 / 1.5
(1830) 725 43 3.6 / 1.5 43 3.7 / 1.6 43 3.8 / 1.6 42 3.3 / 1.4
750 43 3.4 / 1.4 43 3.5 / 1.5 43 3.6 / 1.5 43 3.7 / 1.6
775 44 3.6 / 1.5 44 3.7 / 1.6 43 3.4 / 1.4 43 3.5 / 1.5
7000 800 44 3.4 / 1.4 44 3.5 / 1.5 44 3.6 / 1.5 44 3.7 / 1.6
(2133) 825 44 3.2 / 1.3 44 3.3 / 1.4 44 3.4 / 1.4 44 3.5 / 1.5
850 46 3.8 / 1.6 45 3.8 / 1.6 44 3.2 / 1.4 44 3.3 / 1.4
to
to
to
to
TWO-STAGE FURNACE
(TABULATED DATA BASED ON 20,000 BTUH HIGH-HEAT / 13,000 BTUH LOW-HEAT PER BURNER,
DERATED 2%/1000 FT (305M) ABOVE SEA LEVEL)
ALTITUDE
RANGE
U.S.A. Only
ft (m)
U.S.A. and CanadaU.S.A. Only U.S.A. Only U.S.A. Only U.S.A. and Canada

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
10 440 04 4321 08
Table 3 (Cont.) Orifice Size and Manifold Pressure (in. w.c.) for Gas Input Rate
SAGLARUTANFOYTIVARGCIFICEPSSAG.GVA
HEATVALUE0.580.600.620.64
AT ALTITUDE Orifice Mnfld Press Orifice Mnfld Press Orifice Mnfld Press Orifice Mnfld Press
(Btu/cu ft) No. High/Low No. High/Low No. High/Low No. High/Low
TWO-STAGE FURNACE
(TABULATED DATA BASED ON 20,000 BTUH HIGH-HEAT / 13,000 BTUH LOW-HEAT PER BURNER,
DERATED 2%/1000 FT (305M) ABOVE SEA LEVEL)
ALTITUDE
RANGE
ft (m)
650 42 3.4 / 1.4 42 3.5 / 1.5 42 3.6 / 1.5 42 3.7 / 1.6
7001 675 43 3.8 / 1.6 42 3.2 / 1.4 42 3.3 / 1.4 42 3.4 / 1.5
(2134) 700 43 3.5 / 1.5 43 3.7 / 1.5 43 3.8 / 1.6 42 3.2 / 1.4
725 44 3.8 / 1.6 43 3.4 / 1.4 43 3.5 / 1.5 43 3.6 / 1.5
750 44 3.5 / 1.5 44 3.7 / 1.5 44 3.8 / 1.6 43 3.4 / 1.4
8000 775 44 3.3 / 1.4 44 3.4 / 1.4 44 3.5 / 1.5 44 3.7 / 1.5
(2438) 800 45 3.8 / 1.6 44 3.2 / 1.4 44 3.3 / 1.4 44 3.4 / 1.4
825 46 3.7 / 1.6 46 3.8 / 1.6 45 3.8 / 1.6 44 3.2 / 1.4
625 42 3.4 / 1.4 42 3.5 / 1.5 42 3.6 / 1.5 42 3.7 / 1.6
8001 650 43 3.8 / 1.6 42 3.2 / 1.4 42 3.3 / 1.4 42 3.4 / 1.4
(2439) 675 43 3.5 / 1.5 43 3.6 / 1.5 43 3.7 / 1.6 42 3.2 / 1.3
700 44 3.7 / 1.6 43 3.4 / 1.4 43 3.5 / 1.5 43 3.6 / 1.5
725 44 3.5 / 1.5 44 3.6 / 1.5 44 3.7 / 1.6 44 3.8 / 1.6
9000 750 44 3.3 / 1.4 44 3.4 / 1.4 44 3.5 / 1.5 44 3.6 / 1.5
(2743) 775 45 3.7 / 1.6 44 3.2 / 1.3 44 3.3 / 1.4 44 3.4 / 1.4
9001 600 42 3.3 / 1.4 42 3.4 / 1.5 42 3.6 / 1.5 42 3.7 / 1.6
(2744) 625 43 3.7 / 1.6 42 3.2 / 1.3 42 3.3 / 1.4 42 3.4 / 1.4
650 43 3.5 / 1.5 43 3.6 / 1.5 43 3.7 / 1.6 43 3.8 / 1.6
675 44 3.7 / 1.6 44 3.8 / 1.6 43 3.4 / 1.4 43 3.5 / 1.5
10000 700 44 3.4 / 1.4 44 3.5 / 1.5 44 3.7 / 1.5 44 3.8 / 1.6
(3048) 725 44 3.2 / 1.3 44 3.3 / 1.4 44 3.4 / 1.4 44 3.5 / 1.5
* Orifice numbers shown in BOLD are factory-installed.
to
to
to
U.S.A. OnlyU.S.A. Only U.S.A. Only
A11252B
Adjust Temperature Rise
NOTE: Blower door must be installed when taking temperature
rise reading. Leaving blower door off will result in incorrect
temperature measurements, due to possible changes in duct
static pressure and airflow.
FURNACE DAMAGE HAZARD
Failure to follow this caution may result in:
SOverheating the heat exchangers or condensing
flue gases in heat exchanger areas not designed
for condensate
SShortened furnace life
SComponent damage
Temperature rise must be within limits specified on furnace
rating plate. Recommended operation is at midpoint of rise
range or slightly above.
CAUTION
!
Place SW−1 (TT) in ON position. Jumper R to W/W1 and W2 to
check high−heat temperature rise. To check low−heat
temperature rise, remove jumper to W2. Determine air
temperature rise for both high and low−heat. Do not exceed
temperature rise ranges specified on unit rating plate for high
and low−heat.
This furnace must operate within the temperature rise ranges
specified on the furnace rating plate. Determine the air
temperature as follows:
1. Place duct thermometers in return and supply ducts as
close to furnace as possible. Be sure thermometers do
not “see” heat exchangers so that radiant heat does not
affect thermometer readings. This is particularly
important with straight run ducts.
2. When thermometer readings stabilize, subtract return−air
temperature from supply−air temperature to determine
temperature rise.
If the temperature rise is outside this range, check the
following:
1. Gas input for low and high heat operation.
2. Derate for altitude if applicable.
3. Return and supply ducts for excessive restrictions
causing static pressures greater than 0.50−in. w.c. (125
Pa)
4. Adjust temperature rise by adjusting blower speed.
SIncrease blower speed to reduce temperature rise.
SDecrease blower speed to increase temperature rise.
ELECTRICAL OPERATION HAZARD
Failure to follow this warning could result in personal injury
or death.
Disconnect 115vac electrical power before changing speed
tap.
!WARNING

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
440 04 4321 08 11
Table 4 Blower Speed Taps
COLOR SPEED FACTORY
ATTACHED TO:
Gray 5 Cool
Yellow 4 Spare
Blue 3 HI HT
Orange 2 Spare
Red 1 LO HT/Cont Fan
For low−heat, the following connections can be made at LO HT
on control:
SOrange blower motor lead
SRed blower motor lead −Factory Setting
(Read following caution before changing taps.)
UNIT DAMAGE HAZARD
To avoid operating outside the rise range and avoid
component damage:
Refer to the Air Delivery Tables to determine which airflows
and settings are allowed for proper heating airflow. DO NOT
use the highlighted settings for High−Stage Heating airflow.
The highlighted settings are to be used for Low−Stage
Heating and Cooling ONLY.
CAUTION
!
Change the blower speed taps as required to maintain proper
temperature rise for Heating and Cooling as described above.
To change blower motor speed selections for heating mode,
remove blower motor lead from control HEAT terminal(s). (See
Figure 4) Select desired blower motor speed lead(s) from the
other motor leads and relocate it/them to LOW or HIGH HEAT
terminal(s). Reconnect original lead on SPARE terminal
Adjust Blower Off Delay (Heat Mode)
If desired, the main blower off time delay period may be
lengthened or shortened when operating in the heating mode
to provide greater comfort. For position and location of switches
on control center. (See Figure 4 and Figure 15)
a. Remove blower door if installed.
b. Turn setup switches SW−2 and SW−3 ON or OFF for
desired blower off delay. See Table 5, Figure 4 and
Figure 15.
c. Proceed to “Set Blower Off Delay” before installing
blower door.
Table 5 Blower Speed Taps
DESIRED HEATING MODE
BLOWER OFF DELAY
(SEC.)
SETUP SWITCH
(SW-2 & SW-3)
POSITION
SW-2 SW-3
90 OFF OFF
120 OFF ON
150 ON OFF
180 ON ON
Adjust Cooling Airflow
The cooling airflow can be set from the remaining blower speed
taps. Refer to the Air Delivery Tables in these instructions.
If a two−stage air conditioning or heat pump is installed, the
Low Cooling Speed is the same speed tap as the Low Heat
(LO HT), as well as Continuous Fan.
Adjust Continuous Fan Airflow
The Continuous Fan airflow can be set from the remaining
blower speed taps. Refer to the Air Delivery Tables in these
instructions.
The Continuous Fan speed can be changed to any other
speed tap by toggling th Fan switch at the thermostat. See the
Sequence of Operation for details.
FURNACE OVERHEATING HAZARD
Failure to follow this caution may result in reduced furnace
life.
Recheck temperature rise. It must be within limits specified
on the rating plate. Recommended operation is at the
mid−point of rise range or slightly above.
CAUTION
!
Adjust Thermostat Heat Anticipator
Mechanical thermostat: Set thermostat heat anticipator to
match the amp. draw of the electrical components in the
R-W/W1 circuit. Accurate amp. draw readings can be obtained
at the wires normally connected to thermostat sub-base
terminals, R and W. The thermostat anticipator should NOT be
in the circuit while measuring current. (See Figure 5)
1. Set SW-1 (TT) switch on furnace control board to ON.
(See Figure 4)
2. Remove thermostat from sub-base or from wall.
3. Connect an amp. meter across the R and W sub-base
terminals or R and W wires at wall.
4. Record amp. draw across terminals when furnace is in
low heat and after blower starts.
5. Set heat anticipator on thermostat per thermostat
instructions and install on sub-base or wall.
6. Turn SW-1 (TT) switch OFF.
7. Install blower access door.
Electronic thermostat: Set cycle rate for 3 cycles per hr.
Figure 5 Amp. Draw Check with Ammeter
A96316
R Y W G
10 TURNS
THERMOSTAT SUBBASE
TERMINALS WITH
THERMOSTAT REMOVED
(ANITICIPATOR, CLOCK, ETC.,
MUST BE OUT OF CIRCUIT.)
HOOK-AROUND
AMMETER
EXAMPLE: 5.0 AMPS ON AMMETER
10 TURNS AROUND JAWS
= 0.5 AMPS FOR THERMOSTAT
ANTICIPATOR SETTING
FROM UNIT 24-V
CONTROL TERMINALS

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
12 440 04 4321 08
Table 6 AIR DELIVERY - CFM (with filter1)
UNIT
SIZE
RETURN-AIR
CONNECTION SPEED
TAPS2
EXTERNAL STATIC PRESSURE (in. w.c.)
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0401410 SIDE/BOTTOM
Gray 1120 1080 1030 980 925 875 820 760 690 630
Yellow 880 845 810 780 740 710 680 640 615 570
Blue 695 665 620 575 535 495 455 420 370 280
Orange 640 595 540 495 460 420 370 310 260 230
Red 570 525 475 425 385 330 255 220 - 6- 6
0401712 SIDE/BOTTOM
Gray 1255 1220 1175 1130 1085 1040 990 940 880 825
Yellow 940 905 870 840 805 770 735 695 665 630
Blue 705 670 630 575 540 500 455 410 380 325
Orange 580 535 480 425 380 335 290 235 - 6- 6
Red 555 485 425 375 330 280 215 - 6- 6- 6
0601412 SIDE/BOTTOM
Gray 1265 1225 1185 1140 1090 1030 975 920 850 760
Yellow 1115 1085 1060 1030 1000 970 930 880 810 715
Orange 1000 970 940 910 880 845 815 770 735 695
Blue 945 915 885 855 820 785 745 705 675 635
Red 770 740 700 660 620 575 540 500 455 415
0601714 SIDE/BOTTOM
Gray 1720 1670 1620 1565 1505 1440 1375 1295 1220 1135
Yellow 1325 1285 1255 1220 1185 1145 1115 1075 1040 1000
Blue 1010 970 925 875 835 785 745 690 660 620
Orange 1160 1115 1080 1045 1000 960 920 875 840 785
Red 785 715 655 595 530 490 435 385 340 285
0801716 SIDE/BOTTOM
Gray 1810 1770 1720 1665 1610 1540 1475 1400 1315 1235
Yellow 1535 1500 1475 1435 1405 1370 1340 1310 1245 1160
Blue 1380 1340 1305 1270 1240 1200 1165 1130 1090 1050
Orange 1180 1130 1095 1060 1015 975 935 895 850 800
Red 1100 1045 1010 970 920 885 845 790 745 690
0802120 BOTTOM or
TWO-SIDES 4, 5
Gray 2290 2225 2155 2090 2015 1930 1845 1750 1640 1515
Yellow 1810 1760 1725 1685 1640 1600 1555 1520 1480 1415
Blue 1385 1340 1285 1240 1200 1140 1090 1050 995 950
Orange 1560 1520 1475 1430 1385 1335 1295 1240 1200 1150
Red 1055 985 910 860 795 750 680 615 565 495
1002120 BOTTOM or
TWO-SIDES 4, 5
Gray 2340 2295 2250 2195 2110 2030 1935 1835 1725 1605
Yellow 1950 1900 1855 1800 1755 1705 1655 1605 1560 1485
Blue 1750 1700 1650 1605 1555 1500 1455 1395 1350 1300
Orange 1570 1520 1460 1410 1350 1300 1240 1195 1140 1095
Red 1350 1280 1225 1155 1105 1045 1000 950 895 830
1202422 BOTTOM or
TWO-SIDES 4, 5
Gray 2275 2230 2185 2130 2055 1950 1825 1710 1610 1500
Yellow 1875 1820 1770 1720 1660 1600 1550 1505 1450 1390
Blue 2170 2125 2075 2025 1975 1900 1790 1695 1590 1470
Orange 31475 1420 1350 1280 1215 1165 1105 1050 995 930
Red 31625 1565 1505 1445 1385 1325 1275 1225 1170 1130
NOTE:
1. A filter is required for each return−air inlet. Airflow performance includes a 3/4 in. (19 mm) washable filter media such as contained in factory−authorized accessory
filter rack. See accessory list. To determine airflow performance without this filter, assume an additional 0.1 in. W.C. available external static pressure.
2. ADJUST THE BLOWER SPEED TAPS AS NECESSARY FOR THE PROPER AIR TEMPERATURE RISE FOR EACH INSTALLATION.
3. Shaded areas indicate that this airflow range is BELOW THE RANGE ALLOWED FOR HIGH−STAGE HEATING OPERATION. THESE AIRFLOW RANGES MAY
ONLY BE USED FOR LOW−STAGE HEAT OR COOLING.
4. Airflows over 1800 CFM require bottom return, two−side return, or bottom and side return. A minimum filter size of 20” x 25” is required.
5. For upflow applications, air entering from one side into both the side of the furnace and a return air base counts as a side and bottom return.
6. The ”−” entry indicates an unstable operating condition.
Check Safety Controls
The flame sensor, gas valve, and pressure switch were all
checked in the Start−up procedure section as part of normal
operation.
1. Check Main Limit Switch
This control shuts off combustion system and energizes
air−circulating blower motor, if furnace overheats. By
using this method to check the temperature limit control,
it can be established that the limit is functioning properly
and that the limit will operate if there is a restricted
return−air supply or motor failure. If the limit control does
not function during this test, the cause must be
determined and corrected.
a. Run furnace for at least 5 minutes.
b. Gradually block off return air with a piece of
cardboard or sheet metal until the limit trips.
c. Unblock return air to permit normal circulation.
d. Burners will re−light when furnace cools down.
2. Check Pressure Switch(es)
This control proves operation of the draft inducer blower.
a. Turn off 115−V power to furnace.
b. Disconnect inducer motor lead wires from wire
harness.
c. Turn on 115−V power to furnace.
d. Set thermostat to “call for heat” and wait 1 minute.
When pressure switch is functioning properly, hot
surface igniter should NOT glow and control
diagnostic light flashes a status code 3. If hot surface

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
440 04 4321 08 13
igniter glows when inducer motor is disconnected,
shut down furnace immediately.
e. Determine reason pressure switch did not function
properly and correct condition.
f. Turn off 115−V power to furnace.
g. Reconnect inducer motor wires, replace outer door,
and turn on 115−V power.
h. Blower will run for 90 seconds before beginning the
call for heat again.
i. Furnace should ignite normally.
Checklist
1. Put away tools and instruments. Clean up debris.
2. Verify that the jumper is removed from the TEST/TWIN
terminal. Verify that there is nothing plugged into the PLT
connector.
NOTE: Note: If there is a jumper connector plugged into PLT,
remove it and discard. (See Figure 14)
3. Verify that Heating Operating Mode switch SW−1 is set
properly.. (See Figure 14)
4. Verify that the Blower/Heat Off−Delay SW−2 and SW−3
switches are set as desired. (See Figure 14)
5. Verify that the blower (lower door in upflow position) and
control (“Main” or upper door in upflow position) doors
are properly installed.
6. Verify that the Status LED glows. If not, check that the
power supply is energized and that the blower door is
secure. (See Figure 14) to interpret diagnostic codes.
7. Cycle test furnace with room thermostat to be sure that it
operates properly with the room thermostat. Check all
modes including Heat, Cool and Fan.
8. Check operation of accessories per manufacturer’s
instructions.
9. Review Home Owner’s Information with owner.
10. Attach entire literature packet to furnace.
SERVICE AND MAINTENANCE
PROCEDURES
Untrained personnel can perform basic maintenance functions
such as cleaning and replacing air filters. All other operations
must be performed by trained service personnel. A qualified
service person should inspect the furnace once a year.
FIRE, INJURY OR DEATH HAZARD
Failure to follow this warning could result in personal
injury, death and/or property damage.
The ability to properly perform maintenance on this
equipment requires certain knowledge, mechanical
skills, tools, and equipment. If you do not possess
these, do not attempt to perform any service and
maintenance on this equipment other than those
procedures recommended in the Owner’s Manual.
!WARNING
ENVIRONMENTAL HAZARD
Failure to follow this caution may result in
environmental pollution.
Remove and recycle all components or materials (i.e.
oil, refrigerant, control board, etc.) before unit final
disposal.
CAUTION
!
ELECTRICAL SHOCK, FIRE OR EXPLOSION
HAZARD
Failure to follow this warning could result in personal
injury or death, or property damage.
Before installing, modifying, or servicing system, main
electrical disconnect switch must be in the OFF
position and install a lockout tag. There may be more
than one disconnect switch. Lock out and tag switch
with a suitable warning label. Verify proper operation
after servicing.
!WARNING
ELECTRICAL OPERATION HAZARD
Failure to follow this caution may result in improper
furnace operation or failure of furnace.
Label all wires prior to disconnection when servicing
controls. Wiring errors can cause improper and
dangerous operation.
CAUTION
!
General
These instructions are written as if the furnace is installed in an
upflow application. An upflow furnace application is where the
blower is located below the combustion and controls section of
the furnace, and conditioned air is discharged upward. Since
this furnace can be installed in any of the 4 positions shown in
Figure 6, you must revise your orientation to component
location accordingly.
Figure 6 Multipoise Orientations
A93041
Electrical Controls and Wiring
Pressure Switches
Each pressure switch is labeled with the reference location
(noted as “COLLECTOR BOX−LPS” or “HOUSING−HPS” ON
THE SWITCH). The nominal break point of each switch is
shown on the label below the reference location in inches of
water column, “W.C.” The maximum and minimum break point
of the switch is +/−0.05 inches of water column from the
nominal break point of the switch. The maximum make point of
the switch is 0.10 inches of water above the maximum break
point of the switch
Example: Nominal break point on pressure switch is 0.68−in.
W.C. The minimum break point of the switch is 0.63−in. W.C.
The maximum break point of the switch is 0.73−in. W.C. The
maximum make point of the switch is 0.83−in. W.C.

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
14 440 04 4321 08
ELECTRICAL SHOCK HAZARD
Failure to follow this warning could result in personal
injury or death.
There may be more than one electrical supply to the
furnace. Check accessories and cooling unit for
additional electrical supplies that must be shut off
during furnace servicing. Lock out and tag switch with
a suitable warning label.
!WARNING
The electrical ground and polarity for 115−V wiring must be
properly maintained. Refer to Figure 7 for field wiring
information and to Figure 15 for furnace wiring information.
NOTE: If the polarity is not correct, the STATUS LED on the
control will flash code 10 and prevent the furnace from heating.
The control system also requires an earth ground for proper
operation of the control and flame−sensing electrode.
The 24−V circuit contains an automotive−type, 3−amp. fuse
located on the control. (See Figure 4) Any shorts of the 24−V
wiring during installation, service, or maintenance will cause
this fuse to blow. If fuse replacement is required, use ONLY a
3−amp. fuse. The control LED display will be off when fuse
needs to be replaced.
Troubleshooting
Refer to the service label. (See Figure 14—Service Label)
The Troubleshooting Guide −Flow Chart (See Figure 16) can
be a useful tool in isolating furnace operation problems.
Beginning with the word “Start,” answer each question and
follow the appropriate arrow to the next item.
The Guide −Flow Chart will help to identify the problem or
failed component. After replacing any component, verify correct
operation sequence.
Proper instrumentation is required to service electrical controls.
The control in this furnace is equipped with a Status Code LED
(Light−Emitting Diode) to aid in installation, servicing, and
troubleshooting. Status codes can be viewed at the indicator in
blower door. The green furnace control LED is either be a
heartbeat, off, or a code composed of 1 or 2 digits. The first
digit is the number of short flashes, the second digit is the
number of long flashes.
Figure 7 Heating and Cooling Application Wiring Diagram with Single−Stage Thermostat
115-VOLT FIELD-
SUPPLIED
FUSED
DISCONNECT
JUNCTION
BOX
CONTROL
BOX
24-VOLT
TERMINAL
BLOCK
THREE-WIRE
HEATING-
ONLY
FIVE
WIRE
NOTE 2
NOTE 1
1-STAGE
THERMOSTAT
TERMINALS
FIELD-SUPPLIED
FUSED DISCONNECT
CONDENSING
UNIT
FURNACE
COM
R
WC Y RG
GND
GND
FIELD 24-VOLT WIRING
FIELD 115-, 208/230-, 460-VOLT WIRING
FACTORY 24-VOLT WIRING
FACTORY 115-VOLT WIRING
Connect Y/Y2-terminal as shown for proper operation.
Some thermostats require a "C" terminal connection as shown.
If any of the original wire, as supplied, must be replaced, use
same type or equivalent wire.
208/230- OR
460-VOLT
THREE
PHASE
208/230-
VOLT
SINGLE
PHASE
WHT
BLK
WHT
BLK
W/W1
W2
Y/Y2
G
NOTES: 1.
2.
3.
A95236
For an explanation of status codes, refer to service label
located on control door or Figure 14, and the troubleshooting
guide which can be obtained from your distributor.
Retrieving Stored Fault Codes
NOTE: Fault codes cannot be retrieved if a thermostat signal
(24-V on W, Y, G, etc.) is present, or if any delays such as
blower off-delays are active.
The stored status codes will NOT be erased from the control
memory when 115- or 24-V power is interrupted. See the
Service Label (See Figure 14) for more information. The most
recent fault code may be retrieved as follows:
1. Leave 115-V power connected to furnace.
2. Observe the status LED through the blower door (the
lower door on upflow applications) indicator. Refer to the
Service Label (See Figure 14) to interpret the LED.
3. Remove the Main/Control door (the upper door on upflow
installations).
4. BRIEFLY (2−3 seconds) disconnect and reconnect ONE
of the main limit wires.
5. The LED will flash the last stored fault code. Refer to the
Service Label (See Figure 14) to interpret the LED.
6. A component test sequence will follow.
7. Reinstall the Main/Control door.
Component Self−Test
NOTE: The furnace control component test allows all
components to run for a short time; except the gas valve and
humidifier terminal HUM 24 VAC are not energized. The EAC-1
terminal is energized when the blower is energized. The HUM
terminal is energized when the inducer is energized. This
feature helps diagnose a system problem in case of a
component failure. The component test feature will not operate
if any thermostat signal is present at the control and not until all
time delays are completed.
To begin Component Self−Test:
1. Remove blower access door.
2. Disconnect the thermostat R lead from furnace control.
3. Manually close blower door switch.

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
440 04 4321 08 15
Caution must be taken when manually closing this switch for
service purposes.
ELECTRICAL SHOCK HAZARD
Failure to follow this warning could result in personal
injury, or death.
Blower access door switch opens 115−V power to
control. No component operation can occur unless
switch is closed. Caution must be taken when
manually closing this switch for service purposes.
!WARNING
4. For approximately 2 sec, short (jumper) the C terminal on
control to the TEST/TWIN 3/16—in. (5 mm)
quick-connect terminal on control until the LED goes off.
Remove jumper from terminals. (See Figure 4)
NOTE: If TEST/TWIN and C terminals are jumpered longer
than 2 sec., LED will flash code 10 and ignore component test
request.
Component test sequence is as follows:
a. LED will display previous status code 4 times.
b. Inducer motor starts and continues to run until Step g
of component test sequence.
c. After 7 seconds the hot surface igniter is energized for
15 sec., then off.
d. Blower motor operates on LO−HT speed for 10 sec.
e. Blower motor operates on HI−HT speed for 10 sec.
f. Blower motor operates on COOL speed for 10 sec.
g. Inducer motor goes to low speed for 10 sec., then stops.
5. Reconnect R lead to furnace control, remove tape from
blower door switch, and re-install blower door.
6. Verify furnace shut down by lowering thermostat setting
below room temperature.
7. Verify that furnace restarts by raising thermostat setting
above room temperature.
8. Release blower door switch, reattach wire to “R” terminal
on furnace control board and replace blower door.
Care and Maintenance
FIRE OR EXPLOSION HAZARD
Failure to follow this warning could result in personal
injury, death and/or property damage.
Never store flammable or combustible materials on,
near, or in contact with the furnace, such as:
1. Spray or aerosol cans, rags, brooms, dust
mops, vacuum cleaners, or other cleaning
tools.
2. Soap powders, bleaches, waxes or other
cleaning compounds, plastic or plastic
containers, gasoline, kerosene, cigarette lighter
fluid, dry cleaning fluids, or other volatile fluids.
3. Paint thinners and other painting compounds,
paper bags, or other paper products. Exposure
to these materials could lead to corrosion of the
heat exchangers.
!WARNING
For continuing high performance and to minimize possible
furnace failure, periodic maintenance must be performed on
this furnace. Consult your local dealer about proper frequency
of maintenance and the availability of a maintenance contract.
ELECTRICAL SHOCK AND FIRE HAZARD
Failure to follow this warning could result in personal
injury, death, and/or property damage.
Turn off the gas and electrical supplies to the furnace
and install lockout tag before performing any
maintenance or service. Follow the operating
instructions on the label attached to the furnace.
!WARNING
CARBON MONOXIDE POISONING AND FIRE
HAZARD
Failure to follow this warning could result in personal
injury, death and/or property damage.
Never operate furnace without a filter or filtration
device installed. Never operate a furnace with filter or
filtration device access doors removed.
!WARNING
CUT HAZARD
Failure to follow this caution may result in personal
injury.
Sheet metal parts may have sharp edges or burrs.
Use care and wear appropriate protective clothing,
safety glasses and gloves when handling parts, and
servicing furnaces.
CAUTION
!
The minimum maintenance on this furnace is as follows:
1. Check and clean air filter each month or more frequently
if required. Replace if torn.
2. Check blower motor and wheel for cleanliness each
heating and cooling season. Clean as necessary.
3. Check electrical connections for tightness and controls
for proper operation each heating season. Service as
necessary.
4. Inspect burner compartment before each heating season
for rust, corrosion, soot or excessive dust. If necessary,
have furnace and burner serviced by a qualified service
agency.
5. Inspect the vent pipe/vent system before each heating
season for water leakage, sagging pipes or broken
fittings. Have vent pipes/vent system serviced by a
qualified service agency.
6. Inspect any accessories attached to the furnace such as
a humidifier or electronic air cleaner. Perform any service
or maintenance to the accessories as recommended in
the accessory instructions.
Cleaning and/or Replacing Air Filter
The air filter type may vary depending on the application or
orientation. The filter is external to the furnace casing. There
are no provisions for an internal filter with this furnace. See
“Filter Arrangement” under the “Installation” section of this
manual.
NOTE: If the filter has an airflow direction arrow, the arrow
must point toward the blower.
To clean or replace filters, proceed as follows:

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
16 440 04 4321 08
ELECTRICAL SHOCK, FIRE OR EXPLOSION
HAZARD
Failure to follow this warning could result in personal
injury or death, or property damage.
Before installing, modifying, or servicing system, main
electrical disconnect switch must be in the OFF
position and install a lockout tag. There may be more
than one disconnect switch. Lock out and tag switch
with a suitable warning label. Verify proper operation
after servicing.
!WARNING
1. Turn off electrical supply to furnace.
2. Remove filter cabinet door.
3. Slide filter out of cabinet.
4. If equipped with permanent, washable filter, clean filter by
spraying cold tap water through filter in opposite direction
of airflow. Rinse filter and let dry. Oiling or coating of the
filter is not recommended.
5. If equipped with factory specified disposable media filter,
replace only with a factory specified media filter of the
same size.
6. Slide filter into cabinet.
7. Replace filter cabinet door.
8. Turn on electrical supply to furnace.
Blower Motor and Wheel Maintenance
To ensure long life, economy, and high efficiency, clean
accumulated dirt and grease from blower wheel and motor
annually.
The inducer and blower motors are pre−lubricated and require
no additional lubrication. These motors can be identified by the
absence of oil ports on each end of the motor.
The following items should be performed by a qualified service
technician. Clean blower motor and wheel as follows:
1. Turn off electrical supply to furnace.
2. Remove blower door.
3. All factory wires can be left connected, but field
thermostat and accessory wiring may need to be
disconnected depending on their length and routing.
4. If the vent and combustion air pipe passes through the
blower compartment, it will be necessary to remove the
pipes from the blower compartment.
Disconnect the vent and combustion air pipe by:
a. Loosen the clamps on the vent couplings and
combustion air pipe external to the furnace.
b. Separate the pipes from the couplings and move
them aside.
c. Loosen the clamps on the vent couplings and
combustion air pipe located on the blower shelf.
d. Separate the pipes from the blower compartment and
set aside.
e. Remove the couplings from the pipe adapters and set
aside.
f. After servicing the blower, reverse steps a through e.
g. Tighten all clamps 15 lb−in.
See Figure 8 for steps 5 through 14.
5. Remove screws securing blower assembly to blower
shelf and slide blower assembly out of furnace. Detach
ground wire and disconnect blower motor harness plugs
from blower motor.
NOTE: Blower wheel is fragile. Use care.
6. Clean blower wheel and motor by using a vacuum with
soft brush attachment. Be careful not to disturb balance
weights (clips) on blower wheel vanes. Do not bend
wheel or blades as balance will be affected.
7. If greasy residue is present on blower wheel, remove
wheel from the blower housing and wash it with an
appropriate degreaser. To remove wheel:
a. Mark blower wheel location on shaft before
disassembly to ensure proper reassembly.
b. Loosen setscrew holding blower wheel on motor
shaft.
NOTE: Mark blower mounting arms and blower housing so
each arm is positioned at the same hole location during
reassembly.
c. Mark blower wheel orientation and cutoff plate
location to ensure proper reassembly.
d. Remove screws securing cutoff plate and remove
cutoff plate from housing.
e. Remove bolts holding motor mounts to blower
housing and slide motor and mounts out of housing.
f. Remove blower wheel from housing.
g. Clean wheel per instructions on degreaser cleaner.
Do not get degreaser in motor.
8. Reassemble motor and blower wheel by reversing items
7b through 7f. Ensure wheel is positioned for proper
rotation.
9. Torque motor mounting bolts to 40 +/−10 lb−in. when
reassembling.
10. Torque blower wheel set screw to 160 +/−20 lb−in. when
reassembling.
11. Verify that blower wheel is centered in blower housing
and set screw contacts the flat portion of the motor shaft.
Loosen set screw on blower wheel and reposition if
necessary.
12. Spin the blower wheel by hand to verify that the wheel
does not rub on the housing.
13. Reinstall blower assembly in furnace.
14. Reinstall 2 screws securing blower assembly to blower
deck.
15. Reconnect blower leads to furnace control. Refer to
furnace wiring diagram, and connect thermostat leads if
previously disconnected.
NOTE: Be sure to attach ground wire and reconnect blower
harness plugs to blower motor.

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
440 04 4321 08 17
Figure 8 Blower Assembly
A11584A
SET SCREW
MOTOR WHEEL HUB
MOTOR SHAFT FLAT
A11584B
SCREW
MOTOR ARM
GROMMET
A11584C
SCREW LOCATION
BLO HSG ASSY
BRACKET
BRACKET
ENGAGEMENT
DETAIL A
A11584D
SEE DETAIL A
BRACKET
SCREW
(GND)
BLOWER HSG ASSY
MOTOR, BLOWER
WHEEL, BLOWER
CUTOFF, BLOWER
A11584E
POWER CHOKE
OR CAPACITOR
WHEN REQUIRED
ELECTRICAL OPERATION HAZARD
Failure to follow this warning could result in personal
injury or death.
Blower door switch opens 115−V power to control. No
component operation can occur unless switch is closed.
Caution must be taken when manually closing this
switch for service purposes.
!WARNING
16. Downflow or horizontal furnaces with vent pipe through
furnace only:
a. Install and connect short piece of vent pipe inside
furnace to existing vent.
b. Connect vent connector to vent elbow.
17. Turn on electrical supply. Manually close blower door
switch. Use a piece of tape to hold switch closed. Check
for proper rotation and speed changes between heating
and cooling by jumpering R to G and R to Y/Y2 on
furnace control thermostat terminals. If outdoor
temperature is below 70_F (21_C), turn off circuit breaker
to outdoor unit before running furnace in the cooling
cycle. Turn outdoor circuit breaker on after completing
cooling cycle. (See Figure 4)
NOTE: If R−W/W1 thermostat terminals are jumpered at the
time blower door switch is closed, blower will run for 90 sec
before beginning a heating cycle.
a. Perform component self−test as shown at the bottom
of the SERVICE label, located on the control door.
b. Verify blower is rotating in the correct direction
18. If furnace is operating properly, RELEASE BLOWER
DOOR SWITCH. Remove any jumpers or reconnect any
disconnected thermostat leads. Replace blower door.
19. Turn on gas supply and cycle furnace through one
complete heating cycle. Verify the furnace temperature
rise as shown in Adjustments Section. Adjust
temperature rise as shown in Adjustments Section.
Cleaning Burners and Flame Sensor
The following items must be performed by a qualified service
technician. If the burners develop an accumulation of light dirt
or dust, they may be cleaned by using the following procedure:
NOTE: Use a back-up wrench on the gas valve to prevent the
valve from rotating on the manifold or damaging the mounting
to the burner assembly.
ELECTRICAL SHOCK AND FIRE HAZARD
Failure to follow this warning could result in personal
injury, death, and/or property damage.
Turn off the gas and electrical supplies to the furnace
and install lockout tag before performing any
maintenance or service. Follow the operating
instructions on the label attached to the furnace.
!WARNING
Refer to Figure 9.
1. Disconnect power at external disconnect, fuse or circuit
breaker.
2. Turn off gas at external shut-off or gas meter.
3. Remove control door and set aside.
4. Turn electric switch on gas valve to OFF.
5. Disconnect the gas pipe from gas valve and remove pipe
from the furnace casing.

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
18 440 04 4321 08
6. Remove individual wires from terminals on gas valve.
7. Disconnect Hot Surface Igniter (HSI) wires from HSI.
8. Disconnect Flame Sensor wire from Flame Sensor.
9. Support the manifold and remove the four screws that
secure the manifold assembly to the burner assembly
and set aside. Note the location of the green/yellow wire
and ground terminal.
10. Inspect the orifices in the manifold assembly for
blockages or obstructions. Remove orifice and clean or
replace orifice.
11. Remove the four screws that attach the top plate of the
casing to the furnace.
12. Raise top plate up slightly and prop it up with a small
piece of wood or folded cardboard.
13. Support the burner assembly and remove the screws
that attach the burner assembly to the heat exchanger
cell panel.
14. Remove wires from both rollout switches.
15. Slide one−piece burner out of slots on sides of burner
assembly.
16. Remove the flame sensor from the burner assembly.
17. (Optional) Remove the Hot Surface Igniter (HSI) and
bracket from the burner assembly.
18. Check igniter resistance. Nominal resistance is 40 to 70
ohms at room temperature and is stable over the life of
the igniter.
19. Clean burner with a brush and a vacuum.
20. Clean the flame sensor with fine steel wool (0000 grade).
Do not use sand paper or emery cloth.
Figure 9 Burner Assembly
BURNER ASSY
FLAME SENSOR
(BELOW BURNER)
FLAME ROLL−OUT SWITCH
BURNER SUPT. ASSY
L11F064
Representative drawing only, some models may vary in appearance.
To reinstall burner assembly:
1. Install the Hot Surface Igniter (HSI) and bracket in burner
assembly.
2. Install flame sensor on burner.
3. Align the edges of the one-piece burner with the slots in
the burner assembly and slide the burners forward until
they are fully seated in the burner assembly.
4. Align the orifices in the manifold assembly with the
support rings on the end of the burner.
5. Insert the orifices in the support rings of the burners.
NOTE: If manifold does not fit flush against the burner, do not
force the manifold on the burner assembly. The burners are not
fully seated forward in the burner assembly. Remove the
manifold and check burner positioning in the burner assembly
assembly before re-installing the manifold.
6. Attach the green/yellow wire and ground terminal to one
of the manifold mounting screws.
7. Install the remaining manifold mounting screws.
8. Check the igniter alignment. See Figure 9, Figure 10
and Figure 11.
Figure 10 Igniter Position −Top View
A11405
2-1/2-in.
(64.4)
1-1/4-in.
(31.8)
Figure 11 Igniter Position −Side View
A12392
2−in.
(2.5 mm
3/8 −in.
3/16−in.
, +0.8 -1.5)
(50 mm)
(9.6 mm)
(4.6 mm)
3/32−in., +1/32 -3/64-in.
9. Attach the wires to the roll-out switches.
10. Align the burner assembly with the openings in the
primary cell inlet panel and attach the burner assembly to
the cell panel.
11. Connect the wire for the flame sensor.
12. Connect the wire for the Hot Surface Igniter.
NOTE: Use propane-resistant pipe dope to prevent leaks. Do
not use Teflon tape.
13. Install the gas pipe to the gas valve.
FIRE OR EXPLOSION HAZARD
Failure to follow this warning could result in personal
injury, death, and/or property damage.
Never purge a gas line into a combustion chamber.
Never test for gas leaks with an open flame. Use a
commercially available soap solution made
specifically for the detection of leaks to check all
connections. A fire or explosion may result causing
property damage, personal injury or loss of life.
!WARNING
14. Check for gas leaks with a commercially available soap
solution made specifically for the detection of leaks.
15. Turn gas on at electric switch on gas valve and at
external shut-off or meter

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
440 04 4321 08 19
16. Turn power on at external disconnect, fuse or circuit
breaker.
17. Run the furnace through two complete heating cycles to
check for proper operation
18. Install control door when complete.
Servicing Hot Surface Igniter
The igniter does NOT require annual inspection. Check igniter
resistance before removal. Refer to Figure 9, Figure 10 and
Figure 11.
1. Turn off gas and electrical supplies to furnace.
2. Remove control door.
3. Disconnect igniter wire connection.
4. Check igniter resistance. Igniter resistance is affected by
temperature. Only check resistance when the igniter is at
room temperature.
a. Using an ohm meter, check resistance across both
igniter leads in connector.
b. Cold reading should be between 40 ohms and 70
ohms.
5. Remove igniter assembly.
a. Using a 1/4−in. driver, remove the two screws
securing the igniter mounting bracket to the burner
assembly (See Figure 9)
b. Carefully withdraw the igniter and bracket assembly
through the front of the burner assembly without
striking the igniter on surrounding parts.
c. Inspect igniter for signs of damage or failure.
d. If replacement is required, remove the screw that
secures the igniter on igniter bracket and remove the
igniter.
6. To replace igniter and bracket assembly, reverse items
5a through 5d.
7. Reconnect igniter harness to the igniter, dressing the
igniter wires to ensure there is no tension on the igniter
itself. (See Figure 9)
8. Turn on gas and electrical supplies to furnace.
9. Verify igniter operation by initiating control board self−test
feature or by cycling thermostat.
10. Replace control door.
Flushing Collector Box and Drainage System
ELECTRICAL SHOCK AND FIRE HAZARD
Failure to follow this warning could result in personal
injury, death, and/or property damage.
Turn off the gas and electrical supplies to the furnace
and install lockout tag before performing any
maintenance or service. Follow the operating
instructions on the label attached to the furnace.
!WARNING
1. Turn off gas and electrical supplies to furnace.
2. Remove control door.
3. Disconnect pressure switch tube from pressure switch
port.
NOTE: Ensure the pressure switch tube disconnected from the
pressure switch is higher than the collector box opening or
water will flow out of tube.
4. Remove the collector box plug from the top port on the
upper corner of the collector box. (See Figure 1)
5. Attach a funnel with a flexible tube to port on the collector
box.
6. Flush inside of collector box with water until discharge
water from condensate trap is clean and runs freely.
7. Repeat steps 4 thru 6 with middle plug on upper corner
of collector box.
8. Remove the pressure switch tube from the collector box.
NOTE: Do NOT blow into tube with tube connected to the
pressure switch.
9. Clean pressure switch port on collect box with a small
wire. Shake any water out of pressure switch tube.
10. Reconnect tube to pressure switch and pressure switch
port.
11. Remove the relief tube from the port on the collector box
and the trap.
12. Clean the relief port on collect box and the trap with a
small wire. Shake any water out of the tube.
13. Reconnect relief tube to trap and collector box ports.
Cleaning Condensate Drain and Trap
NOTE: If the condensate trap is removed, a new gasket
between the trap and collector box is required. Verify a
condensate trap gasket is included in the service kit or obtain
one from your local distributor.
1. Disconnect power at external disconnect, fuse or circuit
breaker.
2. Turn off gas at external shut-off or gas meter.
3. Remove control door and set aside.
4. Turn electric switch on gas valve to OFF.
5. Disconnect external drain from condensate drain elbow
or drain extension pipe inside the furnace and set aside.
6. Disconnect the condensate trap relief hose from collector
box port and condensate trap.
NOTE: If condensate has a heat pad attached to the trap,
trace the wires for the pad back to the connection point and
disconnect the wires for the heat pad.
7. Remove the screw that secures the condensate trap to
the collector box, remove the trap and set aside.
8. Remove the trap gasket from the collector box if it did not
come off when the trap was removed.
9. Discard the old trap gasket.
10. Rinse condensate trap in warm water until trap is clean.
11. Flush condensate drain lines with warm water.
Remember to check and clean the relief port on the
collector box.
12. Shake trap dry.
13. Clean port on collector box with a small wire.
To re-install Condensate Drain and Trap:
1. Remove adhesive backing from condensate trap gasket
2. Install gasket on collector box
3. Align the condensate trap with the drain opening on the
collector box and secure the trap with the screw
4. Attach the relief hose to the relief port on the condensate
trap and collector box.
5. Secure tubing to prevent any sags or traps in the tubing.
6. Connect condensate drain elbow or drain extension
elbow to the condensate trap
7. Connect the leads of the condensate heat pad (if used)
8. Connect external drain piping to the condensate drain
elbow or drain extension pipe.
9. Turn gas on at electric switch on gas valve and at
external shut-off or meter
10. Turn power on at external disconnect, fuse or circuit
breaker.

SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
20 440 04 4321 08
11. Run the furnace through two complete heating cycles to
check for proper operation
12. Install control door when complete.
Checking Heat Pad Operation (If Applicable)
In applications where the ambient temperature around the
furnace is 32_F (0_C) or lower, freeze protection measures are
required. If this application is where heat tape has been
applied, check to ensure it will operate when low temperatures
are present.
NOTE: The Heat Pad, when used, should be wrapped around
the condensate drain trap. There is no need to use heat tape
within the furnace casing. Most heat tapes are temperature
activated, and it is not practical to verify the actual heating of
the tape. Check the following:
1. Check for signs of physical damage to heat tape such as
nicks, cuts, abrasions, gnawing by animals, etc.
2. Check for discolored heat tape insulation. If any damage
or discolored insulation is evident, replace heat tape.
3. Check that heat tape power supply circuit is on.
Cleaning Heat Exchangers
The following items must be performed by a qualified service
technician.
Primary Heat Exchangers
If the heat exchangers get an accumulation of light dirt or dust
on the inside, they may be cleaned by the following procedure:
NOTE: If the heat exchangers get a heavy accumulation of
soot and carbon, both the primary and secondary heat
exchangers should be replaced rather than trying to clean them
thoroughly due to their intricate design. A build−up of soot and
carbon indicates that a problem exists which needs to be
corrected, such as improper adjustment of manifold pressure,
insufficient or poor quality combustion air, improper vent
termination, incorrect size or damaged manifold orifice(s),
improper gas, or a restricted heat exchanger (primary or
secondary). Action must be taken to correct the problem.
1. Turn off gas and electrical supplies to furnace.
2. Remove control door.
3. Disconnect wires or connectors to flame rollout switch,
gas valve, igniter, and flame sensor.
4. Using backup wrench, disconnect gas supply pipe from
furnace gas control valve.
5. Remove two screws attaching top filler plate and rotate
upwards to gain access to screws attaching burner
assembly to cell panel.
6. Remove screws attaching burner assembly to cell panel.
(See Figure 9)
NOTE: Burner cover, manifold, gas valve, and burner
assembly should be removed as one assembly.
7. Clean heat exchanger openings with a vacuum and a
soft brush. (See Figure 12)
Figure 12 Cleaning Heat Exchanger Cell
NOTE: After cleaning, inspect the heat exchangers to ensure
they are free of all foreign objects that may restrict flow of
combustion products.
8. Reverse items 6 through 1 for reassembly.
9. Refer to furnace wiring diagram and reconnect wires to
flame rollout switch, gas valve, igniter, and flame sensor.
10. Turn on gas and electrical supplies to furnace.
11. Check furnace operation through two complete heat
operating cycles. Look at burners. Burner flames should
be clear blue, almost transparent. (See Figure 13)
Figure 13 Burner Flame
A11461
Burner Flame
Burner
Manifold
FIRE OR EXPLOSION HAZARD
Failure to follow this warning could result in personal
injury, death, and/or property damage.
Never purge a gas line into a combustion chamber.
Never test for gas leaks with an open flame. Use a
commercially available soap solution made
specifically for the detection of leaks to check all
connections. A fire or explosion may result causing
property damage, personal injury or loss of life.
!WARNING
12. Check for gas leaks.
13. Replace main furnace door.