SERVICE AND TECHNICAL SUPPORT MANUAL
Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
440 04 4321 02
9
Adjust Temperature Rise
NOTE
: Blower door must be installed when taking temperature
rise reading. Leaving blower door off will result in incorrect
temperature measurements.
FURNACE OVERHEATING HAZARD
Failure to follow this caution may result in shortened furnace
life.
Set air temperature rise within limits specified on the rating
plate to prevent reduced life of furnace components.
CAUTION
!
FURNACE DAMAGE HAZARD
Failure to follow this caution may result in overheating the
heat exchangers or condensing flue gases in heat
exchanger areas not designed for condensate.
Temperature rise must be within limits specified on unit
rating plate.
CAUTION
!
UNIT DAMAGE HAZARD
Failure to follow this caution may result in component
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 setup switch (TT) in ON position. Jumper R to W/W1 and
W2 to check highgasheat temperature rise. To check
lowgasheat temperature rise, remove jumper to W2.
Determine air temperature rise for both high and low gas heat.
Do not exceed temperature rise ranges specified on unit rating
plate for high and low gas heat.
This furnace must operate within the temperature rise ranges
specified on the furnace rating plate. Determine the air
temperature as follows:
a. 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.
b. When thermometer readings stabilize, subtract
returnair temperature from supplyair temperature to
determine temperature rise.
If the temperature rise is outside this range, check the
following:
a. Gas input for lowand high gas heat operation.
b. Derate for altitude if applicable.
c. Return and supply ducts for excessive restrictions
causing static pressures greater than 0.50in. w.c.
d. Adjust temperature rise by adjusting blower speed.
e. Increase blower speed to reduce temperature rise.
f. Decrease blower speed to increase temperature rise.
ELECTRICAL OPERATION HAZARD
Failure to follow this warning could result in personal injury
or death.
Disconnect 115V electrical power before changing speed
tap.
!
WARNING
Table 3
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 lowheat, the following connections can be made at
LOHT on control:
a. Speed 2 (Orange)
b. Speed 1 (Red) Factory Setting
(Read following caution before changing taps).
UNIT DAMAGE HAZARD
To avoid operating outside the rise range and avoid
component damage:
1. NEVER connect Speed Tap 1 (Red) wire to “HI HT.”
2. NEVER connect Speed Tap 2 (Orange) wire to “HI
HT” on all models.
CAUTION
!
To change blower motor speed selections for heating mode,
remove blower motor lead from control HIHT terminal. (See
Figure 4
) Select desired blower motor speed lead from one of
the other motor leads and relocate it to HIHT terminal. (See
Table 3
for lead color identification.) Reconnect original lead on
SPARE terminal. Follow this same procedure for proper
selection of LOHT and COOL speed selection.
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 of switches see
Figure 4
and
Figure 15
for location of switches on control
center.
a. Remove blower door if installed.
b. Turn setup switches ON or OFF for desired blower off
delay. See
Table 4, Figure 4
and
Figure 15.
c. Proceed to “Set Blower Off Delay” before installing
blower door.
Table 4
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 Lo Heat speed.
