SERVICE AND TECHNICAL SUPPORT MANUAL
Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
4
440 04 4321 02
STARTUP, ADJUSTMENT, AND SAFETY
CHECK
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.
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.), but must be no less than 0.16 psig (4.5-in.
w.c.).
4. Blower door must be in place to complete 115-V
electrical circuit to furnace.
UNIT OPERATION HAZARD
Failure to follow this caution may result in intermittent unit
operation or performance satisfaction.
These furnaces are equipped with a manual reset limit
switch in burner assembly. This switch opens and shuts off
power to the gas valve is an overheat condition (flame
rollout) occurs in burner assembly. Correct inadequate
combustionair supply 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 low pressure switch is closed
during a call for heat.
There are two humidifier outputs on the furnace control. The
115 VAC HUM terminal is energized when the Induced Draft
Relay closes. The 24 VAC HUM terminal is energized when
the low pressure closes during a call for heat. Connect an
accessory 115 VAC accessory humidifier to the 115 VAC HUM
terminal and L2 on the furnace control. Connect a 24 VAC
humidifier to the 24 VAC HUM terminal and C screw terminal
strip on the control board thermostat strip.
Thermostat Setup Switch
This furnace can be installed with either a singlestage heating
or a twostage heating thermostat. Setup switch (TT) is used to
configure the furnace for single or two stage thermostat
operation. (See
Figure 4
)
For singlestage thermostats, connect thermostat W to W/W1
at furnace control terminal block. (See
Figure 7
) For
singlestage thermostats, the control will operate for 12
minutes on low heat, then switch to high heat if heat call
remains. Setup switch (TT) must be in the factoryshipped OFF
position. See
Figure 7
and
Figure 15
for setup switch
information.
If a twostage heating thermostat is to be used, move setup
switch (TT) to ON position before starting furnace. This
overrides builtin control process for selecting high and low fire
and allows the twostage thermostat to select gas heating
modes. The W2 from thermostat must be connected to W2 on
control terminal block.
Prime Condensate Trap with Water
UNIT OPERATION HAZARD
Failure to follow this caution may result in intermittent unit
operation or performance satisfaction.
Condensate trap must be PRIMED or proper draining may
not occur. The condensate trap has two internal chambers
which can ONLY be primed by pouring water into the
inducer drain side of condensate trap.
CAUTION
!
1. Remove upper and middle collector box drain plugs
opposite of the condensate trap. (See
Figure 1
)
2. Connect field-supplied 1/2-in. (13 mm) OD tube to upper
collector box drain connection.
3. Insert field-supplied funnel into tube.
4. Pour one quart of water into funnel/tube. Water should
run through collector box, overfill condensate trap, and
flow into open field drain.
5. Remove funnel; replace collector box drain plug.
6. Connect field-supplied 1/2-in. (13 mm) OD tube to middle
collector box drain port.
7. Pour one quart of water into funnel/tube. Water should
run through collector box, overfill condensate trap, and
flow into open field drain.
8. 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.
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.
