SERVICE AND TECHNICAL SUPPORT MANUAL Gas Furnace: (F/G)9MXT
Specifications subject to change without notice.
26 440 04 4321 04
15. Two−Stage Thermostat and Two−Stage Heating (See
Figure 17 − Figure 22 for thermostat connections)
NOTE: In this mode the TT switch (SW1−1) must be ON to
select the two−stage thermostat mode in response to closing
the thermostat R−to−W1 circuit. Closing the thermostat
R−to−W1−and−W2 circuits always causes high−heat operation,
regardless of the setting of the thermostat type.
The wall thermostat “calls for heat”, closing the R−to−W1 circuit
for low−heat or closing the R−to−W1 and−W2 circuits for
high−heat. The furnace control performs a self−check, verifies
the low−heat and high−heat pressure switch contacts LPS and
HPS are open, and starts the inducer motor IDM in
high−speed.
The start up and shut down functions and delays described in
item 1. above apply to the two−stage heating mode as well,
except for switching from low− to high−heat and vice versa.
a. Switching from Low− to High−Heat−If the thermostat
R−to− W1 circuit is closed and the R−to−W2 circuit
closes, the furnace control CPU will switch the inducer
motor IDM speed from low to high. The high−heat
pressure switch relay HPSR is de−energized to close
the NC contact. When sufficient pressure is available the
high
−heat pressure switch HPS closes, and the
high−heat gas valve solenoid GV−HI is energized. The
blower motor BLWM will switch to HI HEAT speed five
seconds after the R−to−W2 circuit closes.
b. Switching from High− to Low−Heat−If the thermostat
R−to−W2 circuit opens, and the R−to−W1 circuit
remains closed, the furnace control CPU will switch the
inducer motor IDM speed from high to low. The
high−heat pressure switch relay HPSR is energized to
open the NC contact and de−energize the high−heat
gas valve solenoid GV−HI. When the inducer motor IDM
reduces pressure sufficiently, the high−heat pressure
switch HPS will open. The gas valve solenoid GV−M will
remain energized as long as the low−heat pressure
switch LPS remains closed. The blower motor BLWM
will switch to LO HEAT speed 5 seconds after the
R−to−W2 circuit opens.
16. Cooling Mode−The thermostat “calls for cooling.”
a. Single−Speed Cooling−
(See Figure 17 − Figure 22 for thermostat connections)
The thermostat closes the R−to−G−and−Y circuits. The
R−to−Y circuit starts the outdoor unit, and the furnace
control R−to−G−and−Y/Y2 circuits start the furnace
blower motor BLWM on cool speed.
The electronic air cleaner terminal EAC is energized
with 115 vac when the blower motor BLWM is operating.
When the thermostat is satisfied, the R−to−G and−Y
circuits are opened. The outdoor unit will stop, and the
furnace blower motor BLWM will continue operating on
the COOL speed for an additional 90 seconds. Jumper
Y/Y2 to DHUM to reduce the cooling off−delay to 5
seconds. (See Figure 4)
b. Two−Speed Cooling
The thermostat closes the R−to−G−and−Y1 circuits for
low−cooling or closes the R−to−G−and−Y1−and−Y2 circuits for
high−cooling. The R−to−Y1 circuit starts the outdoor unit on
low−cooling speed, and the furnace control R−to−G−and−Y1
circuit starts the furnace blower motor BLWM on low−cool
speed (same speed as LO HEAT). The R−to−Y1−and−Y2
circuits start the outdoor unit on high−cooling speed, and the
furnace control R−to−G−and−Y1 and −Y2 circuits start the
furnace blower motor BLWM on COOL speed. The electronic
air cleaner terminal EAC is energized with 115 vac whenever
the blower motor BLWM is operating. When the thermostat is
satisfied, the R−to−G−and−Y1 or R−
to−G−and−Y1−and−Y2
circuits are opened. The outdoor unit stops, and the furnace
blower BLWM and electronic air cleaner terminal EAC will
remain energized for an additional 90 seconds. Jumper Y1 to
DHUM to reduce the cooling off−delay to 5 seconds. (See
Figure 4)
17. Dehumidify Mode
(See Figure 17 − Figure 22 for humidity sensing
thermostat connections) The dehumidification output, H
on the humidity sensing thermostat should be connected
to the furnace control thermostat terminal DHUM. When
there is a dehumidify demand, the DHUM input is
activated, (24 vac signal is removed from the H input
terminal). In other words, the DHUM input logic is
reversed. The DHUM input is turned ON when no
dehumidify demand exists.
Activation/Deactivation
Once 24 vac is detected by the furnace control on the
DHUM input, the furnace control operates enables the
dehumidify mode. If the DHUM input is off for more than
48 hours, the furnace control disables the dehumidify
mode.
The cooling operation described in item 3. above also
applies to operation with a humidity sensing thermostat.
The exceptions are listed below:
a. Low cooling − When the R−to−G−and−Y1 circuit is
closed and there is a demand for dehumidification,the
furnace blower motor BLWM will continue running at
low−cool speed (same speed as LO HEAT).
b. High cooling −When the R−to−G−and−Y/Y2 circuit is
closed and there is a demand for dehumidification,the
furnace blower motor BLWM will drop the blower speed
from COOL to HI HEAT for a maximum of 10 minutes
before reverting back to COOL speed. If there is still a
demand for dehumidification after 20 minutes, the
furnace control CPU will drop the blower speed back to
HI HEAT speed. This alternating 10−minute cycle will
continue as long as there is a call for cooling.
c. Cooling off−delay − When the “call for cooling” is
satisfied and there is a demand for dehumidification, the
cooling blower−off delay is decreased from 90 seconds
to 5 seconds.
18. Continuous Blower Mode
When the R−to−G circuit is closed by the thermostat, the
blower motor BLWM will operate on continuous−blower
speed LO HEAT speed. Terminal EAC is energized as
long as the blower motor BLWM is energized. During a
call for heat, the blower BLWM will stop during igniter
warm−up (17 seconds), ignition (7 seconds), and
blower−ON delay (45 seconds in low−heat, and 25
seconds in high−heat), allowing the furnace heat
exchangers to heat up more quickly, then restarts at the
end of the blower−ON delay period at LO HEAT or HI
HEAT speed respectively. The blower motor BLWM will
revert to continuous−blower speed after the heating
cycle is completed. In high−heat, the furnace control
CPU will hold the blower motor BLWM at HI HEAT speed
during the selected blower−OFF delay period before
reverting to continuous−blower speed.
When the thermostat “calls for low−cooling”, the blower
motor BLWM will operate at low−cool speed (same
speed as LO HEAT).
When the thermostat “calls for high−cooling”, the blower
motor BLWM will operate at COOL speed. When the
thermostat is satisfied, the blower motor BLWM will
operate an additional 90 seconds on COOL speed
before reverting back to continuous−blower speed.
When the R−to−G circuit is opened, the blower motor
BLWM will continue operating for an additional 5
seconds, if no other function requires blower motor
BLWM operation.
19. Heat Pump
(See Figure 17 − Figure 22 for thermostat connections)
When installed with a heat pump, the furnace control
automatically changes the timing sequence to avoid long
blower off times during demand defrost cycles. When the
R−to−W/W1−and−Y1 or R−to−W/W1−and−Y1−and−G
circuits are energized the furnace control CPU will switch
to or turn on the blower motor BLWM at low−cool speed
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