International comfort products F9MXT0401410A Service And Technical Support Manual page 25

SERVICE AND TECHNICAL SUPPORT MANUAL
15. TwoïStage Thermostat and TwoïStage Heating (See
Figure 17 ï Figure 22 for thermostat connections)
NOTE: In this mode the TT switch (SW1ï1) must be ON to
select the twoïstage thermostat mode in response to closing
the thermostat RïtoïW1 circuit. Closing the thermostat
RïtoïW1ïandïW2 circuits always causes highïheat operation,
regardless of the setting of the thermostat type.
The wall thermostat "calls for heat", closing the RïtoïW1 circuit
for lowïheat or closing the RïtoïW1 andïW2 circuits for
highïheat. The furnace control performs a selfïcheck, verifies
the lowïheat and highïheat pressure switch contacts LPS and
HPS are open, and starts the inducer motor IDM in
highïspeed.
The start up and shut down functions and delays described in
item 1. above apply to the twoïstage heating mode as well,
except for switching from lowï to highïheat and vice versa.
a. Switching from Lowï to HighïHeatïIf the thermostat
Rïtoï W1 circuit is closed and the RïtoïW2 circuit
closes, the furnace control CPU will switch the inducer
motor IDM speed from low to high. The highïheat
pressure switch relay HPSR is deïenergized to close
the NC contact. When sufficient pressure is available the
highïheat pressure switch HPS closes, and the
highïheat gas valve solenoid GVïHI is energized. The
blower motor BLWM will switch to HI HEAT speed five
seconds after the RïtoïW2 circuit closes.
b. Switching from Highï to LowïHeatïIf the thermostat
RïtoïW2 circuit opens, and the RïtoïW1 circuit
remains closed, the furnace control CPU will switch the
inducer motor IDM speed from high to low. The
highïheat pressure switch relay HPSR is energized to
open the NC contact and deïenergize the highïheat
gas valve solenoid GVïHI. When the inducer motor IDM
reduces pressure sufficiently, the highïheat pressure
switch HPS will open. The gas valve solenoid GVïM will
remain energized as long as the lowïheat pressure
switch LPS remains closed. The blower motor BLWM
will switch to LO HEAT speed 5 seconds after the
RïtoïW2 circuit opens.
16. Cooling ModeïThe thermostat "calls for cooling."
a. SingleïSpeed Coolingï
(See Figure 17 ï Figure 22 for thermostat connections)
The thermostat closes the RïtoïGïandïY circuits. The
RïtoïY circuit starts the outdoor unit, and the furnace
control RïtoïGïandïY/Y2 circuits start the furnace
blower motor BLWM on cool speed.
The electronic air cleaner terminal EAC 1 AMP is
energized with 115 vac when the blower motor BLWM
is operating.
When the thermostat is satisfied, the RïtoïG andïY
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 offïdelay to 5
seconds. (See Figure 4)
b. TwoïSpeed Cooling
The thermostat closes the RïtoïGïandïY1 circuits for
lowïcooling or closes the RïtoïGïandïY1ïandïY2 circuits for
highïcooling. The RïtoïY1 circuit starts the outdoor unit on
lowïcooling speed, and the furnace control RïtoïGïandïY1
circuit starts the furnace blower motor BLWM on lowïcool
speed (same speed as LO HEAT). The RïtoïY1ïandïY2
circuits start the outdoor unit on highïcooling speed, and the
furnace control RïtoïGïandïY1 and ïY2 circuits start the
furnace blower motor BLWM on COOL speed. The electronic
air cleaner terminal EAC 1 AMP is energized with 115 vac
whenever the blower motor BLWM is operating. When the
thermostat is satisfied,
RïtoïGïandïY1ïandïY2 circuits are opened. The outdoor unit
stops, and the furnace blower BLWM and electronic air cleaner
terminal EAC 1 AMP will remain energized for an additional 90
seconds. Jumper Y1 to DHUM to reduce the cooling offïdelay
to 5 seconds. (See Figure 4)
25
the RïtoïGïandïY1 or
Specifications subject to change without notice.
Gas Furnace: (F/G)9MXT
17. Dehumidify Mode
(See Figure 17 ï 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 ï When the RïtoïGïandïY1 circuit is
closed and there is a demand for dehumidification,the
furnace blower motor BLWM will continue running at
lowïcool speed (same speed as LO HEAT).
b. High cooling ïWhen the RïtoïGïandïY/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 10ïminute cycle will
continue as long as there is a call for cooling.
c. Cooling offïdelay ï When the "call for cooling" is
satisfied and there is a demand for dehumidification, the
cooling blowerïoff delay is decreased from 90 seconds
to 5 seconds.
18. Continuous Blower Mode
When the RïtoïG circuit is closed by the thermostat, the
blower motor BLWM will operate on continuousïblower
speed LO HEAT speed. Terminal EAC 1 AMP is
energized as long as the blower motor BLWM is
energized. During a call for heat, the blower BLWM will
stop during igniter warmïup (17 seconds), ignition (7
seconds), and blowerïON delay (45 seconds in
lowïheat, and 25 seconds in highïheat), allowing the
furnace heat exchangers to heat up more quickly, then
restarts at the end of the blowerïON delay period at LO
HEAT or HI HEAT speed respectively. The blower motor
BLWM will revert to continuousïblower speed after the
heating cycle is completed. In highïheat, the furnace
control CPU will hold the blower motor BLWM at HI HEAT
speed during the selected blowerïOFF delay period
before reverting to continuousïblower speed.
When the thermostat "calls for lowïcooling", the blower
motor BLWM will operate at lowïcool speed (same
speed as LO HEAT).
When the thermostat "calls for highïcooling", 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 continuousïblower speed.
When the RïtoïG 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 ï 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
RïtoïW/W1ïandïY1 or RïtoïW/W1ïandïY1ïandïG
circuits are energized the furnace control CPU will switch
to or turn on the blower motor BLWM at lowïcool speed
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