SERVICE AND TECHNICAL SUPPORT MANUAL
Gas Furnace: (F/G)9MAC
Specifications subject to change without notice.
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440 04 4201 01
HPS will open and the gas rate will be changed to
intermediate-heat. The gas valve solenoid GV will remain
energized as long as the low pressure switch LPS
remains closed. When the inducer motor speed gets
within 15% of the required intermediate-heat RPM the
furnace control CPU will start a 5 second blower airflow
change delay. After the 5 second blower airflow change
delay is completed the blower airflow will transition to
minimum-heat airflow. At this point the furnace control
CPU will energize the PSR relay to open the NC contact
and slowly decrease the inducer motor speed to the
required minimum-heat RPM. When the PSR relay is
energized and the NC contact opens the furnace control
CPU will reduce the gas rate to minimum-heat RPM.
Two-Stage Thermostat and Two-Stage Intermediate/
Maximum Heating
NOTE
: In this mode the intermediate-heat only switch SW4-2
must be ON to select the intermediate-heat only operation
mode in response to closing the thermostat R to W1 circuit.
Closing the thermostat R to W1-and-W2 circuits always
causes maximum-heat operation, regardless of the setting of
the intermediate-heat only switch.
The wall thermostat ”calls for heat”, closing the R to W1 circuit
for intermediate-heat or closing the R to W1-and-W2 circuits
for maximum-heat. The furnace control performs a self-check,
and verifies the low, medium and high pressure switch contacts
LPS, MPS and HPS are open , then de-energizes the PSR
relay to close the NC contact.
The start up and shut down functions and delays described
above apply to the 2-stage intermediate/maximum heating
mode as well, except for switching from maximum- to
intermediate-heat.
1.
Switching from Maximum- to Intermediate-Heat
—
If
the thermostat R to W2 circuit opens, and the R to W1
circuit remains closed, the furnace control CPU will
gradually decrease the inducer motor speed to the
required intermediate-heat RPM. When the inducer
motor IDM reduces pressure sufficiently, the high
pressure switch HPS will open and the gas rate will be
changed to intermediate-heat. When the inducer motor
speed gets within 15% of the required intermediate-heat
RPM the furnace control CPU will start a 5 second
blower airflow change delay. After the 5 second blower
airflow change delay is completed the blower airflow will
transition to intermediate-heat airflow.
Cooling Mode
The thermostat “calls for cooling.”
1.
Single-Speed Cooling
The thermostat closes the R to G-and-Y circuits. The R to Y
circuit starts the outdoor unit, and the R to G-and-Y/Y2 circuits
start the furnace blower motor BLWM on cooling airflow.
Cooling airflow is based on the A/C selection shown in
Figure 4
.
The electronic air cleaner terminal EAC-1 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 at cooling airflow for an additional 90
seconds. Jumper Y/Y2 to DHUM to reduce the cooling off-delay
to 5 seconds. (See
Figure 3
).
Two
−
Stage Thermostat and 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 R to G-and-Y1 circuit starts the furnace
blower motor BLWM at low-cooling airflow which is the true
on-board CF selection as shown in
Figure 4
. The R to
Y1-and-Y2 circuits start the outdoor unit on high-cooling speed,
and the R to G-and-Y/Y2 circuits start the furnace blower motor
BLWM at high-cooling airflow. High-cooling airflow is based on
the A/C selection shown in
Figure 4
.
The electronic air cleaner terminal EAC-1 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-1 will remain energized for an additional 90
seconds. Jumper Y1 to DHUM to reduce the cooling off-delay
to 5 seconds. (See
Figure 3
)
Dehumidify Mode
The dehumidification output, H on the Thermostat should be
connected to the furnace control thermostat terminal DHUM.
When there is a dehumidify demand, the DHUM input is
activated, which means 24 vac signal is removed from the
DHUM input terminal. In other words, the DHUM input logic is
reversed. The DHUM input is turned ON when no dehumidify
demand exists. Once 24 vac is detected by the furnace control
on the DHUM input, the furnace control operates in Thermostat
mode. If the DHUM input is low for more than 48 hours, the
furnace control reverts back to non-Thermostat mode.
The cooling operation described above in the Cooling Mode
section also applies to operation with a Thermostat. The
exceptions are listed below:
1.
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 drop the blower airflow to 86%
of low cooling airflow which is the true on-board CF
selection as shown in
Figure 4
.
2.
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 airflow
to 86% of high-cooling airflow. High-cooling airflow is
based on the A/C selection shown in
Figure 4
.
3.
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.
Continuous Blower Mode
When the R to G circuit is closed by the thermostat, the blower
motor BLWM will operate at continuous blower airflow.
Continuous blower airflow selection is initially based on the CF
selection shown in
Figure 4
. Factory default is shown in
Figure 4
. Terminal EAC-1 is energized as long as the blower
motor BLWM is energized.
During a call for heat, the furnace control CPU will transition the
blower motor BLWM to continuous blower airflow,
minimum-heat airflow, or the mid-range airflow, whichever is
lowest. The blower motor BLWM will remain ON until the main
burners ignite then shut OFF and remain OFF for the
blower-ON delay (45 seconds in intermediate heat, and 25
seconds in maximum-heat), allowing the furnace heat
exchangers to heat up more quickly, then restarts at the end of
the blower-ON delay period at modulating or maximum-heat
airflow respectively.
The blower motor BLWM will revert to continuous-blower
airflow after the heating cycle is completed. When the
thermostat satisfies, the furnace control CPU will drop the
blower motor BLWM to minimum-heat airflow during the
selected blower-OFF delay period before transitioning to
continuous-blower airflow.
When the thermostat “calls for low-cooling”, the blower motor
BLWM will operate at low-cooling airflow. When the thermostat
is satisfied, the blower motor BLWM will operate an additional
90 seconds at low-cooling airflow before transitioning back to
continuous-blower airflow.
