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 tran-
sition to high-heat airflow five seconds after the furnace
control CPU switches from low-heat to high-heat.
Switching from High- to Low-Heat -The furnace control
CPU will not switch from high-heat to low-heat while the
thermostat R to W circuit is closed when using a single-
stage thermostat.
g. Blower-Off Delay - When the thermostat is satisfied, the R
to W circuit is opened, de-energizing the gas valve GV-M,
stopping gas flow to the burners, and de-energizing the
humidifier terminal HUM. The inducer motor IDM will
remain energized for a 5-second post-purge period. The
blower motor BLWM and air cleaner terminal EAC-1 will
remain energized at low-heat airflow or transition to
low-heat airflow for 90, 120, 150, or 180 seconds (depend-
ing on selection at blower-OFF delay switches). The
furnace control CPU is factory-set for a 120-second
blower-OFF delay.
2. Two-Stage Thermostat and Two-Stage Heating
See Fig. 30 for thermostat connections.
NOTE:
In this mode the low-heat only switch SW1-2 must be ON
to select the low-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 high-heat operation, regardless
of the setting of the low-heat only switch.
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 2-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 tran-
sition to high-heat airflow 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 transition to low-heat airflow five
seconds after the R to W2 circuit opens.
3. Cooling mode
The thermostat
″
calls for cooling
″
.
a. Single-Speed Cooling-
See Fig. 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 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 Fig. 55.
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
Fig. 32.
b. Single-Stage
Thermostat
and
Two-Speed
Cooling
(Adaptive Mode) -
See Fig. 31 for thermostat connections
This furnace can operate a two-speed cooling unit with a
single-stage thermostat because the furnace control CPU
includes a programmed adaptive sequence of controlled
operation, which selects low-cooling or high-cooling op-
eration. This selection is based upon the stored history of
the length of previous cooling period of the single-stage
thermostat.
NOTE:
The air conditioning relay disable jumper ACRDJ must
be connected to enable the adaptive cooling mode in response to a
call for cooling. (See Fig. 32.) When in place the furnace control
CPU can turn on the air conditioning relay ACR to energize the
Y/Y2 terminal and switch the outdoor unit to high-cooling.
The furnace control CPU can start up the cooling unit in either
low- or high-cooling. If starting up in low-cooling, the furnace
control CPU determines the low-cooling on-time (from 0 to 20
minutes) which is permitted before switching to high-cooling.
If the power is interrupted, the stored history is erased and the
furnace control CPU will select low-cooling for up to 20 minutes
and then energize the air conditioning relay ACR to energize the
Y/Y2 terminal and switch the outdoor unit to high-cooling, as long
as the thermostat continues to call for cooling. Subsequent
selection is based on stored history of the thermostat cycle times.
The wall thermostat
″
calls for cooling
″
, closing the R to G-and-Y
circuits. The R to Y1 circuit starts the outdoor unit on low-cooling
speed, and the R to G-and-Y1 circuits starts the furnace blower
motor BLWM at low-cooling airflow which is the true on-board
CF selection as shown in Fig. 55.
If the furnace control CPU switches from low-cooling to high-
cooling, the furnace control CPU will energize the air conditioning
relay ACR. When the air conditioning relay ACR is energized the
R to Y1-and-Y2 circuits switch the outdoor unit to high-cooling
speed, and the R to G-and-Y1-and-Y/Y2 circuits transition the
furnace blower motor BLWM to high-cooling airflow. High-
cooling airflow is based on the A/C selection shown in Fig 55.
NOTE:
When transitioning from low-cooling to high-cooling the
outdoor unit compressor will shut down for 1 minute while the
furnace blower motor BLWM transitions to run at high-cooling
airflow.
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-Y circuit 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 Fig. 32.)
c. Two-Stage Thermostat and Two-Speed Cooling
See Fig. 31 for thermostat connections
44