79
high-- heat airflow five seconds after the furnace control
CPU switches from low--heat to high --heat.
g. Switching from High-- to Low--Heat --The furnace con-
trol CPU will not switch from high--heat to low--heat while
the thermostat R--to--W circuit is closed when using a sin-
gle-- stage thermostat.
h. Blower--Off Delay --When the thermostat is satisfied, the
R to W circuit is opened, de--energizing the gas valveGV--
M, stopping gas flow to the burners, and de--energizing the
humidifier terminal HUM. The inducer motor IDM will re-
main energized for a 15--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 (depending
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. 40 and 41 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 pres-
sure switch HPS closes, and the high--heat gas valve sole-
noid GV --HI is energized. The blower motor BLWM will
transition 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 pres-
sure 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 sufficient-
ly, 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 sec-
onds after the R --to-- W2 circuit opens.
3. Cooling mode
The thermostat “calls for cooling”.
a. Single--Speed Cooling--
See Fig. 40 and 41 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. 65. The electronic air cleaner
terminal EAC--1 is energized with 115 vac when the blow-
er motor BLWM is operating.
When the thermostat is satisfied, the R--to--G--and--Y cir-
cuits are opened. The outdoor unit will stop, and the fur-
nace blower motor BLWM will continue operating at cool-
ing airflow for an additional 90 seconds. Jumper Y/Y2 to
DHUM to reduce the cooling off-- delay to 5 seconds. See
Fig. 40.
b. Single-- Stage Thermostat and Two--Speed Cooling
(Adaptive Mode) --
See Fig. 40 and 41 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. 40. When ACRDJ is 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. 65.
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. 40.
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. 40.
c. Two--Stage Thermostat and Two--Speed Cooling
See Fig. 40 and 41 for thermostat connections
NOTE: The air conditioning relay disable jumper ACRDJ must
be disconnected to allow thermostat control of the outdoor unit
staging. See Fig. 40.
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 (continuous fan) selection as shown in Fig. 69.
The R -- to -- Y1-- and--Y2 circuits start the outdoor unit on
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