- 4 -
2. TWO-STAGE THERMOSTAT AND TWO-STAGE
HEATING
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 de-energize the high-heat
pressure switch relay HPSR to close the NC contact and
slowly increase the inducer motor speed until the high-heat
pressure switch HPS closes. When the high-heat pressure
switch HPS closes, the high-heat gas valve solenoid GV-
HI is energized and the inducer motor RPM is noted by the
furnace control CPU. The RPM is used to evaluate vent
system resistance. This evaluation is then used to
determine the required RPM necessary to operate the
inducer motor in high-heat mode. The blower motor
BLWM will transition to high-heat airflow five seconds
after the furnace control CPU switches from low-heat to
high-heat.
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 energize the high-heat
pressure switch relay HPSR to open the NC contact and
slowly decrease the inducer motor speed to the required
low-heat RPM . When the high-heat pressure switch HPS
opens, the high-heat gas valve solenoid GV-HI is de-
energized. 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
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 Table 1.
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. 1.)
b. Single-Stage Thermostat and Two-Speed Cooling
(Adaptive Mode)
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
operation. 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. 1.) 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-
Table 1 - Cooling Tonnage vs. Airflow (CFM)
525
2
700
2
700
875
1050
875
700
875
2
1050
1050
1
1225
1225
1225
1400
1400
1225
1750
1
1750
1
1225
1750
2100
DEF.
DEF.
DEF.
040, 060, 3T-080
5T-080, 100
120
BASED ON 350 CFM/TON (SETUP SWITCH SW1-5 OFF)
SETUP SWITCH SW3 POSITIONS
MODEL
SIZE
600
2
800
2
800
1000 1200
1000
800 1000
2
1200
1200
1
1400
1400
1400
1600
1600
1400
2000
1
2000
1
1400
2000
2100
DEF.
DEF.
DEF.
040, 060, 3T-080
5T-080, 100
120
BASED ON 400 CFM/TON (SETUP SWITCH SW1-5 ON)
1. DEFAULT A/C AIRFLOW WHEN A/C SWITCHES ARE IN OFF POSITION
2. DEFAULT CONT. FAN AIRFLOW WHEN CF SWITCHES ARE IN OFF POSITION
3. SWITCH POSITIONS ARE ALSO SHOWN ON FURNACE WIRING DIAGRAM
SETUP SWITCH SW3 POSITIONS
MODEL
SIZE
AIR CONDITIONING (A/C) OR CONTINUOUS-FAN (CF)
AIRFLOW SELECTION CHART
