3. Trial-for-ignition sequence—When ignitor warm-up period
is completed, main gas valve relay contacts MGVR-1 and
-2 (and high-heat pressure switch relay HPSR) close to
energize low-heat gas valve solenoid(s) GV and humidifier
terminal HUM. The gas valve opens, and 24-vac power is
supplied for a field-installed humidifier at terminals HUM
and C
OM
-24V. The low-heat (and high-heat) gas valve
solenoid(s) GV permit(s) gas flow to burners where it is
ignited. After 5 sec, ignitor HSI is de-energized, and a 2-sec
flame-proving period begins. If high-heat pressure switch
HPS fails to close on a call for high-gas heat and low-heat
pressure switch LPS closes, furnace operates at low-heat
gas flow rate until high-heat pressure switch closes.
4. Flame-proving—When burner flame is proved at flame-
proving sensor electrode FSE, the control CPU begins
blower on delay period and continues to hold gas valve GV
open. If burner flame is not proved within 2 sec, the control
CPU closes gas valve GV, and the control CPU repeats
ignition sequence for up to 3 more trials-for-ignition before
going to ignition lockout. LOCKOUT IS RESET AUTO-
MATICALLY after 3 hr, or by momentarily interrupting
115-vac power to furnace, or by interrupting 24-vac power
at SEC1 or SEC2 to control CPU (not at W/W1, G, R, etc.).
Opening thermostat R-W circuit will not reset an ignition
lockout. If flame is proved when flame should not be
present, the control CPU locks out of gas heating mode and
operates inducer motor IDM on high speed until flame is no
longer proved.
5. Blower on delay—If burner flame is proven, 45 sec after
gas valve GV is opened, blower motor BLWM is energized
at appropriate heating airflow, low-gas-heat or high-gas-
heat CFM. Simultaneously, EAC terminals EAC-1 and
EAC-2 are energized with 115 vac and remain energized as
long as blower motor BLWM is operating.
6. Switching from low- to high-gas heat—If furnace control
CPU switches from low-gas heat to high-gas heat, the
control CPU switches inducer motor IDM speed from low
to high. The high-heat pressure switch relay HPSR closes.
When inducer motor IDM provides sufficient pressure to
close high-heat pressure switch HPS, high-heat gas valve
solenoid GV is energized. Blower motor BLWM switches
to high-gas-heat airflow 5 sec after control CPU switches
from low-gas heat to high-gas heat.
7. Switching from high- to low-gas heat—The control CPU
will not switch from high-gas heat to low-gas heat while
thermostat R-W circuit is closed when a single-stage
thermostat is used.
8. Blower off delay—When thermostat is satisfied, R-W
circuit opens, de-energizing gas valve GV, stopping gas
flow to burners, and de-energizing humidifier terminals
HUM and C
OM
-24V. Inducer motor IDM remains energized
for a 5-sec post-purge period. A PSC blower motor BLWM
continues operating at the same airflow for 90, 135, 180, or
225 sec (depending on selection at blower off delay
switches SW-3 and SW-4). An ICM blower motor BLWM
which had been operating at low- or high-gas-heat airflow
operates at low-gas-heat airflow for 90 sec. If selected off
delay period exceeds 90 sec, ICM blower motor BLWM
operates at continuous blower airflow for remainder of off
delay period. The furnace control CPU is factory set for a
135-sec blower off delay.
II.
NON-ADAPTIVE HEATING MODE—TWO-STAGE
THERMOSTAT WITH 2-STAGE HEATING
NOTE:
The low-heat-only switch SW-2 in ON position selects
low-heat-only operation mode in response to closing thermostat
R-W1 circuit when high-heat-only switch SW-1 is in OFF posi-
tion. Closing thermostat R to W1 and W2 circuits is required for
high-gas-heat operation. When high-heat-only switch SW-1 is in
ON position, it always initiates high-gas-heat operation when
R-W1 circuit is closed, regardless of setting of low-heat-only
switch SW-2 and regardless of whether R-W2 circuit is closed or
open.
The start-up and shutdown functions and delays described in
Section I apply to 2-stage heating mode as well, except for
switching from low- to high-gas heat and vice versa.
1. When wall thermostat "calls for heat," R-W/W1 circuit
closes for low-gas heat or R to W1-and-W2 circuits close
for high-gas heat. The furnace control performs a self-
check, verifies low-heat and high-heat pressure switches’
contacts LPS and HPS are open, and starts inducer motor
IDM in low speed or high speed as appropriate.
2. Switching from low- to high-gas heat—If thermostat
R-W/W1 circuit for low-gas heat is closed and R-W2 circuit
for high-gas heat closes, the control CPU switches inducer
motor IDM speed from low to high. High-heat pressure
switch relay HPSR closes. When inducer motor IDM
provides sufficient pressure to close high-heat pressure
switch HPS, high-heat gas valve solenoid GV is energized.
Blower motor BLWM switches to high-gas-heat airflow 5
sec after R-W2 circuit closes.
3. Switching from high- to low-gas heat—If thermostat R-W2
circuit for high-gas heat opens and R-W/W1 circuit for
low-gas heat remains closed, the control CPU switches
inducer motor IDM speed from high to low. High-heat
pressure switch relay HPSR opens to de-energize high-heat
gas valve solenoid GV. When inducer motor IDM reduces
pressure sufficiently, high-heat pressure switch HPS opens.
Low-heat gas valve solenoid GV remains energized as long
as low-heat pressure switch LPS remains closed. Blower
motor BLWM switches to low-gas-heat airflow 5 sec after
R-W2 circuit opens.
III.
COOLING MODE
1. Single-Speed Cooling Outdoor Unit
a. The thermostat closes R to G-and-Y circuits. The R-Y
circuit starts outdoor unit, and R to G-and-Y/Y2 circuits
operate furnace blower motor BLWM on cooling air-
flow.
b. EAC terminals EAC-1 and EAC-2 are energized with
115 vac when blower motor BLWM is operating.
c. When thermostat is satisfied, R to G-and-Y circuits
open. The outdoor unit stops, and furnace blower motor
BLWM continues operating at cooling airflow for an
additional 90 sec.
2. Two-Speed Cooling Outdoor Unit
a. The thermostat closes R to G-and-Y1 circuits for low
cooling or closes R to G-and-Y1-and-Y/Y2 circuits for
high cooling. The R to Y1 circuits operate outdoor unit
on low-cooling speed.
PSC—The R-G circuit operates furnace blower motor
BLWM at low-cooling airflow (same airflow as
for low-gas heat and continuous blower).
ICM—The R to G-and-Y1 circuit operates furnace
blower motor BLWM at low-cooling airflow
(65% of single-speed cooling airflow; different
airflow than for low-gas heat).
The R to Y1-and-Y2 circuits operate outdoor unit on
high-cooling airflow, and R to G-and-Y2 (-and-Y1, for
ICM) circuits operate furnace blower motor BLWM at
high-cooling airflow.
—4—
