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This furnace is equipped with a manual reset limit switch in
the gas control area. The switch will open and shut off power
to the gas valve if a flame rollout or overheating condition
occurs in the gas control area. DO NOT bypass the switch.
Correct inadequate combustion-air supply, component fail-
ure, or restricted flue gas passageway before resetting the
switch.
Before operating furnace, check each manual reset switch for
continuity. If necessary, press and release the button to reset
switch. The blower compartment door must be in place to
complete the 115-v circuit to furnace.
Step 2—Sequence of Operation
Using schematic diagram in Fig. 19, follow the sequence of
operation through the different modes. Read and follow diagram
very carefully.
NOTE: The GE ICM2+ BLWM speeds are infinitely variable
from 300 to 1400 rpm and are dynamically controlled to precisely
control airflow rate (CFM). The ICM2+ motor ramps to speed at
a controlled rate to reduce start-up noise perception (4 to 11 sec,
depending on the target CFM). The ICM2+ motor ramps down
slowly to a stop in the same time as ramp-up time. ICM2+ ramp-up
and ramp-down times are additive to blower-on and -off delays,
respectively. The ICM2+ is 115-v energized whenever power is
available at furnace control, but operates only when 24-v motor
control input(s) are on.
NOTE: If a power interruption occurs during a call for heat
(W/W1 or W/W1 and W2) and if the thermostat is still calling for
gas heating, the control will start a 90-sec blower only on period
2 sec after power is restored. The red LED will flash code 12
during the 90-sec period, after which the LED will be on
continuously as long as no faults are detected. The ICM2+ motor
will operate at low-gas heat CFM after a power interruption in
either low- or high-heat. After 90-sec period, furnace will respond
to the thermostat normally.
The blower door must be installed for power to be conducted
through blower door interlock switch (ILK) to furnace control
(CPU), transformer (TRAN), inducer motor (IDM), blower motor
(BLWM), hot surface ignitor (HSI), and gas valve (GV).
1. Adaptive Heating Mode—Single-Stage Thermostat with
2-Stage Heating (See Fig. 16 for thermostat connections.)
NOTE: In response to thermostat call for heat R to W/W1, control
selects high-heat heating only with HIGH HEAT ONLY switch
(SW1) set to ON regardless of LOW HEAT switch (SW2) setting.
With HIGH HEAT ONLY switch set to OFF, LOW HEAT switch
selects either low-heat heating only mode when set to ON, or the
adaptive heating mode when set to OFF in response to a thermostat
call for heat R to W/W1. (See Tables 6 and 7, and Fig. 18 for
description of switch settings or Fig. 19 for control circuit
diagram.)
This furnace can operate as a 2-stage furnace with a single-stage
thermostat because furnace control CPU includes a programmed
adaptive sequence of controlled operation, which selects low- or
high-gas heat operation. This selection is based upon the stored
history of the lengths of previous gas heating on/off periods of the
single-stage thermostat.
The furnace will start up in either low- or high-gas heat. If the
furnace starts up in low-heat, the control CPU determines low-heat
on time (from 0 to 16 minutes) which is permitted before switching
to high-heat.
If power is interrupted, the stored history is erased. When this
happens, the control CPU selects low-heat for 16 minutes and then
switches to high-heat, as long as the thermostat continues to call
for heat. Subsequent selection is based on stored history of
thermostat cycle times.
When thermostat calls for heat, R-W/W1 circuit closes. The
furnace control performs a self-check, verifies that the pressure
switch contacts for low- and high-heat (LPS and HPS) are open,
and starts motor IDM in low speed or high speed as appropriate.
a. Inducer prepurge period—A 15 sec prepurge period begins
when LPS contacts close. For high-heat operation, both
LPS and HPS must be closed. For low-heat operation, only
LPS must be closed.
b. Igniter warm-up—At end of the prepurge period, the HSI is
energized for a 17-sec igniter warm-up period.
c. Trial-for-ignition sequence—When igniter warm-up period
is completed, the main gas valve relay contacts (MGVR-1
and -2) (and high-heat pressure switch relay HPSR for
high-heat operation) close to energize low- and high-heat
gas valve solenoid(s) (GV) and humidifier terminal
(HUM). The gas valve opens, and 24-v power is supplied
for a field-installed humidifier at terminals HUM and
C
24v. The GV permits gas flow to the burners where it
OM
is ignited. After 5 sec, the HSI is de-energized, and a 2-sec
flame-proving period begins.
If the HPS fails to close on a call for high-gas heat and LPS closes,
the furnace operates at low-heat gas flow rate until HPS closes.
d. Flame-proving—When burner flame is proved at flame-
proving sensor electrode (FSE), the control CPU begins the
blower-on delay period and continues to hold GV open. If
burner flame is not proved within 2 sec, the control CPU
closes GV, and the control CPU repeats the ignition
sequence for up to 3 more trials-for-ignition before going
to ignition lockout. The CONTROL TERMINATES
LOCKOUT AUTOMATICALLY after 3 hr, or by momen-
tarily interrupting 115-v power to furnace, or by interrupt-
ing 24-v power at SEC1 or SEC2 to the control CPU (not
at W/W1, G, R, etc.). Opening thermostat R-W circuit will
not terminate an ignition lockout.
If flame is proved when flame should not be present, the control
CPU locks out of gas heating mode and operates IDM on high
speed until flame is no longer proved.
e. Blower-on delay—If burner flame is proved, 45 sec after
GV is opened BLWM is energized at the appropriate
heating airflow: low- or high-heat CFM. Simultaneously,
the EAC terminals are energized with 115v and remain
energized as long as BLWM is energized.
f. Switching from low- to high-gas heat—If furnace control
CPU switches from low- to high-heat, control CPU
switches the IDM speed from low to high. The high-heat
pressure switch relay (HPSR) closes. When IDM provides
sufficient pressure to close HPS, the high-heat gas valve
solenoid GV is energized. The BLWM switches to high-
heat airflow 5 sec after the control CPU switches from
low-gas heat to high-gas heat.
g. Switching from high- to low-gas heat—The control CPU
will not switch from high- to low-gas heat while the
thermostat R-W circuit is closed when a single-stage
thermostat is used.
h. Blower-off delay—When the thermostat is satisfied, the
R-W circuit is opened, the GV is de-energized, stopping
gas flow to burners, and terminals HUM and C
de-energized. The IDM remains energized for a 5 sec
14
24v are
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