If a 2-stage heating thermostat is to be used, move SW-2 to ON
position at end of furnace installation. This overrides the built-in
control process for selecting high and low fire and allows the
2-stage thermostat to select gas heating modes. W2 from thermo-
stat must be connected to W2 on control terminal block.
CAUTION:
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 failure, 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 the
switch.
SEQUENCE OF OPERATION
Using schematic diagrams in Appendix A, follow the sequence of
operation through the different modes. Read and follow diagram
very carefully.
NOTE:
Permanent-split-capacitor (PSC) motors and G.E. Inte-
grated Control Motors (ICM) operate differently. PSC motors are
basically fixed-multiple-speed motors, that is they operate within
small ranges of speed. The ranges, usually 4, can be selected by
connecting to different 115-vac motor wires. The speed ranges are
small, about 100 RPM wide, and are dependent on the motor’s
synchronous speed (1200 for most furnaces). The characteristics of
PSC motors limit selections to fixed speeds between about 700 and
1100 RPM. The ICM motor speeds are infinitely variable from 300
to 1400 RPM and are dynamically controlled to precisely control
airflow CFM. The start and stop characteristics of PSC and ICM
motors differ in that PSC motors ramp up to speed rapidly and
coast to a stop slowly. ICM motors ramp up to speed at a
controlled rate to reduce start-up noise perception (4 to 11 sec,
depending on target operating CFM). ICM ramp-down time is the
same as ramp-up time. ICM ramp-up and ramp-down times are
additive to blower on and off delays, respectively. The PSC motor
is energized with 115 vac only when operating. The ICM is
energized with 115 vac whenever power is available at furnace
control, but operates only when 24-vac motor control input(s) are
ON. Other motor differences will be included in the following
information.
NOTE:
If a power interruption occurs during a "call for heat"
(W/W1 or W/W1-and-W2) and if thermostat is still calling for gas
heating, the control starts a 90-sec blower-only ON period 2 sec
after power is restored. The red LED will flash status code 12
during the 90-sec period, after which LED will be on continuously
as long as no faults are detected. PSC motors operate at low-gas-
heat or high-gas-heat CFM, respectively. ICM motors operate at
low-gas-heat CFM in both cases. After the 90-sec period, furnace
responds to thermostat normally.
Blower door must be installed for power to be conducted through
blower door interlock switch ILK to furnace control CPU, trans-
former TRAN, inducer motor IDM, blower motor BLWM, hot
surface ignitor HSI, and gas valve GV.
I.
ADAPTIVE HEATING MODE—SINGLE-STAGE
THERMOSTAT WITH 2-STAGE HEATING
NOTE:
With high-heat-only switch SW-1 in OFF position, low-
heat-only switch SW-2 selects either low-heat-only operation
mode when on (see section II below), or adaptive heating mode
when off in response to a call for heat. (See Appendix F.) When
high-heat-only switch SW-1 is in ON position, it always initiates
high-gas-heat operation when R-W/W1 circuit is closed, regardless
of setting of low-heat-only switch SW-2.
These furnaces 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-gas-
heat 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 starts up in either low- or high-gas heat. If furnace
starts up in low-gas heat, the control CPU determines low-gas-heat
on time (from 0 to 16 minutes) which is permitted before switching
to high-gas heat.
If power is interrupted, stored history is erased. When this
happens, the control CPU selects low-gas heat for 16 minutes and
then switches to high-gas heat, as long as thermostat continues to
call for heat. Subsequent selection is based on stored history of
thermostat cycle times.
When wall thermostat "calls for heat," R-W/W1 circuit closes. 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 appropri-
ate.
1. Inducer prepurge period—As inducer motor IDM comes up
to low speed or high speed, low-heat (and high-heat)
pressure switch contacts LPS (and HPS) close to begin a
15-sec prepurge period.
2. Ignitor warm-up—At the end of prepurge period, the hot
surface ignitor HSI is energized for a 17-sec ignitor
warm-up period.
EXAMPLE
START HERE SECTION
STEP
ACTION
YES
NO
GO TO
1.
Step 1 tells us to remove control door first and NOT TO REMOVE THE
BLOWER DOOR because it will erase status codes stored in memory. It then
asks the question, "Is RED LED status light on?" If low-heat pressure switch
was defective, a low-heat pressure switch did not open, and a status code
would be flashing, so the answer is YES. We go to Step 2.
2
19
—
2.
Step 2 asks the question, "Is RED LED status light blinking rapidly without a
pause?" If low-heat pressure switch was defective, a low-heat pressure switch
did not open, and a status code would be flashing, so the answer is NO. We go
to Step 4.
3
4
—
4.
Step 4 asks the question, "Is RED LED status light blinking ON/OFF slowly with
a combination of short and long flashes?" If low-heat pressure switch was de-
fective, a low-heat pressure switch did not open, and a status code would be
flashing, so the answer is YES. We go to Step 5.
5
7
—
5.
Step 5 tells us to determine the status code. The status code is a 2 digit num-
ber with the first digit determined by the number of short flashes and the sec-
ond digit by the number of long flashes. So we count the short and long flashes
and see that status code 23 is flashing and go to Step 6.
—
—
6
6.
Step 6 tells us to go to Status Code 23 section.
—
—
INDEX
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