WARNING: FIRE OR EXPLOSION HAZARD
Failure to follow this warning could result in a fire,
explosion, personal injury, or death.
Never purge a line into a combustion chamber. Never test
for gas leaks with an open flame. Use a commercially
available soap solution made specifically for the detection
of leaks to check all connections, as specified in ″Gas
Piping″ and ″Start-up Adjustment, and Safety Check″
sections of these instructions.
IV. SEQUENCE OF OPERATION
Using schematic diagram in Fig. 31, follow the sequence of
operation through the different modes. Read and follow diagram
very carefully.
NOTE: If power interruption occurs during ″call for heat″ (W/W1
or W/W1 and W2), the control will run the blower for the selected
blower off delay period after power is restored, if the thermostat is
still calling for gas heating. The amber LED will flash code 12
during this period, after which the LED will be ON continuous as
long as no faults are detected. After this period, the 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 igniter HSI, and gas valve GV.
A. Two-Stage Heating With Single-Stage Thermostat
(Adaptive Mode)
(See Fig. 27 for thermostat connections.)
NOTE: Low-heat-only switch, SW-1, selects either the low-heat-
only operation mode when ON, or adaptive heating mode when
OFF, in response to a call for heat. (See Fig. 32.)
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-gas-
heat or high-gas-heat operation. This selection is based upon the
stored history of the length 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-gas heat, the control CPU determines the
low-gas 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 will initially select low-heat for up to 16
minutes and then switch 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 wall thermostat ″calls for heat″, R-W/W1 circuit closes. The
furnace control performs a self check, verifies the low-heat and
high-heat pressure switch contacts LPS and HPS are open, and
starts inducer motor IDM in high speed.
1. Inducer Prepurge Period- If the furnace control CPU
selects low-heat operation the inducer motor IDM comes up
to speed, the low-heat pressure switch LPS closes, and the
furnace control CPU begins a 15-second prepurge period.
After the low-heat pressure switch re-closes the furnace
control CPU will begin a 15-second prepurge period, and
continue to run the inducer motor IDM at high-speed.
Fig. 50—Inducer Housing Drain Cap
A99118
Fig. 51—Filling Condensate Trap
A99119
—37—
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