FIRE AND EXPLOSION HAZARD
Failure to follow this warning could result in a fire, explosion,
personal injury, or death.
Never purge a gas 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.
Step 4—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.
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.
If the furnace control CPU selects high-heat operation, the
inducer motor IDM remains running at high-speed, and the
high-heat pressure switch relay HPSR is de-energized to close
the NC contact. When sufficient pressure is available the
high-heat pressure switch HPS closes, and the high-heat gas
valve solenoid GV-HI is energized. The furnace control CPU
begins a 15-second prepurge period after the low-heat pressure
switch LPS closes. If the high-heat pressure switch HPS fails
to close and the low-heat pressure switch LPS closes, the
furnace will operate at low-heat gas flow rate until the
high-heat pressure switch closes for a maximum of 2 minutes
after ignition.
2. Igniter Warm-Up-At end of the inducer prepurge period, the
Hot Surface Igniter HSI is energized for a 17-sec igniter
warm-up period.
3. Trial-For-Ignition Sequence-When the igniter warm-up pe-
riod is completed the main gas valve relay contacts GVR close
to energize the gas valve GV, the gas valve opens. The gas
valve GV permits gas flow to the burners where it is ignited by
the Hot Surface Igniter HSI. Five seconds after the GVR
closes, a 2-second flame period begins. The HSI igniter will
remain energized until the flame is sensed or until the
2-second flame proving period begins.
4. Flame-Proving-When burner flame is proved at the flame-
proving sensor electrode FSE, the inducer motor IDM
switches to low-speed unless running at high-speed, and the
furnace control CPU begins the blower-ON delay period and
continues to hold the gas valve GV-M open. If the burner
flame is not proved within two seconds, the control CPU will
close the gas valve GV-M, and the control CPU will repeat the
ignition sequence for up to three more Trials-For-Ignition
before going to Ignition-Lockout. Lockout will be reset
automatically after three hours, by momentarily interrupting
115 vac power to the furnace, or by interrupting 24 vac power
Fig. 51—Filling Condensate Trap
A99119
38
