47
441 01 2121 01
The wall thermostat “calls for heat”, closing the R−to−W
circuit. The furnace control performs a self−check, verifies the
low−heat and high−heat pressure switch contacts LPS and
HPS are open, and starts the inducer motor IDM in
high−speed.
a. Inducer Prepurge Period
(1.) 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. If the low−heat
pressure switch LPS fails to remain closed the
inducer motor IDM will remain running at high−
speed. 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.
(2.) If the furnace control CPU selects high−heat
operation, the inducer motor IDM remains run-
ning at high−speed, and the high−heat pres-
sure 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.
Figure 55 − Cooling (A/C) or Continuous−Fan (CF) Airflow Selection Chart
SWITCH SW2 or SW3 POSITIONS
SWITCH SW2 or SW3 POSITIONS
b. Igniter Warm−Up −At the end of the prepurge peri-
od, the Hot−Surface Igniter HSI is energized for a
17−second igniter warm−up period.
c. Trial−For−Ignition Sequence −When the igniter
warm−up period is completed the main gas valve
relay contact GVR closes to energize the gas valve
solenoid GV−M. The gas valve solenoid GV−M per-
mits gas flow to the burners where it is ignited by
the HSI. Five seconds after the GVR closes, a
2−second flame proving period begins. The HSI
igniter will remain energized until the flame is
sensed or until the 2−second flame proving period
begins. If the furnace control CPU selects high−
heat operation, the high−heat gas valve solenoid
GV−HI is also energized.
d. Flame−Proving − When the burner flame is proved
at the flame−proving sensor electrode FSE, the in-
ducer motor IDM switches to low−speed unless the
furnace is operating in high−heat, and the furnace
control CPU begins the blower−ON delay period
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