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OPERATING SEQUENCE
The operating sequences for the heating, cooling, and
fan modes are described below. Refer to the field and
unit wiring diagram (Figure 12, page 32).
Heating Mode
1. On a call for 1st stage heat, the thermostat closes and
applies 24 VAC to terminal W1 on the control board.
2. Safety Circuit Proving - The control board verifies
continuity through the 24VAC limit control circuit. If the
high temperature limit switch, inducer limit, and flame
roll-out switch are closed and both the inducer hall
effect sensor and vent pressure switches are open, the
control board will then begin pressure switch proving. If
an open limit is detected, the control board will energize
the indoor blower motor and operate it continuously
while the limit switch circuit is open. All other system
functions will be inoperable until the limit circuit closes.
While the limit circuit is open, the control will display
Open Limit Switch Fault. See Table 5, (page 22).
3. The furnace control checks for continuity (24 VAC) across
the hall effect sensor and pressure switch. If either the
hall effect sensor or vent pressure switch is closed, the
heat mode sequence will not continue. If either remains
closed, the control will display a Pressure Switch Closed
with Inducer Off Fault until the fault condition clears.
4. Hall Effect Sensor and Pressure Switch Proving - The
control energizes the induced draft motor on High speed
and waits for the hall effect sensor and vent pressure
switch to close. If the hall effect sensor switch does
not close within 12 seconds, the control will display a
Pressure Switch Open with Inducer On Fault. If the hall
effect sensor switch does not close before 60 seconds
expires, the control will de-energize the inducer for 5
minutes and continue to display a Pressure Switch Open
with Inducer On Fault during the 5 minute inducer off
period. After the inducer has been off for 5 minutes, the
control energizes the inducer motor and restarts the 60
second pressure switch proving period.
5. Pre-Purge - If the hall effect sensor switch closes the
control runs the inducer on high speed for a 30 second
pre-purge time while monitoring the thermostat call for
heat, limit switch circuit, vent pressure switch, and flame
sense.
6. Igniter Warm Up - When the 30 second pre-purge time
has elapsed, the control begins the Hot Surface Ignition
(HSI) warm-up as follows:
Initial Power up: After 30 seconds of igniter warm-up, the
gas valve will open. The igniter circuit will stay energized
for 3 seconds after the gas valve opens.
After Initial Power up: The control has a programmed
adaptive ignition feature which varies the warm-up
period as follows: If ignition is successful the warm-up
is reduced by 3 seconds on each subsequent call for
heat until ignition failure occurs. Upon ignition failure,
the warm -up is increased by 3 seconds on the next try.
If successful, the timing remains fixed at this level. In
general, whenever ignition failure occurs the warm-up
interval is increased by 3 seconds on the next try.
If successful, it remains there. Minimum and maximum
warm-up time limits are set at 9 and 54 seconds.
7. Flame Proving / Ignition / Low Heat Operation - The
furnace control must prove flame via the flame sensor
5 seconds after the high main gas valve is energized.
• If flame is sensed, burners are on and the igniter cools
off. The Yellow LED will be lit.
• If no flame is sensed, the gas valve closes immediately
and the inducer continues to run on high. A second
trial for ignition (step 6) begins. If no flame is sensed
on the fifth try for ignition, the furnace control is locked
and the control will display 1 Hour Lockout Fault. The
thermostat must be opened for at least 10 seconds to
reset the furnace control after a lock out. Otherwise,
the furnace will attempt another ignition sequence
in 1 hour.
Low Fire Heat (Stage 1): If flame is present at the end
of the trial for ignition time, the control leaves High main
gas valve energized and inducer motor on High speed
for the first 15 seconds, and begins the heat blower
22 second on delay. Control inputs are continuously
monitored while the call for heat remains. The control
will always energize the gas valve on High fire with the
inducer on High speed for the first 15 seconds of any
heating cycle. After this 15 second period, the control
switches the gas valve to Low fire and inducer to Low
speed if no second stage (W2) demand is present.
8. Blower Start-Up - After flame is proven, the furnace
control energizes the circulating air blower after 22
seconds. The blower will then ramp up to the airflow
selected for heating.
9. High Heat Staging (with 2 Stage Heat Thermostat) - Unit
will always start in High heat operation. See step 7 for
start up sequence. After the initial 15 second start up
sequence is satisfied, the control will move to Low heat
and wait for a W2 call from a 2-stage thermostat. After a
second stage heat demand is present (W2 energized),
the gas valve will open to High heat mode increasing
the flow of gas to the burners while the inducer and
blower motor ramp to high speed.
Single Stage Heat Thermostat with Automatic Heat
Staging - Automatic heat staging allows the unit to
operate as a two stage gas furnace with the use of a
single stage heating thermostat. With the ignition control
AUTO HEAT STAGE jumper repositioned from the OFF
position to the ON position, see Figure 11 (page 31), the
gas furnace will automatically switch to High heat mode
of operation after a 10 minute delay. The induced draft
motor and blower motor will both ramp to high speed.
10. When the thermostat has been satisfied, gas valve is
de-energized.
11. The inducer motor is de-energized after a 30 second
post purge.
12. The circulating air blower will remain energized for
approximately 120 seconds (factory setting) after the
demand for heat is no longer present.
13. Abnormal Conditions - If a limit switch opens during
heating operation the gas valve is de-energized
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