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Sequence of Operation
NOTE: The ignition control thermostat selection DIP switch
is factory-set in the "TWO-STAGE" position.
Applications Using a Two-Stage Thermostat
A - Heating Sequence -- Control Thermostat Selection
DIP switch in "Variable Capacity" Position
1. On a call for heat, thermostat first-stage contacts close
sending a signal to the integrated control. The integrated
control runs a self-diagnostic program and checks high
temperature limit switches for normally closed contacts
and pressure switches for normally open contacts.The
combustion air inducer is energized at ignition speed,
which is approximately the same as the inducer speed
at 70 percent firing rate.
2. Once the control receives a signal that the low-fire
pressure switch has closed, the combustion air inducer
begins a 15-second pre-purge in ignition speed.
3. After the pre-purge is complete, a 20-second initial
ignitor warm-up period begins. The combustion air
inducer continues to operate at the ignition speed.
4. After the 20-second warm-up period has ended, the gas
valve is energized and ignition occurs. At the same time,
the control module begins an indoor blower 45-second
ON-delay. When the delay ends, the indoor blower motor
is energized at a speed that matches the firing rate.
After the 10-second ignition stabilization delay expires,
the inducer speed is adjusted to the appropriate target
rate. If the furnace is operating in the initial heating cycle
after power-up, the initial firing rate will be approximately
40 percent. The firing rate on subsequent cycles will be
automatically adjusted by the integrated control based
on thermostat cycles. The firing rate will vary and will
range from 40 percent to 90 percent. The furnace will
continue this operation as long as the thermostat has a
first-stage heating demand.
5. If second-stage heat is required, the thermostat
second-stage heat contacts close and send a signal
to the integrated control. The integrated control either
increases the firing rate to 70 percent (if the current rate
is at or below 60 percent) or increases the firing rate by
10 percent (if the current rate is above 60 percent). If
the call for heat continues 5 minutes beyond this initial
upstage, the rate will be increased by 10 percent very 5
minutes until the call for heat is satisfied or the furnace
reaches 100 percent rate. As the firing rate increases,
the indoor blower motor is adjusted to a speed which is
appropriate for the target rate.
6.
If second-stage heat demand is satisfied, but first stage
is still present, the furnace will continue to operate at the
present firing rate until the heat cycle ends.
7. When the demand for first- and second-stage heat is
satisfied, the gas valve is de-energized and the field-
selected indoor blower off delay begins. The combustion
air inducer begins a 20-second post-purge period.
8. When the combustion air post-purge period is complete,
the inducer is de-energized. The indoor blower is de-
energized at the end of the off delay.
Applications Using a Single-Stage Thermostat
B - Heating Sequence -- Control Thermostat Selection
DIP switch in "Single-Stage" Position
1. On a call for heat, thermostat first-stage contacts close
sending a signal to the integrated control. The integrated
control runs a self-diagnostic program and checks high
temperature limit switches for normally closed contacts
and pressure switches for normally open contacts.
The combustion air inducer is energized at the ignition
speed, which is approximately the same as the inducer
speed at 70 percent firing rate.
2. Once the control receives a signal that the low-fire
pressure switch has closed, the combustion air inducer
begins a 15-second pre-purge at the ignition speed.
3. After the pre-purge is complete, a 20-second initial
ignitor warm-up period begins. The combustion air
inducer continues to operate at the ignition speed.
4. After the 20-second warm-up period has ended, the gas
valve is energized and ignition occurs, which initiates a
10-second ignition stabilization delay. At the same time,
the control module sends a signal to begin an indoor
blower 45-second ON-delay. When the delay ends,
the indoor blower motor is energized at a speed which
is appropriate for the firing rate. After the 10-second
ignition stabilization delay expires, the inducer speed
is adjusted to 40 percent speed. The integrated control
also initiates a second-stage on delay (factory-set at 7
minutes; adjustable to 12 minutes).
5. If the heating demand continues beyond the secondstage
on delay, the integrated control energizes the combustion
air inducer at 70 percent speed. The indoor blower motor
is adjusted to a speed which matches the target rate. A
fixed, 10-minute third-stage on delay is initiated.
6. If the heating demand continues beyond the thirdstage
on delay, the integrated control energizes the inducer
at high speed. The indoor blower motor is adjusted to a
speed which is appropriate for the target rate.
7. When the thermostat heating demand is satisfied,
the gas valve is de-energized and the combustion air
inducer begins a 20-second post-purge. The field-
selected indoor blower off delay begins.
8. When the combustion air post-purge period is complete,
the inducer is de-energized. The indoor blower is de-
energized at the end of the off delay.
Page 64
31-5000723 Rev. 0
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