Page 59
31-5000724 Rev. 0
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.
