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31-5000723 Rev. 0
Applications Using a Two-Stage Thermostat
C - Heating Sequence -- Control Thermostat Selection
DIP switch in “Two-Stage” Position (Factory Setting)
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 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. 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
that matches the firing rate. After the 10-second ignition
stabilization delay expires, the inducer speed is adjusted
to the appropriate target rate. The inducer will remain at
the 70 percent speed 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 initiates a
30-second second-stage recognition delay.
6.
At the end of the recognition delay and on all subsequent
calls for heat in the same heating cycle, the integrated
control energizes the combustion air inducer at high
speed. The control also checks the high-fire pressure
switch to make sure it is closed. As the inducer speed is
increased to high, the indoor blower motor is adjusted to
a speed which is appropriate for the target rate.
7.
When the demand for high-fire (second stage) heat
is satisfied, the gas valve is de-energized and the
fieldselected 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.
