508146-01
Issue 2051
Page 6
1
of 7
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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.
