Allied Air A96US2V Installation Instructions Manual page 66

Applications Using a Two-Stage Thermostat
A-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 prepurge in the ignition speed.
3. After the prepurge 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 30 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
field selected indoor blower off delay begins. The
combustion air inducer begins a 20 second post purge
period.
8. When the thermostat demand for low-fire (first stage)
heat is satisfied, the gas valve is de-energized and the
field-selected indoor blower OFF delay bergins. The
combustion air inducer begins a 5 second port-purge
period.
9. When the combustion air post purge period is complete,
the inducer, the HUM contacts as well as the 120V ACC
terminals are de-energized. The indoor blower is de-
energized at the end of the OFF delay.
Page 66 of 80 Issue 1621 507404-04