Allied Air A97USMV Installation Instructions Manual page 61

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 every
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
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.
Applications Using A Single-Stage Thermostat
C - 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.
507028-03
2. Once the control receives a signal that the low fire
pressure switch has closed, the combustion air inducer
begins a 15 second prepurge at 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, which initiates a
10 second ignition stabilization delay. 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 which
is appropriate for the firing rate. After the 10 second
ignition stabilization delay expires, the inducer speed is
adjusted to 35 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, 1O 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.
Issue 1622 Page 61 of 74