Heating Sequence Of Operation - Lennox Allied Air A96UHMV Installation Instructions Manual

Heating Sequence of Operation

The integrated control initiates a pressure switch calibration
at the initial unit start-up on a call for heat. The ignition
control will also initiate a calibration any time main power is
turned off and back on and a heating demand is present.
Additional calibrations may be initiated by the service
technician during field test sequence. The following heating
sequence of operation assumes completion of a successful
calibration.
NOTE: The thermostat selection DIP switch on the
integrated control is factory set in the "TWO STAGE" position.
Applications Using a Two Stage Thermostat
A-Heating Sequence - Control Thermostat Selection DIP
switch in "Two Stage" Position (Factory Settling)
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 secon stage recognition delay.
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6. At the end of the recognition delay and on all subsequent
calls for heat in the same heating cycle, the intergrated
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 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.
Application Using a Two Stage Thermostat
B - 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 prepurge in low speed.
3. After the prepurge is complete, a 20 second initial ignitor
warm-up period begins. The combustion air inducer
continues to opeate 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 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. If the furnace is operating in the initial heating
cycle after power up, the initial firing rate will be
approximately 35 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 35 percent to 90 percent. The
furnace will continue this operation as long as the
thermostat has a first stage heating demand.
Issue 1230 506550-01