Ends; Heating Sequence Of Operation - Lennox G71MPP Installation Instructions Manual

Failure To Operate
If the unit fails to operate, check the following:
1 − Is the thermostat calling for heat?
2 − Are access panels securely in place?
3 − Is the main disconnect switch closed?
4 − Is there a blown fuse?
5 − Is the filter dirty or plugged? Dirty or plugged filters will
cause the limit control to shut the unit off.
6 − Is gas turned on at the meter?
7 − Is the manual main shut-off valve open?
8 − Is the internal manual shut-off valve open?
9 − Is the unit ignition system in lock out? If the unit locks out
again, inspect the unit for blockages.
10 − Is blower harness connected to ignition control? Fur-
nace will not operate unless harness is connected.

Heating Sequence of Operation

The integrated control initiates a pressure switch calibra-
tion 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 tech-
nician during field test sequence. The following heating se-
quence 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 Setting)
1 − On a call for heat, thermostat first−stage contacts close
sending a signal to the integrated control. The inte-
grated 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 ener-
gized 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 induc-
er begins a 15−second pre−purge in the ignition speed.
3 − After the pre−purge is complete, a 20−second initial ig-
nitor warm−up period begins. The combustion air in-
ducer 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 ad-
justed to the appropriate target rate. The inducer will
remain at the 70 percent speed as long as the thermo-
stat has a first−stage heating demand.
5 − If second−stage heat is required, the thermostat sec-
ond−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 subse-
quent calls for heat in the same heating cycle, the inte-
grated control energizes the combustion air inducer at
high speed. The control also checks the high−fire pres-
sure 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 combus-
tion air inducer begins a 20−second post−purge period.
8 − When the combustion air post−purge period is com-
plete, the inducer is de−energized. The indoor blower
is de−energized at the end of the off delay.
Applications 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 inte-
grated 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 ener-
gized 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 induc-
er begins a 15−second pre−purge in low speed.
3 − After the pre−purge is complete, a 20−second initial ig-
nitor warm−up period begins. The combustion air in-
ducer 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 begins 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 appropri-
ate target rate. If the furnace is operating in the initial
heating cycle after power−up, the initial firing rate will
be approximately 40 percent. The firing rate on subse-
quent cycles will be automatically adjusted by the inte-
grated control based on thermostat cycles. The firing
rate will vary and will range from 40 percent to 90 per-
cent. The furnace will continue this operation as long
as the thermostat has a first−stage heating demand.
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