Service - Lennox G71MPP Installation Instructions Manual

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 either in-
creases 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 ini-
tial 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 in-
creases, 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 oper-
ate 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 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 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 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 the 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 at 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, which initiates a
10−second ignition stabilization delay. 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 which
is appropriate for the firing rate. After the 10−second
ignition stabilization delay expires, the inducer speed
is adjusted to 40 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 second−
stage on delay, the integrated control energizes the
combustion air inducer at 70 percent speed. The in-
door blower motor is adjusted to a speed which
matches the target rate. A fixed, 10−minute third−stage
on delay is initiated.
6 − If the heating demand continues beyond the third−
stage on delay, the integrated control energizes the in-
ducer at high speed. The indoor blower motor is ad-
justed 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 in-
ducer begins a 20−second post−purge. The field−se-
lected indoor blower off delay begins.
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.

Service

ELECTRICAL SHOCK, FIRE,
OR EXPLOSION HAZARD.
Failure to follow safety warnings exactly could result
in dangerous operation, serious injury, death or
property damage.
Improper servicing could result in dangerous opera-
tion, serious injury, death, or property damage.
Before servicing, disconnect all electrical power to
furnace.
When servicing controls, label all wires prior to dis-
connecting. Take care to reconnect wires correctly.
Verify proper operation after servicing.
At the beginning of each heating season, system should be
checked as follows by a qualified service technician:
Blower
Check the blower wheel for debris and clean if necessary.
The blower motors are prelubricated for extended bearing
life. No further lubrication is needed.
WARNING
The blower access panel must be securely in place
when the blower and burners are operating. Gas
fumes, which could contain carbon monoxide, can
be drawn into living space resulting in personal inju-
ry or death.
Page 58
WARNING