Lennox SLP98UHV Installation Instructions Manual page 52

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 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 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 pre-purge in low speed.
3 - After the pre-purge 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 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.
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
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.
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.
2 - Once the control receives a signal that the low-
fire pressure switch has closed, the combustion
air inducer begins a 15-second pre-purge at the
ignition speed.
3 - After the pre-purge 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 second-
stage 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, 10-minute third-
stage on delay is initiated.
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