Lennox SLP98UHV Installation Instructions Manual page 52

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 ther­
mostat 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 blow­
er 30-second ON-delay. When the delay ends, the in­
door 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. 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 automati­
cally adjusted by the integrated control based on ther­
mostat cycles. The firing rate will vary and will range
from 35 percent to 90 percent. The furnace will contin­
ue this operation as long as the thermostat 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 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 initi­
ates 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 in­
ducer speed is adjusted to 35 percent speed. The inte­
grated 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.
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