Lennox SL280DF060NV36B Instruction Manual page 38

CAUTION
Failure to use properly sized wiring and circuit
breaker may result in property damage. Size wiring
and circuit breaker(s) per Product Specifications
bulletin (EHB) and unit rating plate.
Applications Using a Two-Stage Thermostat
A - Heating Sequence -- Integrated Control Thermostat
Selection DIP Switch 1 OFF 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 low speed.
2. Once the control receives a signal that the low pres­
sure switch has closed, the combustion air inducer be­
gins a 15-second pre-purge in low speed.
NOTE - If the low fire pressure switch does not close
the combustion air inducer will switch to high fire. After
a 15 second pre-purge the high fire pressure switch
will close and the unit will begin operation on high fire.
After 10 to 20 seconds of high fire operation the unit
will switch to low fire..
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 low speed.
4 - After the 20-second warm-up period has ended, the gas
valve is energized on low fire (first stage) and ignition oc­
curs. At the same time, the control module sends a sig­
nal to begin an indoor blower 30-second ON-delay.
When the delay ends, the indoor blower motor is ener­
gized on the low fire heating speed, the HUM contacts
close energizing the humidifier and 120V ACC termi­
nal is energized. The furnace will continue this opera­
tion as long as the thermostat has a first-stage heating
demand.
NOTE - If the indoor thermostat is set on CONTINU­
OUS FAN ON mode, the furnace will light on high fire
(second-stage) for 60 seconds to improve heat ex­
changer warm up. After 60 second warm-up period,
furnace will switch to low fire (first-stage).
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, the integrated con­
trol energizes the combustion air inducer at high
speed. The control also checks the high fire (second
stage) pressure switch to make sure it is closed. The
high fire (second stage) gas valve is energized and the
indoor blower motor is energized for operation at the
high fire heating speed.
7 - When the demand for high fire (second stage) heat is
satisfied, the combustion air inducer is switched to the
low-fire heating speed and the high-fire (second
stage) gas valve is de-energized. The low-fire (first
stage) gas valve continues operation. The indoor
blower motor is switched to the low-fire heating speed.
8 - When the thermostat demand for low-fire (first 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 5-second post-purge
period.
9 - When the combustion air post-purge period is com­
plete, the inducer, the HUM contacts as well as the
120V ACC terminals are de-energized. The indoor
blower is de-energized at the end of the off delay.
Applications Using A Single-Stage Thermostat
B - Heating Sequence -- Integrated Control Thermostat
Selection DIP Switch 1 ON in "Single-Stage" Position
NOTE - In these applications, two-stage heat will be initi­
ated by the integrated control if heating demand has not
been satisfied after the field adjustable period (7 or 12 min­
utes).
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 low speed.
2. Once the control receives a signal that the low pres­
sure switch has closed, the combustion air inducer be­
gins a 15-second pre-purge in low speed.
NOTE - If the low fire pressure switch does not close
the combustion air inducer will switch to high fire. After
a 15 second pre-purge the high fire pressure switch
will close and the unit will begin operation on high fire.
After 10 to 20 seconds of high fire operation the unit
will switch to low fire.
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 low speed.
4 - After the 20-second warm-up period has ended, the gas
valve is energized on low fire (first stage) and ignition oc­
curs. At the same time, the control module sends a sig­
nal to begin an indoor blower 30-second ON-delay.
When the delay ends, the indoor blower motor is ener­
gized on the low fire heating speed and the HUM con­
tacts are energized. The integrated control also initi­
ates a second-stage on delay (factory-set at 7 min­
utes; 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 high speed. The control also
checks the high fire (second stage) pressure switch to
make sure it is closed. The high fire (second stage)
gas valve is energized and the indoor blower motor is
energized for operation at the high fire heating speed.
6 - When the thermostat heating demand is satisfied, the
combustion air inducer begins a 5-second low speed
post-purge. The field-selected indoor blower off delay
begins. The indoor blower operates at the low-fire
heating speed.
7 - When the combustion air post-purge period is com­
plete, the inducer, the HUM contacts as well as the
120V ACC terminals are de-energized. The indoor
blower is de-energized at the end of the off delay.
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