Lennox SL280DFNV Series Unit Information page 38

Applications Using a Two-Stage Thermostat
A - Heating Sequence -- Integrated Control Thermo-
stat Selection DIP Switch 1 OFF in "Two-Stage" Posi-
tion (Factory Setting)
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 for normally
closed contacts and pressure switch for normally
open contacts. The combustion air inducer is
energized at low speed.
2 - Once the control receives a signal that the 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 low speed.
4 - After the 20-second warm-up period has ended,
the gas valve is energized on low fire and ignition
occurs. 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 on the low fire heating speed,
the HUM contacts close energizing the humidifier
and 120V ACC terminal is energized. The furnace
will continue this operation as long as the thermostat
has a first-stage heating demand.
NOTE - If the indoor thermostat is set on
CONTINUOUS FAN ON mode, the furnace will
light on high fire (second-stage) for 60 seconds to
improve heat exchanger 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
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, the integrated
control energizes the combustion air inducer at high
speed. The gas valve is energized on high fire and
the indoor blower motor is energized for operation
at the high fire heating speed.
7 - When the demand for high fire heat is satisfied, the
combustion air inducer is switched to the low-fire
heating speed and the gas valve reduces to low
fire. The low-fire gas valve continues operation.
The indoor blower motor is switched to the low-fire
heating speed.
8 - When the thermostat demand for low-fire 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
complete, 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
minutes).
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 low speed.
2 - Once the control receives a signal that the 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 low speed.
4 - After the 20-second warm-up period has ended,
the gas valve is energized on low fire (first stage)
and ignition occurs. 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 on the low fire
heating speed and the HUM contacts are energized.
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
secondstage on delay, the integrated control
energizes the combustion air inducer at high speed.
The gas valve is energized on high fire 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
complete, 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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