Other Unit Adjustments; Heating Sequence Of Operation - Lennox SL280V Installation Instructions Manual

Other Unit Adjustments

Primary and Secondary Limits
The primary limit is located on the heating compartment
vestibule panel. The secondary limits (if equipped) are lo-
cated in the blower compartment, attached to the back side
of the blower. These limits are factory set and require no
adjustment.
Flame Rollout Switch
This manually reset switch is located on the burner box. If
tripped, check for adequate combustion air before reset-
ting.
Pressure Switches
The pressure switch assembly (includes two switches) is
located in the heating compartment adjacent to the com-
bustion air inducer. These switches check for proper com-
bustion air inducer operation before allowing ignition trial.
The switches are factory−set and require no adjustment.
Temperature Rise
Place the unit into operation with a second−stage heating de-
mand. After supply and return air temperatures have stabi-
lized, check the temperature rise. If necessary, adjust the
heating blower speed to maintain the temperature rise within
the range shown on the unit nameplate. Increase the blower
speed to decrease the temperature rise. Decrease the blow-
er speed to increase the temperature rise. Failure to properly
adjust the temperature rise may cause erratic limit operation
and may result in premature heat exchanger failure.
Thermostat Heat Anticipation
Set the heat anticipator setting (if adjustable) according to
the amp draw listed on the wiring diagram that is attached
to the unit.

Heating Sequence of Operation

Electronic Ignition
The two−stage, variable speed integrated control used in
SL280DFV units has an added feature of an internal Watch-
guard control. The feature serves as an automatic reset de-
vice for ignition control lockout caused by ignition failure.
This type of lockout is usually due to low gas line pressure.
After one hour of continuous thermostat demand for heat,
the Watchguard will break and remake thermostat demand
to the furnace and automatically reset the control to begin
the ignition sequence.
NOTE − The integrated control thermostat selection DIP switch
is factory−set in the TWO−STAGE" position.
Applications Using a Two−Stage Thermostat
See figure 28 for ignition control sequence
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.
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.
Page 42