Other Unit Adjustments; Sequence Of Operation - Lennox SL280DFNV Installation Instructions Manual

Other Unit Adjustments

Primary Limit
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.
Thermal Switch
The auto-reset switch is located on the front of the air gas
intake. The switch will safely shut the unit down if exces-
sive temperatures are detected.
Pressure Switches
The pressure switch is located in the heating compartment
adjacent to the gas valve. The switch checks for proper
combustion air inducer operation before allowing ignition
trial. The switch is factory-set and requires no adjustment.
Temperature Rise
Place the unit into operation with a second-stage heating
demand. After supply and return air temperatures have
stabilized, check the temperature rise. If necessary, ad-
just the heating blower speed to maintain the tempera-
ture rise within the range shown on the unit nameplate.
See Table 22 for allowable heating speeds. Increase the
blower speed to decrease the temperature rise. Decrease
the blower speed to increase the temperature rise. Failure
to properly adjust the temperature rise may cause erratic
limit operation.
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 SL280DFNV units has an added feature of an internal
Watchguard control. The feature serves as an automat-
ic reset device for ignition control lockout caused by ig-
nition 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 27 for ignition control sequence
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 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 low pres-
sure switch has closed, the combustion air inducer
begins 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 opera-
tion the unit will switch to low fire..
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, 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.
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 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
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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