Sequence Of Operation - Lennox DAVE SIGNATURE SL297UHNV Installation Instructions Manual

Other Unit Adjustments
NOTE - See troubleshooting flow charts if any safety
switches are found to be open.
Primary Limit.
The primary limit is located on the heating compartment
vestibule panel. This limit is factory set and requires no
adjustment.
Roll Out Switch
This auto-reset switch is located on the front of the intake
air elbow.
Pressure Switch
The pressure switch is located in the heating compartment
on the cold end header box. This switch checks for proper
combustion air inducer operation before allowing ignition
trial. The switch is factory-set and must not be adjusted.
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, adjust
the heating blower speed to maintain the temperature rise
within the range shown on the unit nameplate. See TABLE
23 on page 66 for allowable heating speeds. Increase
the blower speed to decrease the temperature rise. De-
crease the blower speed to increase the temperature rise.
Failure to properly adjust the temperature rise may cause
erratic limit operation.
Electrical
1 - Check all wiring for loose connections.
2 - Check for the correct voltage at the furnace (furnace
operating). Correct voltage is 120VAC + 10%
3 - Check amp-draw on the blower motor with blower
access panel in place.
Unit Nameplate__________Actual__________
Exhaust and Air Intake Pipe
1 - Check exhaust and air intake connections for
tightness and to make sure there is no blockage.
2 - Is pressure switch closed? Obstructed exhaust pipe
will cause unit to shut off at pressure switch. Check
termination for blockages.
3 - Obstructed pipe or termination may cause rollout
switch to open. Check termination for blockages.
Heating Sequence of Operation
Electronic Ignition
The two-stage, variable speed integrated control used in
SL297UHNV units has an added feature of an internal
Watchguard control. The feature serves as an automatic
reset device for ignition control lockout caused by ignition
failure. After one hour of continuous thermostat demand
for heat, the Watchguard will break and remake thermo-
stat demand to the furnace and automatically reset the
control to begin the ignition sequence.
NOTE - The ignition control thermostat selection DIP
switch is factory-set in the "TWO-STAGE" positiwon.
Applications
See figure 51 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
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
pressure 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 and the HUM contacts are de-
energized. The indoor blower is de-energized at the
end of the off delay as well as the 120V ACC terminals.
Page 50
Using a Two-Stage Thermostat