Other Unit Adjustments; Heating Sequence Of Operation - Lennox SL280UHNV Installation Instructions Manual

Other Unit Adjustments

Primary 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 auto reset 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 Switch
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 20 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.
NOTE - Do not secure the electrical conduit directly to the
air ducts or structure.

Heating Sequence of Operation

Electronic Ignition
The two-stage, variable speed integrated control used in
SL280UHNV 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" position.
Applications Using
See Figure 27 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 switch 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.
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 (second stage) heat is satisfied,
the combustion air inducer is switched to the low-fire
heating speed and the gas valve is re-duced 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.
Page 41
a Two-Stage Thermostat