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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 auto reset limits are factory set
and require no adjustment.
Flame Rollout Switches (Two)
These manually reset switches are located on the inside
of the the burner box. If tripped, check for adequate com-
bustion air before resetting.
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
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. 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.
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 SL280UHV 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 a Two-Stage Thermostat
See figure 36 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 operation
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.
