Heating Sequence Of Operation - Lennox SLP98UHV Installation Instructions Manual

Pressure Switches (Two)
The pressure switches are located on the cold end header
box. These switches check for proper combustion air in-
ducer operation before allowing ignition trial. The switches
are factory−set and require no adjustment. Pressure switch
tubing installation is critical for safe operation. See figure
55.
PRESSURE SWITCH TUBING INSTALLATION
(shown in upflow position)
Gas Valve
Cold End Header Box
4
High−Fire Pressure
Switch
Low−Fire Pressure
Switch
1 − Black hose from front port on low−fire pressure switch to
positive port on the gas valve.
2 − Red hose from rear port on low−fire pressure switch to the
negative port on the gas valve.
3 − Red hose from front port on high−fire pressure switch to
negative port on cold end header box.
4 − Black hose from rear port on high−fire pressure switch to
positive port on cold end header box.
FIGURE 55
Temperature Rise
After the furnace has been started and supply and return air
temperatures have been allowed to stabilize, check the
temperature rise with the unit operating at 100 percent fir-
ing rate. If necessary, adjust the blower speed to maintain
the temperature rise within the range shown on the unit
nameplate. Increase the blower speed to decrease the
temperature. Decrease the blower speed to increase the
temperature rise. Failure to adjust the temperature rise
may cause erratic limit operation.
2
3
1
Page 52
Electronic Ignition
The integrated control has an added feature of an internal
Watchguard control. The feature serves as an automatic re-
set device for ignition control lockout caused by ignition fail-
ure. This type of lockout is usually due to low gas line pres-
sure. After one hour of continuous thermostat demand for
heat, the Watchguard will break and remake thermostat de-
mand to the furnace and automatically reset the control to
begin the ignition sequence.
Exhaust and Air Intake Pipe
1 − Check exhaust and air intake connections for tightness
and to make sure there is no blockage.
2 − Are pressure switches closed? Obstructed exhaust
pipe will cause unit to shut off at pressure switches.
Check termination for blockages.
3 − Reset manual flame rollout switches on burner box
cover.

Heating Sequence of Operation

The integrated control initiates a pressure switch calibra-
tion at the initial unit start−up on a call for heat. The ignition
control will also initiate a calibration any time main power is
turned off and back on and a heating demand is present .
Additional calibrations may be initiated by the service tech-
nician during field test sequence. The following heating se-
quence of operation assumes completion of a successful
calibration.
NOTE − In icomfort Toucht communicating applications, the
sequence of operation is the same but all DIP switch settings
are overidden by the thermostat.
NOTE − The thermostat selection DIP switch on the integrated
control is factory−set in the TWO−STAGE" position.
Applications Using a Two−Stage Thermostat
A − Heating Sequence −− Control Thermostat Selection
DIP switch 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 ignition speed, which is approximately the
same as the inducer speed at 70 percent firing rate.
2 − Once the control receives a signal that the low−fire
pressure switch has closed, the combustion air induc-
er begins a 15−second pre−purge in the ignition speed.
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 the ignition speed.
4 − After the 20−second warm−up period has ended, the gas
valve is energized 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 at a speed that
matches the firing rate. After the 10−second ignition