Heating Operation And Adjustments; Electric Heat Start-Up - Lennox LGT036 Installation Instructions Manual

12- If the furnace does not light the first time (gas line not
fully purged), it will attempt up to two more ignitions
before locking out.
13- If lockout occurs, repeat steps 1 through 10.
14- If the appliance will not operate, follow the
instructions "Turning Off Gas to Appliance" and call
your service technician or gas supplier.
Turning Off Gas to Unit
1- If using an electromechanical thermostat, set to the
lowest setting.
2- Before performing any service, turn off all electrical
power to the appliance.
3- Open or remove the control access panel.
4- Move gas valve switch to OFF.
5- Close or replace the control access panel.
WARNING
Danger of explosion. Can cause inju­
ry or death. Do not attempt to light
manually. Unit has a direct spark igni­
tion system.

Heating Operation and Adjustments

(Gas Units)
A-Heating Sequence of Operation
Two-Stage
1- On a heating demand the combustion air inducer
starts immediately.
2- Combustion air pressure switch proves inducer
operation. After a 30-second pre-purge, power is
allowed to ignition control. Switch is factory set and
requires no adjustment.
3- Spark ignitor energizes and gas valve solenoid
opens.
4- Spark ignites gas, ignition sensor proves the flame
and combustion continues.
5- If flame is not detected after 8 seconds, the ignition
control will repeat steps 3 and 4 two more times. The
ignition control will wait 5 minutes before the ignition
attempt recycles.
B-Ignition Control Diagnostic LEDs
TABLE 14
IGNITION CONTROL HEARTBEAT LED STATUS
LED
Flashes
Steady Off No power or control hardware fault.
Steady On Power applied. Control OK.
3 Flashes Ignition lockout from too many trials.
Ignition lockout from too many flame
4 Flashes
losses within single call for heat.
5 Flashes Control hardware fault detected.
LGT/LCT036, 048, 060, 072
Indicates
Page 35
C-Limit Controls
Limit controls are factory-set and are not adjustable. The
primary limit is located to the right of the combustion air
inducer. See figure 30.
D-Heating Adjustment
Main burners are factory-set and do not require adjustment.
The following manifold pressures are listed on the gas valve.
Natural Gas Units - Low Fire - 2.0" w.c.
Natural Gas Units - High Fire - 3.5" w.c.
LP Gas Units - Low Fire - 5.9" w.c.
LP Gas Units - High Fire - 10.5" w.c.
Electric Heat Start-Up (LCT Units)
Optional electric heat will stage on and cycle with
thermostat demand. See electric heat wiring diagram on
unit for sequence of operation.
SCR Electric Heat Controller (LCT Units)
Optional factory-installed SCR (A38) will provide small
amounts of power to the electric heat elements to
efficiently maintain warm duct air temperatures when
there is no heating demand. The SCR maintains duct
air temperature based on input from a field-provided
and installed thermostat (A104) and duct sensor
(RT20). SCR is located in the compressor section on
the left wall. Use only with a thermostat or specified
DDC control system.
Use the instructions provided with the thermostat to set
DIP switches as follows: S1 On, S2 Off, S3 Off. Use the
instructions provided with the duct sensor to install sensor
away from electric element radiant heat and in a location
where discharge air is a mixed average temperature.
Once power is supplied to unit, zero SCR as follows:
1- Adjust thermostat (A104) to minimum position.
2- Use a small screwdriver to slowly turn the ZERO
potentiometer on the SCR until the LED turns solid red.
3- Very slowly adjust the potentiometer the opposite
direction until the LED turns off.