Lennox KGC Series Unit Information page 26

IGNITION CONTROL A3
FLAME
LED
CAI Line
Line voltage
FIGURE 16
IGNITION CONTROL A3
S18 COMBUSTION
AIR PROVING SWITCH
FIGURE 17
Flame rectification sensing is used on all KGC units. Loss
of flame during a heating cycle is indicated by an absence
of flame signal (0 microamps). If this happens, the control
will immediately restart the ignition sequence and then lock
out if ignition is not gained after the third trial. See System
Service Checks section for flame current measurement.
The control shuts off gas flow immediately in the event of
a power failure. Upon restoration of gas and power, the
control will restart the ignition sequence and continue until
flame is established or system locks out.
Operation
On a heating demand, the ignition control checks for a
closed limit switch and open combustion air prove switch.
Once this check is complete and conditions are correct, the
ignition control then allows 30 seconds for the combustion
air inducer to vent exhaust gases from the burners. When
the combustion air inducer is purging the exhaust gases,
the combustion air prove switch closes proving that the
combustion air inducer is operating before allowing the
ignition control to energize.
When the combustion air prove switch is closed and the
delay is over, the ignition control activates the gas valve,
the spark electrode and the flame sensing electrode.
Once the gas valve is energized, the non-adjustable
40-second indoor blower delay period begins. Sparking
stops immediately after flame is sensed or at the end of
the 8-second trial for ignition.
The control then proceeds to "steady state" mode where
all inputs are monitored to ensure the limit switch, rollout
switch and prove switch are closed as well as flame is
present. When the heat call is satisfied and the gas valve is
de-energized, a combustion air inducer post purge period
of 5 seconds begins along with a 120-second blower off
delay.
A3 IGNITION
CONTROL
Page 26