Lennox LGM092U - Ignition Sequence Timing Diagrams

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NORMAL IGNITION SEQUENCE - TIMINGS NOMINAL
THERMOSTAT DEMAND
COMBUSTION AIR BLOWER
GAS VALVE
IGNITION SPARK
BLOWER
IGNITION TRIAL
SECONDS 0 30 35 0 110
ON / CLOSED
OFF / OPEN
END OF
DEMAND
1
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COMBUSTION AIR PROVE SWITCH
5 570
FIGURE 16
FIGURE 17
RETRIALS - IGNITION ATTEMPT SEQUENCE - TIMINGS NOMINAL
ÉÉÉ
ÉÉÉ
ÉÉ
ÉÉ
ÉÉ
THERMOSTAT DEMAND
COMBUSTION AIR BLOWER
GAS VALVE
IGNITION SPARK
BLOWER
IGNITION TRIAL
SECONDS 0 30 35 70
ON
OFF
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉ
ÉÉ
ÉÉ
ÉÉ
ÉÉ
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65 100 105
WATCHGUARD
LOCKOUT
COMBUSTION AIR PROVE SWITCH
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5
FIGURE 18
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THERMOSTAT DEMAND
COMBUSTION AIR BLOWER
GAS VALVE
IGNITION SPARK
BLOWER
IGNITION TRIAL
SECONDS 0 30
ON
OFF
IGNITION CONTROL TIMING
É
É
É
É
ÉÉ
ÉÉ
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ÉÉ
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30
30
HEATING
CYCLE
6 (+3.4, -2.0) SEC.
END OF
THERMOSTAT
DEMAND
123
42 SEC. 122 SEC.
Flame rectification sensing is used on all 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 Ser
vice 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 con
trol will restart the ignition sequence and continue until
flame is established or system locks out.
On a heating demand, the ignition control is energized by
the A55 Unit Controller. The ignition control then allows 30
to 40 seconds for the combustion air blower to vent exhaust
gases from the burners. When the combustion air blower is
purging the exhaust gases, the combustion air prove switch
is closing proving that the combustion air blower is operat
ing before allowing the ignition control to energize. When
the combustion air prove switch is closed and the delay is
over, the ignition control activates gas valve, the spark elec
trode and the flame sensing electrode. Sparking stops im
mediately after flame is sensed. The combustion air blower
continues to operate throughout the heating demand. If the
flame fails or if the burners do not ignite, the ignition control
will attempt to ignite the burners up to two more times. If ig
nition cannot be obtained after the third attempt, the control
will lock out. The ignition control is not adjustable.