Page 27
FIGURE 17
ÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉ
NORMAL IGNITION SEQUENCE - TIMINGS NOMINAL
THERMOSTAT DEMAND
COMBUSTION AIR BLOWER
GAS VALVE
IGNITION SPARK
BLOWER
SECONDS 0 30 40 0 110
ON / CLOSED
OFF / OPEN
END OF
DEMAND
ÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉ
COMBUSTION AIR PROVE SWITCH
5 45
FIGURE 18
RETRIALS - IGNITION ATTEMPT SEQUENCE - TIMINGS NOMINAL
THERMOSTAT DEMAND
COMBUSTION AIR BLOWER
GAS VALVE
IGNITION SPARK
BLOWER
IGNITION TRIAL
SECONDS 0 30 40 75
ON
OFF
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
65 100 110
WATCHGUARD
LOCKOUT
COMBUSTION AIR PROVE SWITCH
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
5
FIGURE 19
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
THERMOSTAT DEMAND
COMBUSTION AIR BLOWER
GAS VALVE
IGNITION SPARK
BLOWER
IGNITION TRIAL
SECONDS 0 30
ON
OFF
IGNITION CONTROL TIMING
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
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.
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
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 gas valve, the
spark electrode and the flame sensing electrode. Sparking
stops immediately 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 ignition cannot be obtained after the third
attempt, the control will lock out. The ignition control is not
adjustable.
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