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d) After the prepurge, the control module energizes the gas control valve and the spark for 4.5 seconds. If a fl ame is
established and proved, the control allows the fl ame to stabilize for 5 seconds at the combustion fan ignition speed
setting. If the fl ame fails to prove, the control module will attempt to light the burner 4 more times. If a fl ame is still
not established, the control will lockout.
e) Once the fl ame stabilization period has ended, the MCBA allows the burner to modulate. The actual fi ring rate
is dependent upon the measured current and recent differences between the set point temperature and the supply
temperature. If an outdoor sensor is connected to the control module and the boiler is responding to a call for heat,
the set point temperature will be determined by the outdoor reset curve shown in Figure 12.4.
f) Once the set point temperature is reached, the MCBA will turn the burner off and allow the combustion fan to operate
in postpurge for 35 seconds before it turns off.
g) The central heating pump will continue to operate until the room thermostat has been satisfi ed.
h) A demand for domestic hot water (DHW) is given priority on FCM series boilers. If a call for DHW is received while
the boiler is responding to a call for heat, the heating circulator is deenergized until the call for DHW is satisfi ed.
Figure 12.4: Outdoor Reset Curve