d)After the prepurge, the control module energizes the gas control valve and the spark for 4.5 seconds. If a flame is established and proved, the control allows the flame to stabilize for 5 seconds at the combustion fan ignition speed setting. If the flame fails to prove, the control module will attempt to light the burner 4 more times. If a flame is still not established, the control will lockout.
e)Once the flame stabilization period has ended, the MCBA allows the burner to modulate. The actual firing 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 satisfied.
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 satisfied.
Figure 12.4: Outdoor Reset Curve
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