Could somebody please explain the ignition sequence. (eg common, normally open, normally closed wires).
Also what would happen if the fan was to fail and the air pressure switch/micro switch? was stuck down.
General boiler workings and sequences are in plenty of books etc.
You'd be better off with a specific problem on a certain boiler for more info.
In order to your OP. The fan wont be sent power from the pcb if the APS is still in the N/O position. Most have 3 wires Common, NO and NC. It checks it is in the NC before powering fan.
ignition sequence of a direct burner ignition for fan flued combi boiler
* demand for heat
*demand reaches PCB via interlocks
*Air pressure switch proves in normally closed position
*Fan runs
*Air pressure switch moves to normally open position
*Spark generator on
*Low fire gas solenoid opens
*Burner ignnites at low fire
*Flame proved by flame rec
*High fire gas solenoid opens
*Burner operates at high gas rate
*Flame constantly monitored by flame rec
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