Air Pressure Switch
The air pressure switch serves various purposes on a fan flu-ed boiler, it tells the Printed Circuit Board (PCB) that the fan is running or if it is not. It also stops the boiler firing if the primary heat exchanger flue-way is blocked. Basically it proves that the fan and the flue are sound. There are two types of air pressure switching - 3 wire and 2 wire method. Essentially what happens is that the PCB checks that the air pressure switch is in the normally closed position which indicates that the fan is not running. The PCB then runs the fan and checks that the switch has moved over to the normally open position, this sequence of operation prevents the fan running even if the switch is stuck in the normally open position. Before one replaces the fan check that the air pressure switch is operational - it costs far less to replace.