Main Combi HE - Changing The Air Pressure Switch 

On the older boilers the fan was used to push out the POC from the boiler to outside via the flue.
Therefore the air pressure switch acts as a safety device which will prevent the boiler from firing up if it can not detect the fan is spinning.
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When the fan spins on the boiler it activates a switch on the APS which then sends a signal to the PCB. If the APS detects the fan then it will tell the PCB that the boiler is safe to ignite whereas if the APS can not detect the fan then it will tell the PCB that the boiler is not safe to ignite.
If the flue is not blocked and the fan is spinning (&the pcb is all good) then the APS should allow the boiler to fire up.
On the air pressure switch there are 3 connections:
1.) C - Common
2.) NC - Normally Closed
3.) NO - Normally Open
At rest, the switch should be at NC. Therefore you should have continuity between C & NC, if you don't then this is a sign that the APS is faulty.
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