In message , John Stumbles
writes
Phillip Brown wrote:
Just to update the thread, I tested the fan on separate mains and it
ran fine. Turned out the air pressure switch was stuck, which the
Potterton troubleshooting flow chart pointed me to. Cleaning it out
resulted in a working heater.

Are you saying that because the air pressure switch wasn't working the
fan wasn't coming on? That's puzzling me because AIUI on these boilers
the fan runs whenever mains is applied regardless of the air pressure
switch, but the latter has to operate to make the boiler light.

No, it's a safety feature which also tests the air pressure switch, as
we have seen here. The air pressure switch must be in the normally
closed position and have good ohmic contact.

If the APS had welded itself in the normally open position, in your
understanding, the pcb could light the boiler if the fan wasn't working
--
geoff