I have a fairly new (2yrs) Baxi condensing boiler which has an intermittent fault which is now getting worse.
The fault has definately to do with the flue fan pressure switching logic.
Often the boiler having shut down will not relight as the fan does not run, sometimes removing the pressure tubes and blowing down then fixes the problem temperory.
Now I find that removing and then replacing the three wires to the Pressure switch the boiler will then start and run until it once again calls for heat.
Could it be that one of the micro switches is stuck and therefore relaying the incorrect logic back to the control board (ie the boiler thinks the fan is running but will not light due to lack of pressure failing to operate the other switch ?)
am I correct in assuming one switch is normaly closed whilst the other is normaly open ?

In message , Michael A Scott
writes

I have a fairly new (2yrs) Baxi condensing boiler which has an
intermittent fault which is now getting worse.
The fault has definately to do with the flue fan pressure switching
logic.
Often the boiler having shut down will not relight as the fan does not
run, sometimes removing the pressure tubes and blowing down then fixes
the problem temperory.
Now I find that removing and then replacing the three wires to the
Pressure switch the boiler will then start and run until it once again
calls for heat.
Could it be that one of the micro switches is stuck and therefore
relaying the incorrect logic back to the control board (ie the boiler
thinks the fan is running but will not light due to lack of pressure
failing to operate the other switch ?)
am I correct in assuming one switch is normaly closed whilst the other
is normaly open ?


not mentioning the boiler type makes it difficult to give an accurate
answer, pressure switch sensing works in two ways, normally that the APS
microswitch has to be in the normally closed position for the fan to be
powered, then the pressure differential causes the APS to change to
normally open for the pcb to move on to open the valve and spark

There are some boilers which just energise the fan and look for the
common to normally open contacts to go from open circuit to short
circuit - but your three wire APS microswitch would indicate the former

You should test the resistance of the contacts

Is there a possibility that your removing and replacing the connectors
is giving the APS a mechanical kick because the pressure differential is
not quite enough to move from one position to the other ?

.... fan problem, loss of seal integrity, birds nest in the flue, clogged
tubes etc








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geoff