Testing an air pressure switch

bster · 7 Sep 2006

Is it possible totest an air pressure switch is working whilst off the boiler.

I brought one home from work with me and was playing around with a continuity tester - which showed the micro switch is working.

cheers from a wannabee fault finder!

Gasguru · 7 Sep 2006

Get some silicon tube to allow you to blow/suck into the APS positive/negative connections.

Test both ports and check continuity of switch contacts. Test all combinations and expect to get less than an ohm resistance (with short meter leads).

Check switch contacts several times. Duff APS contacts will give higher resistance contact readings.

APS's are generally reliable with a few exceptions (older Biasi's and newer Vaillants). Most air problems are associated with partially blocked venturi's, pitot tubes, tubes and clogged fan impellers.

bster · 7 Sep 2006

My multimeters lowest ohm range is 200 - do i need a more sensitive\accurate meter?

Gasguru · 7 Sep 2006

Normally sufficient

Paul Barker · 7 Sep 2006

I know you brought it home, but useful to test live whether the switches even though switching and even though showing a low resistance on your meter, do actually pass the required current to the pcb to allow the next job on it's list to be performed. This is only achieved by bridging the two terminals which should make at the appropriate time, if before the boiler didn't go on to the next stage and after it did, yet the aps is aparently working no it isn't.

A phone call to manufacturers will tell you at what pressure it should switch, check that with your guage. If not sufficient pressure you are looking at the mechanics of the fan and impediments to flow like blockages in flue and the small venturi passage from which the aps get's it's information.

I would say that a test off the boiler of an aps is interesting but not conclusive.

bster · 7 Sep 2006

cheers Paul.

I'm always bringing parts home - it makes me do a search or read up on their function and operation.

do you think ,y multimeter is ok for boiler fault finding as it only goes down to the 200 ohm scale?

Rob · 7 Sep 2006

bster said:
cheers Paul.

I'm always bringing parts home - it makes me do a search or read up on their function and operation.

do you think ,y multimeter is ok for boiler fault finding as it only goes down to the 200 ohm scale?

generally apart from continuity (15ohms or less) the only thing which require testing under 200 are fan appprox 30ohms and pump maybe 100 -200 ohms

gas valve solenoids are usually in the thousands

bster · 7 Sep 2006

Actually i think it goes down .1 ohm (i think) when in '200' ohm mode the display reads 000.0 (the dot is marked with a little 200 symbol), so i think its alright.

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Testing an air pressure switch

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