Baxi Solo 3 Sitting and Doing Nothing..Sometimes

[stem]

I see how you craftily avoided any diagnosis, but I understand why

I was not craftily avoiding any diagnosis! But I actively refrain from advising DIYers on gas/combustion related matters both because its common sense not to and its contrary to the forum rules.

I know, and that's what I was referring to.

If the NC connections on the APS are not made then most boiler control circuits will be programmed to treat that as a fault condition and prevent the fan from starting.

Thank you. That makes sense.

Looks like I've got to go down the RGI part swapping exercise then.....unless anyone knows a good technical RGI in Nottingham they can recommend.

 

[Agile]

This might even prompt you to give me a push on the thanks button!

Whilst I cannot suggest you open the combustion area or touch the APS itself, there is something that you can do safely.

That is to check the APS NC contact resistance at the plug when it is connected on the PCB !!!

Tony

 

[stem]

Tony, you are a star, and do indeed earn a push of the thanks button.

I waited until the fault occurred, pulled off the connector from the PCB, and measured the resistance between the black and white wires (pressure switch normally closed contact) and it was open circuit. I left the meter connected until the boiler cooled and low and behold, it dropped to 0.3 of an ohm. When I plugged it back in the boiler fired straight up.

So now I can call my RGI who services the boiler and ask him simply to change the pressure switch.

Interestingly whilst measuring the n/c contact when it was open circuit, I also checked the n/o contact and that was 1.3 ohms, which is a bit of an odd reading. However after the cooling period it became open circuit.

Odd how the fault occurs only after it's been running for a while.

Thanks also, to brennan50 for pointing out the same thing, even if I didn't understand it at the time.

 

[Agile]

Its actually a very run of the mill common fault.

Surprising though how many engineers get confused about it though and so few actually measure the contact resistances! To me its the bread and butter of fault finding!

I sometimes try to get a few engineers to a curry

Tony

 

[tinymole]

Thanks to everyone on this thread, especially Agile, and the fantastic reverse engineered wiring schematic from underwurlde - //www.diynot.com/forums/viewtopic.php?t=207176&start=0

You helped me to diagnose a faulty APS on my Baxi Solo 3 PFL 50. I was concerned that the replacement main control board that I had fitted 16 months ago was faulty and I would have to fork out another €200. I was able to confirm using the multimeter that the APS was not resetting when the fan stopped, until the boiler cooled down again.

Not being one to waste scarce cash, I wanted to see if I could repair the APS itself (€50 to €60). It consists of a differential pressure diaphragm actuator which operates a microswitch attached to the side of the APS.

Blowing gently into the positive pressure tube confirmed the diaphragm was working ok, but the switch contacts were not returning to their normal position when the pressure was removed. It looked as if the actuator wasn't pushing the small cam which operates the microswitch enough to toggle it. The movement of the cam is measured in millimetres and the movement of the actuator was just short of the tipping point of the switch by a fraction of a millimetre.

I carefully removed the small melted plastic domed heads on the retaining posts on the microswitch, allowing me to lift it off the body of the APS. Examining the small green plastic cam showed that it was quite loose, very worn and perished looking. I would guess that the high temperatures that it operates at would degrade the plastic.

I was lucky enough to have another microswitch that I had recovered from my old dishwasher before scrapping it (Hate throwing anything away ). It had the same electrical characteristics and rating as the faulty one (VERY IMPORTANT). I popped it on to the APS and tested its operation again with the multimeter - it worked as per spec. The plastic retaining posts were re-melted with a heated small tipped screwdriver to ensure the microswitch didn't fall off.

Apologies if the above is a bit long-winded, I just want others to be able to follow it blow-by-blow. Anyway, on re-assembling everything, the boiler is working fine again and it cost me nothing but time! As an DIY kind of person I can devote time to these things, but not cash. I know things are different for those who make their living at this.

Thanks again to the forum users and I hope this feedback may prove useful to others.

Best regards,
Mark

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• tinymole
• 21 Jan 2015
• 1

Pic 1: APS with faulty microswitch removed - note retaining posts

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• tinymole
• 21 Jan 2015
• 1

Pic 2: APS with old (black) and new (white) microswitches.

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• tinymole
• 21 Jan 2015
• 1

Pic 3: APS with new microswitch fitted and plastic retaining posts melted to ensure it doesn't fall off.