Baxi Solo 3 Sitting and Doing Nothing..Sometimes

stem · 2 May 2013

I seem to have an intermittent fault with my Solo 3 60PF.

When the programmer turns on the heating it fires up, but after about an hour, it stops working. The room stat is calling for heat, the "Boiler on" neon remains illuminated, and the pump is running. But that's it. No fan, ignition or burner activity.

If I switch the programmer off, allow everything to cool down, turn the programmer on again and it will fire up.

The red overheat light does not illuminate at any point. (I tried a reset just in case the neon had failed, but still nothing)

About 5 years ago, it developed an intermittent fault where it kept firing up then shut down immediately, the guy that installed it and who services it for me came and fitted, a new fan, a new gas valve, and new PCB all to no avail, eventually it was found to be the thermostat sensor on the flow pipe. This trial and error exercise was rather expensive and I was a bit miffed that in the end it was probably the cheapest component that had failed and I had forked out big bucks.

So this time I would appreciate some advance feedback from the experts, hoping that you may have come across this before and prevent another trial and error scenario.

stem · 10 May 2013

Ok, after another week, I can add a bit more information now.

The boiler operates OK for about 1 to 1.5 hours, then just stops firing, "boiler on" light still illuminated, (room stat and / or cylinder stat calling for heat) and pump running.

After this time, the whole system cools down naturally, then when almost completely cold, it fires up again.

When the problem occurs, turning the potentiometer up/down doesn't result in a 'click' at any point. My feeling is that either the thermistor isn't detecting the temperature correctly, or that the circuit board is not responding to the signal from the thermistor. It's the fact that it works for a short period that is confusing, so it would appear that either time or temperature is having an effect.

I tested the thermistor and got 0.9 M ohms at ambient. I think this is OK. The manual says should be 1 M, but my multimeter is not of the finest quality, so may explain the slight discrepancy.

Tested the potentiometer and also got 0.9 M to 0 ohms clockwise and anticlockwise, also on/off switch OK.

Hope that the above symptoms mean something to someone.

Slugbabydotcom · 10 May 2013

Could be an over sensitive overheat stat

If it gets replaced and you still have a problem then you would probably be looking at a fan or maybe flue problem but that's gas safe engineer territory.

stem · 10 May 2013

Slugbabydotcom said:
Could be an over sensitive overheat stat

I wondered about that, but eliminated it because if the overheat thermostat does trip, a red neon lights and it has to be reset manually. It doesn't reset of it's own accord.

brennan50 · 10 May 2013

i've come across this problem and it caused me a fortune with unneccessary parts. it is a common problem with Baxi solos and the cure is a replacement Air pressure switch. The other possibility is a gas valve and you need to check the resistance across the solonoids however I would go for the air pressure switch first.

stem · 10 May 2013

Thanks brennan50, please correct me if I'm wrong, but I thought that if the pressure switch had failed, the fan would still start, but the switch wouldn't sense this and so the burner would not light. In my case the fan doesn't even start.

Agile · 10 May 2013

brennan50 said:
i've come across this problem and it caused me a fortune with unneccessary parts. it is a common problem with Baxi solos and the cure is a replacement Air pressure switch. The other possibility is a gas valve and you need to check the resistance across the solonoids however I would go for the air pressure switch first.

That is very likely but is not appropriate advise to encourage a Diyer to attempt to access or replace it.

It needs an RGI.

Tony

stem · 10 May 2013

Thanks for that Agile, but there's no need to worry. I have no intention of replacing anything that involves opening the boiler myself. I just want to diagnose the fault to avoid a repeat of my previous experience involving lots of expensive unnecessary new parts. Last time I had a problem, a new PCB, Fan, Pressure Switch, and Gas Valve costing £100's were fitted before the faulty Thermistor was eventually replaced. The part cost less than £10 I seem to remember and finally solved the problem. Although in fairness, the fan was replaced because it was noisy, not as part of the fault finding process.

Whilst I accept RGI's are mustard when it comes to gas safety, those that have attended my boiler in the past didn't appear to have an understanding of the basic principles of electronics, or even the functional sequence of the operation of the boiler, which is explained really simply in the boiler installation manual.

I would be very interested to hear your diagnosis of these symptoms, but don't worry, I won't go changing the gas valve or anything.

brennan50 · 10 May 2013

Air pressure switch hasnt totally failed but works untill it switched off and moves to normally closed circuit but doesnt return to normally open circuit to restart the boiler.

Agile · 10 May 2013

stem said:
Whilst I accept RGI's are mustard when it comes to gas safety, those that have attended my boiler in the past didn't appear to have an understanding of the basic principles of electronics, or even the functional sequence of the operation of the boiler, which is explained really simply in the boiler installation manual.

Very unfortunately I have to agree with you there.

All the training the average one gets is the compulsory gas safety training to become gas registered.

My background is in electronics and I worked for BBC Engineering for some while until it was privatised and I and many others left.

Many are less interested in boiler operation more interested in getting home early and getting into the pub!

I take an active interest in boiler malfunctions and guess what? My favourite TV program is the Air Crash Disasters! Airplane malfunctions!

Tony

stem · 10 May 2013

brennan50 said:
Air pressure switch hasnt totally failed but works untill it switched off and moves to normally closed circuit but doesnt return to normally open circuit to restart the boiler.

I think I must be missing something here. Doesn't the fan have to start in order to operate the pressure switch? But the fan isn't starting up, so there is no pressure to operate the switch, working or not.

stem · 10 May 2013

Agile said:
My favourite TV program is the Air Crash Disasters! Airplane malfunctions!

I'm glad to find that I'm not the only one

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

Slugbabydotcom · 10 May 2013

Have you checked the 'call for heat' side of things? it could be a faulty diverter valve or even roomstat!

Agile · 11 May 2013

stem said:
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.

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.

stem · 11 May 2013

slugbabydotcom said:
Have you checked the 'call for heat' side of things? it could be a faulty diverter valve or even roomstat!

stem said:
The room stat is calling for heat, the "Boiler on" neon remains illuminated, and the pump is running. But that's it. No fan, ignition or burner activity.

Yes, checked that. I'm sitting here now typing & the thermostat is reading 18 degrees (set at 20). The heating has been on for one hour & 15 mins and is cooling down slowly now. I can hear the water is being circulated through the rads, and the "boiler on" light is lit (it only lights when the heating or HW are calling for heat) but at the moment, there's no activity.

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Baxi Solo 3 Sitting and Doing Nothing..Sometimes

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