Boiler Problem. PCB or Pressure Switch??

Hi All
I'm having a now long standing problem with my boiler ( I have posted recently which people have very kindly replied to) but the problem is gettting worse and costing me lots of money!!
In short, the boiler does not stay on long, cutting out once igniting and then we have to manually switch it off and restart it. It can be fine for days with no problems and then just refuse to work for the next few days (and this continues!!) There is an audible clicking noise at times before the boiler flame goes out.
I had a gas fitter out and as is always the case, it was working fine and he could find no fault. He came a few days later and said it was likely to be my gas valve, this was changed and cost me just over £250. It was not the gas valve, it played up again the same day and has continued to do so. The gas fitter has said it is likely to be the PCB and is going to change this for me. But a mate of mine in convinced it's not as he said they just usually go and stop working rather than be a intermittent fault. He said it is likely to be the pressure switch.
Does anyone have any knowledge on this and know the most likely cause??
Thanks in advance
19:01 Tue 31st Jan 2012
 
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Don't you just hate intermittent faults, Slidders? Why don't they just blow up and get it over with?

Anyway, faulty pressure switch would prevent the boiler from firing. It could easily have an intermittent fault as well. I don't know whether faulty thermistors (boiler flow etc) could do this, but they're cheap enough to replace. They're often changed at a service, just in case.

All guesswork of course. Perhaps a call to the manufacture's tech dept. might help. That's what boiler engineers do when they're unsure about something.
It could be due to a very poor water flow rate, either due to the pump on the way out and/or crud within the system. It sounds like the over temperature thermostat is operating (shutting down the whole boiler). If it is due to the lack of water flow, you should notice a ‘kettleing’ sound – as the water in the heat exchanger starts to boil, followed by the operation of the boiler cut out.
Question Author
Thank you both for your answers. I don't think it is over heating though as for the last few days it had been fine, surely if it was over heating it would be like that all the time?? I'm not sure.
In some ways I wish it would completely give up the ghost and be done with like you say Builder. At least then I can bite the bullet and get a new boiler. At this rate I would have spent enough to be part way there !!!
When it next happens, make careful observations of the boiler operation, which will help with the diagnosis of the problem.

If the pump is on the way out – it is possible that at times it pumps OK, then for almost no reason will fail/stall. Similarly, crud could pass through the boiler OK, but at other times cause an obstruction.

Away next time; look, listen & touch (don’t bother too much with taste & smell).
Question Author
Thanks Hymie. To be honest I've spent many a day listening to it. It starts up, the fan kicks in, I hear the ignition go and then the flame ignites. It will run for sometimes for an hour no problems but other times only for 5 mins. When it plays up, it will click and the flame goes out. The fan will continue to run as well as the pump as far as I can tell but with no flame there is obviously no heat. No other noise is heard from the boiler once the flame goes out, it appears to be running normally ( but no flame)
It is possible that a faulty pressure switch is causing the problems you are encountering – it equally could be that the boiler seals are leaking causing a genuine lack of pressure differential (at times), shutting down the boiler.

Definitely an area worth more consideration – given your last post.
Forgot to mention – had a similar problem on my boiler which was traced to perished pipery connected to the pressure sensor.
I had a similar episode and it turned to be a temperature sensor.
Question Author
Hi all
Just an update. The gas fitter has been out again today and has been testing it like crazy for 2 hours. As the problem is so intermittent it has been very difficult for him to pin point it exactly. He is reluctant to fit a new PCB as it could well not be this and he is aware of more cost which are adding up. He thought he had it down to the condensation cut off but sadly, after he left it played up again. Maybe this needs changing???
Thank you all so much for taking the time to reply. It really is a help to get some ideas from experts or people who are feeling my pain!! Please feel free to continue answering if you should have any other ideas.. believe me, its a great help!! I was thinking of getting Glow Worm out to diagnose the problem but not sure if this would really cost me!!
Thanks again
Is it a condensing boiler? From your description it sounds as if it is. Have you made sure the condensate (drain) pipe is clear and not backing up water. If it's not a condensing boiler ignore this!
Question Author
Hi Eastern
Yes it is a condenser boiler but the pipe is clear. It seems to be the orange electrical wire or the plastic condenser cup that maybe the issue. It's still not certain that this is the problem though!!
I got a local "plumber" that advertised as a Boiler specialist so I called him when I was having trouble with my Bosch Worcester. After about an hour of scratching his head, I just knew he didn't have clue what he was doing, the last straw was when he asked me if I had the manual for the boiler! That's when I asked him to leave and I called the Bosch service engineer, the whole reason I hadn't called them in the first place was because I thought they were expensive (£175 call out fee) but they came out next day and fixed my boiler, turns out the callout fee included three spare parts and mine needed a new PCB. So I think you know what the moral to the story is. Call the A team!
What model exactly?
So .. It's a Glow-Worm Gas Combi ... what?
The clicking is the gas valve cutting the gas off .. then the flame goes out.
Have the combustion casings ever been removed for cleaning .. or disturbed at all?

I have a couple of ideas.

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