gloworm boiler cutting out

Dear All,
I am really sorry to be a pain. A few people have read my previous messages but no-one has replied so I'm not really sure if I'm doing something wrong. This is my last attempt to seek some advice... promise.

My query is :
can an airlock cause a boiler to cut out after about a sort period of time, say 10-20mins and need resetting?

An engineer called and did nothing to the system except for oil the fan. I tried bleeding all the radiators last night, just water. If it could be an air lock is there anything I can do to get rid of it?

If alternatively this is something complicated (beyond my scope and therefore Corgi territory),such as the PCB/thermocouple/overheating stat, should this engineer have checked it when he came out.

Part of the problem is that the system will probably work when an engineer comes to the house... he will phone me, I will go home, turn the heating on and it will work for about 20 mins then the problems start. Is there a better way that I can explain this to the engineer? I don't want to waste anyone's time.

I'll be very grateful for any replies,

Best wishes,
AM

First Q, do the water pipes from the boiler go up or down. It's common to have trapped air build up over several years if the pipes go up then down, say. Often there's no bleed point at the top and air gets stuck.
If they (any) go up, then it's unlikely air is sticking in the boiler.

Try turning the boiler thermostat down a bit to see if it makes a difference (how far up is it?).

Obvious "normal" cause would be failure of a boiler thermostat - which GW model is it?

Just called my brother in the house- it is a Gloworm Swiftflow E.
All the pipes are below the boiler as far as I can tell.

Yes, I think it is a thermostat problem.... I'm not sure if and how it could be connected to the previous "pressure falling" problem, or if replacing the AAV could have caused an airlock.

Next, we can't change the temp at all, or switch it from hi to low which also seems to have been suggested to others.

Thanks a million for responding,
AM

it is a combi could be scaled up heat/exch. dodgy thermistor/ sticking pump/air press. switch etc, you need an eng. to diagnose problem get glow-worm out.

Thanks gasgeezer.

I know I need an engineer. There have been 5 visits by engineers from the same company in the last few months. I'm trying to establish if I am explaining things wrongly. I phoned another company the week but they said I should get Heatforce to sort it out, so I did. The engineer arrived, the system seemed to be working, so I am trying to establish if there is any justification in him making his last visit and saying
"it's an airlock" and doing nothing.

Like all people who work full time I don't want to take half a day of work to have someone come round and do nothing.

An example of how it works:
Came home last night- needed reset button setting. It then worked fine for a few hours. Came on fine this morning on timer. Has not needed resetting since last night.

Other days, it works for about 20mins, then needs redetting, then 5mins maybe-->off, reset----> off. So effectively no heating.

The intermittent nature of the problem suggests that it might not be the thermostat? Could it be an airlock? If so is there anything I can do to speed it up getting out of the system. The engineer said it would eventually find it's way out through the AAV.
Is that plausible?

If so I will put up with the inconvenience ( I'm off for a weeks holiday in Goa- but my nrother will be chilly!) until this happens, otherwise I will call out a different company, maybe Gloworm. Or I could call out this company, Heatforce, but I'm wary of wasting our time doing that.

Thanks
AM

Don't you hate intermittant faults...

I'm not familiar with the Glowworm unit but...
Could be an airlock so the water is getting too hot in the boiler as it cannot circulate properly.
Why though would it be fine sometimes and not others. If the airlock is moved then bleeding the rads / boiler (is there is a bleed point on it) would sort that.
Do the all radiators / Hot water tank get warm - even luke warm as the bolier cuts out way before temp is reached? If they do then water is flowing round - less likely to be an airlock perhaps.
Can you vary the pump speed. What is it normally set on?
Most are 3 speed, putting on max for a while might shift an airlock.

Once the problem occurs does the reset work? or does it cut out again in 20mins?
If it works fine after a reset then that would give less weight to the airlock idea, unless this airlock comes and goes!

If it does not work after the reset all too hot? or keeps cutting out after 20 mins each time then I would suspect the overheat thermostat tripping too soon.
Rather that than the other way, so I would think they are designed to fail on the side of tripping sooner rather than later.

I guess its hard for an engineer to resolve an intermittant fault and they don't perhaps have the pondering / measuring time that we DIY heating engineers have.

Let up know what happens.

Hello speedysteve,
Well, what happened? We put up with it until today. Sometimes it worked fine for a weekend but on average it had to be reset once or twice a day with lots of accompanying noises that I'm not sure how to describe.
But today we phoned engineers again nd they were able to come! We had also been putting up with not having hot water just barely lukewarm showers... although the odd time that is normal too! My boyfriend (fiance since January!) had taken to showering at work.
Why did we put up with it for so long? Who knows... probably what feels like bad experiences with engineers.
So today, the engineer told the fiance that he found a loose connection on that was probably causing the heating to cut out and that should now be fixed. He said that the problem with the hot water was due to the gas inlet valve being blocked.... it wouldn't open enough to let more gas in when necessary when heating water. He said that changing this and the PCB (it's not clear why this needs changed... I'm getting this secondhand from the other half) would cost £400 and we would be better advised to get a new boiler at £1300 plus vat, as this system is >10 years old. He also said that he couldn't guarantee that there wasn't something else wrong with the system.

So does this make sense? I don't want to go and spend a lot of money (saving for wedding and everything) that isn't necessary. Surely we could just get the air inlet valve changed and take it from there and forget about the PCB, or am I missing something?

Any advice greatly appreciated.
Thank you
AM