Combi pilot flame keeps going out

[wiffers2003]

I did actually get a bit brave and following the manual that i found, i took out the burner and hoovered it. While i was there i measured the resistance of the safety thermostat, the water temp was 'slightly warm' and it measured short circuit across the terminals (one side disconnected). I am actually an electronics engineer so am happy to play with a DVM.
Is that correct for the safety thermostat to be S/C ??

Do you know what the temp sensor should measure across its terminals?

thanks again.

 

[gas4you]

I bow to your electronics knowledge, as a lowly gas engineer I rely on MI's to give me the resistance values at a given ambient temp.

I would not expect an ntc to be short circuit completely.

 

[Gasguru]

You need to tell us what overheat thermostat you have. The early ones have a small reset button in the center of the thermostat. The later ones don't.

If you have the later type (without the button), then the boiler could lockout on ignition failure or an overheat condition.

The stat should measure less than an ohm it sits just above the flow thermistor.

And confirm the boiler is a Performa 24 and not a Performa 24i

 

[Agile]

All simple boiler o/h stats are NC.

The NTC should be about 10K at room temp and 2k at 80 C.

Your boiler is physically overheating and you need to determine why.

Can you monitor the temperature on the flow pipe with a fairly short time constant??

Tony

 

[wiffers2003]

My safety thermostat does have the red button in the middle, although its fairly solid. I tried to push it in but there was no movement !
This would be easy to change, by the looks of it, which is why i'm trying to determine if this is the cause.

Its defintely a Performa 24 and not a 24i.

How would i measure the temp on the pipe, shall i just use a temp probe and hold it on there. Over what time constant, would 30 secs be ok or would it need to be longer than that?

Many thanks, Carl

 

[Agile]

This would be easy to change, by the looks of it, which is why i'm trying to determine if this is the cause.

How would i measure the temp on the pipe, shall i just use a temp probe and hold it on there. Over what time constant, would 30 secs be ok or would it need to be longer than that?

Many thanks, Carl

You have to understand that fixing a boiler is not usually just a simple matter of changing a small part.

Almost certainly the boiler is overheating! You dont stop it overheating by changing the stat! You have do do work on the hydraulic circuit.

Measuring the temperature of the flow pipe will amongst other things show at what temperature the o/h stat trips. Guessing but it should be aroung 100 C.

You will need to attach a probe permanently and see how the temperature varies up to the point the o/h stat trips.

Tony

 

[wiffers2003]

Mmm starting to get a bit involved.
I think i'll call out a Potterton engineer to see if he can diagnose it any further.
I'll keep you posted once i have a conclusion, in the mean time thanks again.

 

[Agile]

You can buy a simple temperature capable meter at Maplins for about £20-£30.

Without knowing the temperatures it difficult for us to guess whats wrong.

Its also VERY important to see whats happening in the 10 sec BEFORE it trips.

Tony

 

[wiffers2003]

Thanks, but i can't make it trip and it only seems to do it once every 2-3 days so its almost impossible to see what happens in the 10 secs before hand.

 

[baxpoti]

10 seconds is the ignition sparking sequence.

You'll probably find the rectification probe is only in the flame on dhw (high gas rate) and goes out on c/h (low gas rate)?

 

[wiffers2003]

I watched the flames through the window in the front (main front cover removed) and while the CH was on and it lights for 10 secs then goes off, lights again for 10 secs etc etc. I noticed that the flames seem to start properly and then as the seconds increase up to 10 the flame seemed to get shorter until only the top was alight and then eventually it went out.
Almost as though there was too much pressure from below and the entire flame didn't have enough time to ignite !!
Is that normal.

 

[Gasguru]

Almost certainly the boiler is overheating!

The lockout light on this model is not used to indicate an overheat condition UNLESS its had the modified overheat stat (without the manual reset button) which this boiler doesn't have.

It must be ignition failure - so sense etc problem.

 

[baxpoti]

You'll probably find the rectification probe is only in the flame on dhw (high gas rate) and goes out on c/h (low gas rate)?

The flame rectification probe needs to be in the flame.

If you don't understand this, get someone in who does!

 

[wiffers2003]

Baxpoti, if you mean the electrodes (as they're called in the service manual) then they do both appear to be in the flame although its a bit hard to tell properly.
A bit more information though, i just noticed that the left hand of the two electrodes continuously sparks for the 10 seconds, whereas the right hand one does nothing.
Is this normal behaviour or are we getting closer to a cause?!?!?

 

[mehran]

if it continues to spark even tho a flame is present then it is a flame detection failure.