Boiler not staying lit

[DIYcurious]

I have a very old but well maintained Glow worm back boiler that I need to nurse through one last winter (I think it knows!) On firing up this weekend for the first time since March, the boiler lit - stayed lit for 30 seconds/minute then the gas valve shut everything off - even the pilot.

I relit the pilot, fired up again and the same happened.

The pump is operational with water flowing. The gas valve was replaced 2 years ago but I suppose could be at fault. If I turn the boiler thermostat to off/on repeatedly I can make the boiler light and turn off in sync - but if I leave on for longer than 30 secs then the whole thing shuts down.

Any ideas of what to look at first - sounds electrical rather than mechanical but .... My electrics are on RCD and nothing trips out, the controller is switching for HW/Heat OK. Wall thermostat was wapped up to max to eliminate that. It's the pilot going off as well that's worrying.

Thanks

 

[Slugbabydotcom]

Could be a few things but the odds on favourite going by what you told us is that your thermocouple is not getting enough heat from the pilot is because the pilot injector or burner is partially blocked.
Some later back boilers have an air sensing device which would be the next thing to check.

Check that out and post back

 

[DIYcurious]

Thanks for starting this off.

Pilot injector seems ok - good steady flame. If the pilot thermocouple wasn't getting to temperature then the pilot would go out - correct? but it stays lit ok. It's just the main burner only staying lit for a few seconds that's foxing me.

There is no air sensor on this boiler.

 

[gas4you]

If the pilot injector is partially blocked then it will look ok when just this is burning.

When the main burner comes on the gas pressure to the pilot is naturally less and as it is partially blocked will not envelop the thermocouple fully.

 

[DIYcurious]

Cheers,

I'll give it a check tomorrow once the sun comes out again - brrrr

 

[Agile]

Could be a restricted gas supply or meter regulator.

Needs gas pressure at inlet to boiler checking when boiler fires.

Might be dirt in the gas valve inlet filter.

All CORGI though!

Tony

 

[DIYcurious]

Thanks a bunch - Cause was pilot not contacting thermocouple when main burner was lit. All working now thanks to you all.

 

[Agile]

Thats what Slug told you six days ago!

What you said later was not in keeping with this because you said the pilot stayed on and it was only the main burner which went out!

Tony

 

[Slugbabydotcom]

The tricky bit which I didn't explain is that there can be sometimes enough heat to keep the pilot alight but when the boiler lights the airflow inside the boiler is such that the flame is drawn away from the thermocouple