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with the front cover removed the flame died out quietly, making it look like I was inventing the problem. However, when he replaced the cover it started popping again.
That might be significant. How does the air flow to the burner with the cover on? Perhaps there's an obstruction, dust build-up or something, in the air path
 
That might be significant. How does the air flow to the burner with the cover on? Perhaps there's an obstruction, dust build-up or something, in the air path
That's a good point. It's not a sealed unit. There's a 2" gap along the bottom of the inner cover at the front of the boiler allowing free flow of air beneath the burners. The outer cover has a cutout grille on front and sides and there aren't any obstructions. In fact it's in a pretty draughty outhouse so I thought at first it might be the wind blowing out the pilot, but the pilot will stay lit for weeks when the boiler's not being used and it wasn't until I heard it pop that I realised what was causing the problem. The HE says it's not dangerous and has issued his safety check certificate but it's annoying to come down on a cold morning when the boiler has gone out. Our paying guests are a bit prickly, too, when they feel they aren't being looked after properly.
 
Thanks Gasmk1, that sounds logical too. Since it was okay with plenty of air does that mean there could be more gas than there should be? Would you think the jets might have become enlarged and are letting too much through? The flame doesn't seem too fierce though.
Has your engineer checked the injectors
 

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