boiler woes- goes 'out' on its own.....

have a 20 year old boiler- gravity flow, pilot light, with a hot water cylinder in the airing cupboard. recently (a week ago) had a plumber drain the boiler to sort out problems with radiators (also fitting thermostatic valves all over the house on the radiators). since then (maybe coincidentally?) the boiler is misbehaving. on manually lighting it, the pilot comes on, the boiler fires up, can see the flames clearly, then the flames gutter and go out. (i can see a lot of yellow flames- is that unusual? i thought they would be all blue.) after repeated attempts, the boiler stays on when left on a continous timer setting. leaving it on a twice a day setting, the pilot goes out and the boiler does not heat up. what could the possible problems be? is the boiler kaput? any help appreciated.
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If you told us what make and model of boiler it is we might be able to make a sensible contribution. Also when was the boiler last serviced and is all ventilation clear?
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its a glow-worm balanced flue boiler 100B last serviced 2 years ago all ventilation clear.
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problems
see
If it's a Glow Worm balanced flue 100B (like mine) and the flames are all yellow, the heat exchanger fins are all blocked up with soot. This is marginally dangerous as there will be partial combustion and poisonous fumes (but only marginal as it's a balanced flue and they all go outside.
It is possible to do it DIY (I've just done mine) but I cannot recommend this to anyone for obvious reasons. It MUST be done, however. Whoever does it has to remove all the internal covers and laboriously clean between the fins (lolly sticks just fit between them, and a stiff paint brush). The stuff that comes out is ultrafine carbon, the dirtiest substance known to man. You are in for a right pasting from SWMBO (probably). Don't use the best vacuum cleaner (I did - once :o{ ).
As an aside they ususally last 5 years before sooting up (this is my one's second event). This means the "service" you had was the usual quick hoover round and didn't involve cleaning the heat exchanger properly.
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Bob Mannix
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Wonder what influences how fast this happens? My experience is that my boiler does over twice that before needing attention. Mine's on the first floor, so may get a cleaner air supply?
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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It sounds as though the flue is not discharging the products of combustion so the main flame is "smothering" itself because either the outlet is blocked or the combustion air inlet is blocked. Can you see any possible obstruction at the balanced flue terminal? If not and you are competent to do so I suggest you look inside the boiler to check if the burner and its air supply ways are clean and clear, the heat exchanger is clean and clear, and the path for the egress of the flue gases via the balanced flue is also clear. I can't recall the exact construction details of this model but it may be that a fibrefrax combustion chamber lining could have fallen off and be lying on the burner upsetting the combustion. Balanced flue boilers are inherently very reliable and simple so its most likely something stupid
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If the pilot keeps goin out I'd suspect the thermocouple. That's the bit that sticks into the pilot flame and is connected by a thin copper tube to the gas valve. It's job is to keep the gas valve open (and hence the pilot lit) as long as it is 'warm'.
You can replace the thermocouple yourself for about 4 quid. (See www.screfix.com and search for 'thermocouple' )
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Think it's possibly more likely the pilot jet is partially blocked so the flame is too weak to keep the thermocouple happy.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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There shouldn't be any yellow when it's burning correctly - apart, perhaps, for a brief moment at ignition.
Does the pilot light look a healthy size? It's possible the act of ignition of the main burner is blowing it out.
First thing I'd try is to remove it and make sure it's clean. Give it a good scrub in hot water and washing up liguid, and use a bristle from the brush down the jet. Examine the jet with a magnifying glass to see it's clear - but don't use anything metallic to try and clear any crud.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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