"Zax" wrote in message
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"John" wrote in message
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"Zax" wrote in message
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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.

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?

its a glow-worm balanced flue boiler 100B
last serviced 2 years ago
all ventilation clear.

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