"Zax" wrote in message
...
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?