Alan wrote in message ...
My 10-year old boiler (conventional, open flue) has started smelling
of gas when it fires up (and recently set my CO detector off). The
flue and ventillation are all fine. I've checked the input and burner
pressures and gas rate - all spot on.

The flames don't look too good - very yellow and look too big. All the
burners etc. have been properly serviced recently and this hasn't
fixed the problem.

regards,
Alan

Yellow flames indicate insufficient air flow either because the air inlet to
the burner is restricted in some way or the flue is not allowing all the
combustion gasses out. As your CO detector is triggering I would suspect the
latter.
Yellow flames means soot which could well helpblock the flue even more. Do
get this checked out before there are serious consequences.
In the short term you couldback off the gas flow until the flames turn blue
again. You will get less heat output but at least it should be safe.

Whoever checked the boiler over did not do a good job. Either get him back
or use another reputable one to inspect and report the first one to CORGI