Potterton boiler switching on and switching off again in few seconds?

I had a similiar problem last year. in my case it turned out that the chimney was blocked and the "spill switch" on the flue stack was shutting down the boiler because it sensed the backdraft of flue glass.

paul
January 2007

feel pipes at top of boiler if the are warm then boiler is igniting if its cold then boiler poss not igniting.can u see flame thru site glass.

paul
January 2007

have u got zone valves or a zone valve with 3 pipes to it .does pump have water at it [undo screw on top of pump].i think at moment boiler is firing then as pump isnt working water is not being force round system.causing it to over heat and go off

paul
January 2007

dear paul,

I've repeatedly tried to ignite it, it starts producing noises (like it is heating up the water) but then it dies...it happened few years ago on another boiler and the reason was that there was too much air into it...

also, there's a switch: pumped or gravity, might touching this make anything happen?... sorry for stupid questions
[[the name of the model is 'potterton netaheat 40e']]

dan
January 2007

poss pump

paul
January 2007