I turn on the hot water and the boiler fires up but then the flame dies
> down an so it lights up etc in a cycle at maybe once every second or > quicker.
> Its almost like the boiler fires up thinks the water is hot enough cuts
> the gas notices the water is being demanded and fires up and so it goes
> round and round.
>
> some times it just does it 3 times and then runs okay.
> what could it be?
> I know the boiler well.
> seems to be a dodgy sensor as I gave it a thump this morning, mid shower
> and it worked okay for a while after that.
> which should I check first, pressure sensor, pump, flame detector? > thanks
> Ben
>
First, check the water level or pressure gauge as it could just be getting low in water. Have you bled air from the whole system recently ? Remember the air vent at the top of the boiler.