The hot water heats up above the thermostat setting when the central heating is on as well. Think it does it also when water not on but not sure as it's the summer and we only use heating first thing in the morning. Am worried that in the winter, we'll end up with scalding water.

Jo B, May 2011

Manwithnoname, thank you very much for your answer. The boiler is a British Gas 330+. The boiler and the hot water tank are in the cellar.The hot water tank has a thermostat on the front. Water tank in the loft. Radiator in every room of a two storey house.

Jo B, May 2011

i'm assuming you have:-
hot water cylinder
motorized divertor valve

could be something as simple as divertor valve passing
BUT
would need to know
make & model of boiler
system configuration i.e. S plan, Y plan etc
fully pumped or gravity circulation

the more information you can give the more accurate the diagnosis of fault

manwithnoname, May 2011