Hi Dheron,
Most likely the zone valve that is used to isolate the CH loop when operating in HW only is stuck open. This could be down to a number of things. If the system has been recently installed or drained/refilled check that the valve isn't stuck open on manual override for filling. (This should disengage automatically once the heating has been activated, but they can get stuck if the valve is old or it's been put on upside down)
Forgive me if you already know what the zone valve is, but if you don't you'll most likely find it close to the boiler sitting in the CH feed pipe out of the boiler. It'll look like a brass valve with a plastic box on top with an electrical cable coming out that goes to the wiring centre and should have a little flat metal tab sticking out of the bottom of the plastic box. The tab should be able to slide in a slot that has a knotch cut out in it. If it is stuck in override, the tab will be in the knotch. Just flick it out of the knotch and the valve (which is spring loaded) should close itself (you'll see the metal tab slide to the closed position).
The other possibilities are that the actuator is broken (not likely as the spring should result in it failing to the closed position) more likely is that it's just got jammed try sliding the slider back and forth to free it if thats the case. A wiring fault to the valve is another posibility.
!! NOTE the valve is controlled by switched mains voltage so make sure power is OFF before investigating any potential (pardon the pun) wiring problems !!
If the system has no zone control at all then you will find that some of the radiators do get warm when the hot water is running, this is normal and there's nothing to be done about it, other than adding some zone control to to the system!
Jon, September 2006
