furlong

if its doing as i said all you can do long term is drain the system and fit a new valve.

its poss the valve is letting by. crud blocking it from shutting or spindle seizing up.
when system cold put programmer on hw so the boiler kicks in then after 5 mins feel the pipe to the cylinder will be hot.
then feel the pipe on the out side to the heating it should be cold.
if its hot then the valve is not shutting fully. maybe crud or the rubber paddle in the valve is worn.
new brass valve.
what valve is it ? drayton, honeywell etc.
how old approx is the system or valve?

hi furlong sounds like you have a bypassing motorised valve.

have you got a gravity system with a hot water cylinder in the airing cupboard ?
if so you should see a pump next to cylinder then after the pump will be a motorised valve.
have you one or two valves?

I'm not sure really, I'm new to the boiler world... Sounds like the wiring is wrong... but ask on the forum. When I heat up the radiators it heats up the hot water as well which I find odd, but I dont mind that too much.