On 08/11/2017 09:06, Tricky Dicky wrote:
Got to agree that it is probably a faulty zone valve. If you need heat whilst waiting repair you should be able to manually open the valve by moving a lever on the motor housing as long as it not the valve itself that is stuck. Manually opening the valve means you will not have any temperature control except for those radiators fitted with TRVs but the boiler stat will continue to control water temp and by turning any manual valves down you should be able to ensure you do not get runaway temp rises. This all assumes the switch in the zone valve which switches on the pump is not what is at fault. A good indicator of a faulty switch is the motor turning but the pump not switching on.

Richard

Thanks for all the replies.

It was the switch in the zone valve. The valve seems to move all the
way across but the switch wasn't quite closing. I've bent the springy
level on the switch a little bit and the boiler and pump now respond to
the room thermostat and still respond to the cylinder thermostat.

Thanks to everyone who replied.

Gareth