Central Heating Not Switching Off!

[DurantUK]

Hi,
My Central heating will not switch off (hot water seems to work normally).
The heating stays on despite being turned to cold on the thermostat and off at the controller.
I think I may have tracked the problem to the Meyson MPE 222 Power Extra valve, when the heating sticks on there is no resistance in the return spring if I slide it to manual. If I slide it back and forth between auto and manual eventually the spring responds and pushes the lever back to auto then the central heating works fine? (for a while).
Any suggestions if this may the valve or the actuator?

Boiler is a Potterton Suprima HE and the system is a sealed system with an expansion vessel.

Thanks

[grasscarp]

You may get better suggestions but I would call in an expert quickly. (with the cost of heating for a minute longer than you want to). I am with British gas for maintenance for a monthly charge that covers call outs.

[VHG]

Have you got a freeze device to check if it is freezing and it switches the central heating on (to stop the pipes freezing)

Our bolier and some of the pipes are in the garage and often our heating is on all night as it is freezing outside.

[SpikeyBush]

My OFCH exhibited the same issues. It was a relay that was fused on in the electronic box on the wall. Ten minute job to fix once diagnosed.

[buildersmate]

Depends how handy you are with a multimeter.
Your initial diagnosis sounds spot-on, but it is what is or is not causing the mid-way valve to switch across to 'hot water only' side once the demand instructions for CH are removed.
It may be the frost-stat malfunctioning, as VHG suggests, or it may be a malfunctioning valve, which is incapable of responding to the instruction from the controller to shift back. There is a position-sensing system (often 2 micro-switches which run on a plastic cam) inside the gubbins of the actuator.

[DurantUK]

Thanks for your replies,
yes the system has a freeze setting that kicks in and raises it to 15 degrees but this thing stays on and on even when its 9 outside, I am pretty handy with a mutimeter i used to be a spark 25 odd years ago before they made these damn houses out of plasterboard. I will give it a test but at the moment I am expecting to change the whole valve thanks for the advice all.

[buildersmate]

Although you may have to buy the whole mid-way valve assembly, its a damn sight easier to whip the actuator assembly off it and merely change that. Saves phaffing about with 'stuff what leaks water'.
I've done this is the past.