My GC heating won't switch off! Is it mid postition valve?

Hi

Hope you can help. The last couple of times I boosted my heating for 1 hour - it stayed on for hours! (although the programmer had switched off after an hour as normal). Can this be down to the mid postition valve? On the side of it, it has a switch marked W M H, and was still on H. I tried gently to move it but it didn't budge. There is also a switch at the bottom of it.

The make of the mid postition valve is ACL Lifestyle. Please can you advise it you think it is this that is sticking on, and if I should force it to M? Also, what does the switch do at the bottom?

Thanks

It is probably stuck

dont force it pop a new one in

Thanks for your reply
Isn't that the last resort though? I wouldn't know how to put a new one on.
Does anyone know if they can just be fixed in-situ please?

It's almost certainly a spring-return valve. SO if you turn off the power to the heating at it's isolator, or plug, then the valve should return to the W position. If it doesn't, it's stuck! It will also stay in the Manual position on a latch, if you put it there - don't. If it isn't stuck you have a Controls problem - programmer, thermostat....

What you refer to as a switch at the bottom is almost certainly the button to release the plastic head from the brass valve. AFAIK you can't buy just the head, so you'll have to buy a whole valve and just use the head.