Central heating not shutting down.

Hi,

I have a Potterton system boiler (Performa 12e) which is controlled by a Myson timer and a Myson thermostat (MRT1).

I always have my heating set at 20c and let it run 24hrs a day. I have came home on two occassions now to find that the house is roasting and that the radiators are on full.

I assumed that it must be a fault with the MRT1 since when I turn the control knob both ways the heating switches off again, almost as if it has just stuck a bit!

The thermostat seems to be clicking on and off quite nicely though and before I go and replace it I was wondering if it could be a problem with the timer electronics not picking up the signal from the stat or if it might even be a problem with the boiler electronics or a micro switch or something.

What would your initial thoughts be?

Many thanks

Steve.

You need to get your motorised valve checked out along with the room stat,motorised valve micro switch could be faulty

As the rads don't shut off I would suspect a motorised valve problem, although it could well be the room 'stat. Try setting the timer to timed CH - if the heating goes off then the stat is most likely, if it's the same then mv.

Update!

I dont think it is the thermostat. I suspect now its the Honeywell motorised valve that may be the problem (Replacement part no 4000 3916 001). This used to make a screeching noise every time the thermostat switched off but no longer does.

Would this cause the effects mentioned above, ie house heating over the set temperature?

This little valve is very easy to get at, right in front of water tank, if I ordered the part would this be a diy job?

Thanks

Steve.

Update again!

After reading another post on here, I think its the mpe222 motorised valve!!!

//www.diynot.com/forums/viewtopic.php?t=154526

Just replaced the MPE222 valve - problem sorted!