Thermostat clicks off but CH stays on

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Hi. I have a problem with my Central Heating.

When the room temperature drops and the room thermostat clicks on then all starts up correctly, motorised valve opens and when fully open the boiler fires up.

But when room temperature is eventually raised and the thermostat clicks off the motorised valve stays open and the boiler continues running and house gets too hot and gas bill increases.

But when the CH is turned off at the controller then the motorised valve shuts and the boiler turns off.

So I surmise that the motorised valve is OK and that it might be the room thermostat. But what can go wrong with a thermostat?

Any advice welcomed.

Thanks,
Max

 

[Spark123]

What type of thermostat, is it digital or mechanical?

 

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What type of thermostat, is it digital or mechanical?

Mechanical. Everything about the system is quite old!

 

[Spark123]

Firstly turn the central heating off at the mains and make sure it is electrically dead.
Open the cover on the thermostat. Make a note of where the wires go. Disconnect the wires. With a multimeter set to ohms measure across the thermostat connections. Adjust the thermostat until it clicks. You should see the ohms change state, i.e. infinity to short or viceversa. If ohms do not change, you may be across the incorrect terminals or the thermostat is broken.
Reconnect wires and replace cover before power up.

 

[toplist]

OK, I'll need to borrow a multimeter from someone tomorrow. But do you think it is possible for a faulty thermostat to cause the symptoms I described? I don't see how it can create the circuit when it clicks on but not break the circuit when it clicks off. Or is there something more that must happen to make the motorised valve close and the boiler switch off?
Thanks, Max

 

[ollski]

Its more common for the microswitch in the valve head to stick to be honest.

 

[Spark123]

The fact that the thermostat is mechanical and switching off / on by the timer appears to reset the problem makes me think in my opinion it is unlikely to be the thermostat but one never knows, it is worth checking as a first port of call.

 

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Thanks all for your replies. I agree that I can't quite understand how it could be the thermostat but I'll check it out. And also I can't quite understand how it could be the microswitch in the valve head when switching off at the controller there is no problem. Hmm, confused!

 

[JPC]

its a feature fitted by BG (when u were not looking) to increase their profits by selling u more gas !!

 

[toplist]

its a feature fitted by BG (when u were not looking) to increase their profits by selling u more gas !!