Thermostat stopped working - help please!

[RedskyAtnight]

Help please! We put the heating on yesterday for the first time since the spring and the radiators basically got hotter and hotter and hotter ... even when we turned the thermostat down to the very lowest setting. So it looks like the thermostat is not working properly.

We have an "old-fashioned" manual thermostat that was put in with the house (which is about 25 years old). Never had any issues with it before.

The only thing that's happened since spring is that we had our annual boiler service. The heating engineer found the heating wasn't working properly and changed a valve. Could he have done something that broke the thermostat when he was doing this? Do thermostats have on/off switches that I'm unaware of?

We've turned the heating off for now and are thinking we will have to call an electrician, but wondered if there was anything we could do/check first?

[hedgehoglurker]

Not an expert at all, but our radiators were roasting hot, including ones with the individual thermostatic valves turned to Off.

When I checked the boiler, I saw that the engineer had turned the heating temperature up to 75 degrees. I turned it down to 63 and our system is working normally again.

(This included the 2 new radiators we'd had fitted in our garage conversion that had never worked as there were apparently now "too many radiators" for our boiler. I can't say if 63 degrees is optimal for our condensing boiler, and am happy to be advised.)

[PigletJohn]

It is very unlikely to be the thermostat at fault. Show us a photo of it please.

Wireless thermostats often go wrong, and ones with batteries run down every two or three years, but an old manual one has practically nothing in it that can go wrong.

Much more likely it is a motorised valve.

You've got a hot-water cylinder, haven't you? What colour?

Does your motorised valve look like one of these?

Post a photo please, including whatever wiring you can see.

Is it the part that was replaced?

Did your heating engineer appear to be over the age of 14, and do you know if he was qualified?