Central heating stays on even when switched off

Question

Three bed semi, combi boiler about eight years old, and a wireless thermostat we had fitted last year. Even with the heating turned off at the stat and the programmer, the radiators still warm up in the evening. Our gas bill has jumped and I can't work out what's causing it. Is it the thermostat or a valve?

Kamran · Accepted Answer

It’s difficult to diagnose for sure without being there to inspect the system, and I’d need a little more information to narrow it down. From what you’ve described, I suspect it could be related to the wireless thermostat or receiver.

I’d recommend getting a heating engineer out to diagnose the fault properly and check the controls and boiler.

Joseph · Answer

It could be related to the thermostat.

But going off your description of it warming up in the evening (assuming evening only), I suspect it could be the diverter valve, again assuming radiators warming up coincides with you running the hot water in the evening

Of course it's hard to make a proper diagnosis without testing/diagnosing in person. Call an engineer out.

Anthony · Answer

Good morning,
I would suspect a passing diverter valve.
Even with everything off for heating, I would run a hot tap and then see 8f the rads get hot while the tap is running. When it happens in the evening, does it coincide with a bath or shower running?
The other way to check is to run a tap and if the pipes to the Boiler are accessible, I would feel the flow pipe, normally on the far left in 22mm pipe, and see if that gets hot while the tap is running. Thanks

Zac · Answer

Yeah, you're right. Based on what she's actually said, I'd suspect a motorised heating valve is sticking or failing to close.

If it's a system boiler with a hot water cylinder, it could be the 2-port heating valve or, depending on the setup, the 3-port valve.

If it is a combi, then I'd look at the diverter valve.

The thermostat itself is less likely if the heating is showing as OFF but the radiators are still warming up. The valve can remain open or be receiving a false signal from the controls.

Iain · Answer

It appears to me that the thermostat is wired incorrectly you must be getting a signal for the heating to be on .

Central heating stays on even when switched off

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