Radiators Hot When The Heating Is Off

What's happening?

When the heating is off, radiator valves should have nothing hot to give. If radiators are warm or hot, hot water is still going round, and where it comes from separates the causes. On a combi, a diverter valve that does not close fully sends some of the boiler's output to the radiators whenever the hot water runs, which is why the heating seems to come on with the hot water. On a system with a cylinder, a motorised valve that is stuck open does the same, and the hot water schedule itself can warm radiators on a gravity or primary circuit. Frost protection can fire the boiler briefly in cold weather. And the heating programmer may simply be on constant or boost, with the thermostat setting too high to ever stop. Residual warmth that fades in an hour is normal.

What usually causes it?

• A combi diverter valve stuck partly open or worn
• A motorised zone valve on a system boiler stuck open or its actuator failed
• A hot water schedule sending heat through a circuit that includes a radiator or towel rail
• Gravity circulation on an older system pushing hot water round the heating pipes
• Frost protection firing the boiler in a cold house
• A programmer or smart thermostat left on boost, constant or an override
• A faulty thermostatic valve, though that usually affects one radiator only

Is it dangerous?

No. It wastes gas and warms rooms you do not want warm, but there is no risk beyond hot pipework.

Can I fix it myself?

Check the programmer is off for heating and not on boost or constant, and that the smart thermostat app is not overriding it. Switch the boiler off at the spur for an hour: if the radiators cool down, the boiler is still sending heat, and if they stay warm for many hours something is holding them. Turn the heating up and down once to see whether a valve is stuck. Do not open the boiler.

When should you call a heating engineer?

Call a heating engineer if radiators heat whenever the hot water runs, if one or all stay hot for hours with the heating off, or if the motorised valve head is warm or buzzing. Diverter and zone valves are parts of the heating system the engineer will replace.

What will the heating engineer actually do?

• Run the heating off, then hot water only, and watch which radiators warm and when
• Test the diverter valve cartridge or motorised valve actuator for sticking or passing
• Check the programmer and thermostat wiring and settings
• Look for gravity circulation and fit a check valve or zone valve where needed
• Replace the failed valve, flush if debris is the cause and recheck

Who fixes this

• Heating not working
• Boiler repair

Questions people ask

Sources & References

1. Vaillant: how to fix a boiler diverter valve
2. Vaillant: what is a system boiler?
3. Vaillant: heating controls
4. Vaillant: most common boiler problems