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Quick answer
If your radiators get warm when the heating is switched off, hot water from the boiler is leaking into the heating circuit. On a combi, the usual cause is a sticking diverter valve, often noticed when someone runs a hot tap. On system or regular boilers, it is usually a zone valve or three-port mid-position valve that is not closing properly. Less commonly, gravity circulation in older systems causes the same effect. All of these waste gas and should be repaired.
Key takeaways
- Warm radiators with the heating off mean heat is reaching the heating circuit when it should not.
- On combis, check whether the radiators warm when a hot tap runs; if so, suspect the diverter valve.
- On system and regular boilers, a stuck zone valve or three-port valve is the usual cause.
- Older gravity-fed systems can circulate heat without a pump.
- The fault wastes gas, especially in summer when only hot water is needed.
Combi boilers: the diverter valve
In a combi, the diverter valve sends heat either to the radiators or to the hot water heat exchanger. If it is not closing fully on the heating side, radiators get warm whenever you use hot water. Test it: with heating off, run a hot tap for a few minutes and feel the radiator nearest the boiler. If it warms, the diverter valve is the prime suspect. We explain the part in diverter valve problems.
System and regular boilers: zone and three-port valves
These systems usually have motorised valves outside the boiler, often in the airing cupboard. A two-port zone valve opens and closes to control heating or hot water. A three-port mid-position valve can send water to heating, hot water or both. If the valve sticks open, or the actuator head fails, heating gets heat whenever hot water is being made. Our explainer on zone valves shows what they look like.
Gravity circulation in older systems
Hot water naturally rises. In some older systems, especially where the boiler sits below the radiators and there is no anti-gravity valve, heat can circulate by convection even with the pump off. This is more common on older open-vented systems; see gravity vs pumped central heating.
Controls and wiring
Occasionally the problem is not mechanical. Incorrect wiring after a controls change, or a programmer that is still calling for heating, can make radiators run. Check the programmer first before assuming a valve fault.
Why it matters
Heat leaking into radiators in summer means your boiler is heating rooms you do not want heated while making hot water, wasting gas. It can also make the hot water supply weaker, because part of the heat is going elsewhere. See hot water only in summer for how an efficient setup should behave.
Book with Gasmaster
Radiators running when they should not be? Gasmaster Plumbing & Heating, Gas Safe registered (220932), repairs diverter valves, zone valves and central heating faults across Stoke-on-Trent, Newcastle-under-Lyme and Biddulph.
Frequently asked questions
Is it normal for radiators to be slightly warm with the heating off?
A radiator near the boiler may feel a little warm from pipes passing nearby. Radiators getting properly hot means a fault.
Can I fix a sticking zone valve myself?
Some valves have a manual lever that can free them temporarily, but replacing the head or body is best left to an engineer.
Will this damage my boiler?
Not directly, but it wastes energy and a failing valve can eventually stop heating or hot water working.
Sources
- Energy Saving Trust, heating controls guidance (energysavingtrust.org.uk)
- Manufacturer servicing manuals for combi and system boilers
- Gas Safe Register (gassaferegister.co.uk)
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
