Radiators warm even with the heating switched off, often when you run hot water? Usually a stuck diverter or zone valve. What to check and when to call an engineer.
By AJ, certified Gas Safe engineer and lead plumber at 247 Emergency Plumbers Edinburgh · Checked
If your radiators get warm when the heating is off, often when you run the hot water, a valve is usually letting hot water into the heating circuit. On a combi boiler this is often the diverter valve inside the boiler. On a system or regular boiler it can be a motorised zone valve, or a faulty anti-gravity or check valve.
Quick guide: cold at the top means air (bleed it). Cold at the bottom means sludge (needs flushing). Cold all over while others are hot usually means a stuck valve.
What it could be and what to do
Controls calling for heat You may be able to fix this
Likely cause: A programmer, thermostat or smart control may still be set to heat, or set to constant.
What to do: Check the programmer and thermostat settings, including any boost or holiday mode, before anything else.
Likely cause: HomeServe says a stuck diverter or zone valve keeps sending hot water to the radiators even when the heating is set to off. On a combi the diverter valve is inside the boiler.
What to do: Do not open the boiler. Book a Gas Safe engineer to test and replace the valve.
Stuck zone valve or anti-gravity valve Engineer needed
Likely cause: On older or system boilers, a motorised zone valve, check valve or anti-gravity valve can stick or clog, letting hot water circulate when the pump is off.
What to do: Book a heating engineer to find which valve is passing water and replace it.
Safe radiator checks you can do
These are the checks we suggest without an engineer. Never open the boiler casing.
Heating off first: turn the heating off and let radiators cool before you bleed them or touch a valve. The water can scald.
Feel the radiator: with the heating on, note whether it is cold at the top, the bottom or all over. That tells you which page above to use.
Valves open: check the thermostatic valve is turned up and the lockshield at the other end is not closed.
Bleed: open the bleed valve slightly with a radiator key, catch the water and close it when water flows steadily.
Pressure: after bleeding, check the boiler gauge shows about 1 to 1.5 bar when cold, and top up as your manual shows. Low boiler pressure guide →
Radiator still cold or leaking? We can sort it.
Our callout is from £75 in standard hours (out of hours £100–£180) for attendance and diagnosis. Repair labour and any parts are agreed with you before work starts.
Our emergency line is open 24 hours a day, 7 days a week.
Every heating system is different. This page is general guidance, not a diagnosis.
YOUR QUESTIONS, ANSWERED
Helpful answers before you call.
Why do my radiators get hot when I run the hot tap?
On a combi boiler, this usually means the diverter valve is stuck and sending hot water to the radiators as well as the tap. It needs a Gas Safe engineer, because the valve is inside the boiler.
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