Updated October 2026 · Fraser Bond
Repairs · Heating
Radiators hot with heating off?
Combidiverter valve
Systemzone valve
Settingsprogrammer
SafetyGas Safe
In short: If your radiators get hot when the heating is off, the most common cause on a combi boiler is a sticking diverter valve, which sends hot water to the radiators when you run a hot tap. On system or regular boilers it's often a zone valve stuck open, or the programmer set so heating is on. Check your programmer and thermostat first. Valve repairs need a Gas Safe registered engineer. In rented homes, the landlord is responsible for repairing heating and hot water.
| Symptom | Likely cause |
|---|---|
| Radiators heat when you run a hot tap (combi) | Diverter valve sticking |
| Radiators warm all the time (system boiler) | Zone or motorised valve stuck open |
| Upstairs radiators warm only | Gravity circulation; may need a check valve |
| Heating comes on at odd times | Programmer or thermostat settings |
| Radiators warm after heating switches off | Normal pump overrun for a few minutes |
Common causes
Safe checks to try
1
Check the programmerMake sure heating isn't set to "on" or "boost".
2
Turn the thermostat downSee if radiators cool.
3
Run a hot tapIf radiators heat up, suspect a diverter valve.
4
Turn radiator valves offA temporary fix while you wait for an engineer.
5
Call a Gas Safe engineerFor valve repairs or replacement.
Frequently asked questions
Is it dangerous?
Usually not, but it wastes energy; get it fixed.
How much does a diverter valve cost to fix?
It varies by boiler; get a quote from a Gas Safe engineer.
Who pays in a rented home?
The landlord is responsible for repairing heating and hot water.
Landlord? Let us handle repairs
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