Why Does a Radiator Heat Up When Only the Hot Water Is On?
For one or more radiators that become warm when the programmer is set to hot water only and central heating is supposed to be off.
What this usually means
If radiators heat up when only hot water is selected, hot primary water is reaching the heating circuit when it should normally be directed to the cylinder only. On conventional Y-plan or S-plan systems this can happen when a motorised valve is sticking, passing internally or being driven incorrectly. However, a bathroom radiator or towel rail may sometimes be deliberately connected so it warms with hot-water operation, so the first step is to identify whether the behaviour is new or has always been part of the system design.
What may be causing this
A motorised valve is sticking or passing internally
Clues: The behaviour is new, several radiators warm during hot-water-only demand, or the valve has become noisy or unreliable.
What to do: A heating engineer can check whether the two-port or three-port valve is reaching and sealing the correct position, then replace the actuator or valve body as required.
A three-port mid-position valve is not moving fully to hot-water-only
Clues: The system uses a hot-water cylinder and a Y-plan valve, and radiators warm whenever the cylinder calls for heat.
What to do: The valve motor, gearing or internal paddle may need servicing or replacement. Electrical checks belong with a competent heating engineer or electrician.
The controls or wiring are calling for heating at the wrong time
Clues: The fault appeared after a programmer, thermostat, wiring-centre or valve change.
What to do: Have the control sequence checked against the system wiring plan. Do not move wires experimentally.
A bathroom radiator or towel rail is intentionally connected to the hot-water circuit
Clues: Only one bathroom emitter warms, it has always behaved this way, and the rest of the heating stays cold.
What to do: This may be intentional on some older or bespoke systems. Confirm the design before paying to alter something that is not actually faulty.
How to check what is happening
Heat is likely leaking or being directed into the central-heating circuit.
It may be intentionally piped to warm during hot-water production.
A sticking actuator or valve is plausible and needs competent testing.
Incorrect control wiring or valve setup becomes more likely.
Can you fix this yourself?
You can safely establish the pattern by using the programmer and noting which radiators warm, but diagnosing a motorised valve normally involves hot pipework and mains-voltage controls. Once the symptom is confirmed, a heating engineer should test the valve and control sequence rather than having you remove covers or probe wiring.
- Phone camera for valve and programmer labels
- Heating programmer instructions
- Notebook for recording which radiators warm
What you can safely try
- 1
Run a controlled hot-water-only test
Set central heating off and hot water on. Wait for the cylinder to call for heat, then note exactly which radiators become warm.
- 2
Check whether it is one bathroom emitter or the whole circuit
A single towel rail that has always warmed may be intentional. Several normal room radiators warming points much more strongly to unwanted flow into the heating circuit.
- 3
Identify the visible valve arrangement
Without removing any covers, photograph the labels on accessible two-port or three-port motorised valves near the cylinder or boiler. This helps a heating engineer identify whether the system is S-plan, Y-plan or another arrangement.
- 4
Check whether the fault followed recent work
If a thermostat, programmer, valve or cylinder was recently changed, tell the engineer. Timing is a useful clue for wiring or commissioning errors.
- 5
Have the valve and controls tested
A competent heating engineer can confirm whether the valve is passing mechanically, failing to travel, or being energised incorrectly before replacing parts.
Call a heating engineer if…
- Any check would require removing a motorised-valve cover or wiring-centre lid.
- There are exposed electrical conductors, scorch marks or a burning smell.
- The hot-water cylinder or pipework is leaking.
- The boiler repeatedly locks out or overheats during the fault.
- You are unsure whether the system contains an unvented hot-water cylinder requiring qualified work.
Get the right professional.
This problem can have more than one cause. Start with the checks above: a heating engineer is the usual first route, while a electrician may be more appropriate when the clues point that way.
Sources used for this guide
We prefer primary UK guidance, regulators and manufacturers for technical and safety-sensitive claims.
- V4073A 3 Port Mid Position ValveResideo / Honeywell Home
- Heating Controls Wiring GuideResideo
- Motorised Valve – 3 Port DiverterDrayton Controls
Last reviewed: 31 August 2026. Home Manual updates review dates only when the substance of a guide is checked or changed.