Why is my radiator hot even with the valve turned off?

A radiator that heats with its valve closed wastes fuel and makes a room uncomfortable. It is nearly always mechanical and nearly always cheap to fix, but there is one case where it is telling you something about the whole system.

What you'll need

• Pliers or an adjustable spanner
• Penetrating spray
• A cloth

Step by step

1. Set the TRV to its minimum or off position.Give it fifteen minutes. TRVs are deliberately slow, and a radiator still cooling is not a fault.
2. Take the head off and look at the pin.With the head removed, the pin should stand proud under spring pressure. If it is sunk down flush, the valve is stuck open.
3. Free the pin.Grip it with pliers and work it up and down. Penetrating spray and a light tap on the valve body help. It should spring back briskly when released.
4. Check the head is actually closing.A failed wax capsule inside the head means it never presses the pin. If the pin is free but the radiator stays hot with the head fitted and set low, replace the head.
5. Close the lockshield as a test.Take the cap off the other valve and close it fully, counting the turns. If the radiator still warms with both valves shut, the valves are not the problem.
6. Suspect gravity circulation if both are shut.On some systems, hot water rises naturally through the pipework even with the pump off, warming radiators on the flow side. It shows up most on a towel rail or a radiator directly above the boiler.
7. Check whether it is the pipes, not the radiator.Feel the panel and the pipes separately. Hot pipes under a floor can warm a room and make an otherwise cool radiator feel warm at the bottom.
8. Consider whether it is meant to be on.Many systems deliberately leave one radiator or towel rail without a TRV so there is always a circulation path. If yours is that one, it is doing its job.
9. Reopen the lockshield afterwards.Return it to exactly the number of turns you counted, or the balance of the whole system changes.

Tips

• Work the pin under every TRV head once a year. It prevents both the stuck-open and stuck-closed versions of this problem.
• Do not leave a lockshield closed as a permanent fix — it unbalances everything else and the underlying fault stays.
• A towel rail that is warm all year is often the system's bypass radiator and is meant to be.

Common mistakes

• Closing the lockshield permanently to keep a room cool — It changes the flow distribution to every other radiator and hides a faulty valve. The valve should be repaired instead.
• Forcing a stuck pin with excessive leverage — The pin is thin and it seals the water in. Snapping or bending it turns a free fix into a valve replacement with a system drain-down.

If it doesn't work

Questions people ask