Why is one radiator cold when the rest of the house is warm?

One cold radiator in a warm house is a local problem, and the useful information is in exactly where the cold is. Working through it in order takes about twenty minutes and usually ends with something free.

What you'll need

• A bleed key
• A pair of pliers or a small adjustable spanner
• A cloth and a small container

Step by step

1. Run the heating properly first.Give it half an hour with the room thermostat well up and all the TRVs open, so you are diagnosing a genuinely cold radiator rather than a satisfied one.
2. Feel the whole radiator with a flat hand.Top, bottom, and both ends. The pattern is the diagnosis, and going straight to bleeding without doing this is why so many people bleed a radiator that has sludge.
3. Cold at the top, warm at the bottom: bleed it.Air rises and collects along the top. Open the bleed valve with a key and a cloth until water comes out steadily, then close it.
4. Cold at the bottom, warm at the top: not air.That is sludge settled in the base. Bleeding does nothing. It needs flushing, which is a job for a heating engineer with the right equipment.
5. Cold at one end only: flow is restricted.Warm at the valve end and cold at the far end usually means that radiator is starved relative to the others, which is a balancing problem.
6. Cold all over: go to the valves.Check the thermostatic head is not set to a low number and not covered by a curtain or furniture. Turn it fully open and give it ten minutes.
7. Take the TRV head off and check the pin.Unscrew the collar and lift the head away. Underneath is a small metal pin. Press it — it should spring back. If it is stuck down, the valve is closed however you set the head.
8. Free a seized pin gently.Grip it with pliers and work it up and down a few millimetres, or tap the valve body lightly. Penetrating spray helps. Do not lever hard — the pin can snap.
9. Check the lockshield at the other end.Take the cap off and turn the spindle anticlockwise a couple of turns to open it. Count the turns so you can put it back if that was not the problem.
10. If both valves are open and it is still cold, balance the system.That radiator is losing the competition for flow with the others. Balancing means closing the lockshields on the radiators nearest the boiler to push flow further out.
11. Stop if every radiator is cold.That is not a radiator problem. Check the programmer, the room thermostat, the boiler pressure and whether the boiler is firing.

Tips

• Check the TRV pin before touching the lockshield. It is the most common single cause and it costs nothing to check.
• Push and release TRV heads a few times each autumn, and leave them fully open over summer. Pins seize from sitting still.
• Count and note every turn you make on a lockshield so you can put the system back exactly as it was.

Common mistakes

• Bleeding a radiator that is cold at the bottom — Air collects at the top. A radiator cold along its base has sludge in it, and bleeding just lets water out and drops the system pressure for nothing.
• Opening the lockshield fully to fix one cold radiator — It takes flow from every other radiator on the system, so you trade one cold room for several lukewarm ones. Balancing is about relative settings, not opening everything.

If it doesn't work

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