How to bleed a radiator

Bleeding a radiator lets trapped air out of the system, and it is one of the few plumbing jobs that is genuinely safe to do yourself with no tools beyond a radiator key and an old towel. It takes five minutes. The part worth knowing first is which symptom it actually fixes, because bleeding the wrong radiator problem wastes the five minutes and leaves you thinking the fix didn’t work.

The one symptom bleeding actually fixes

If a radiator is cold at the top and hot at the bottom, that is air sitting in the top of the radiator, and bleeding it works every time. If it is cold at the bottom and hot at the top, that is a different thing entirely — sludge, corrosion debris, sitting in the bottom of the radiator — and no amount of bleeding will move it. That one needs a power flush, not a radiator key.

How to do it

1. Turn the heating on and let the radiators warm up, so you can feel which ones have cold patches.
2. Turn the heating off. Bleeding a hot, pressurised system sprays hot water; let it cool for a few minutes first.
3. Find the bleed valve, a small square-socket screw at the top corner of the radiator, opposite the thermostatic valve.
4. Hold a cloth under it, and turn the radiator key anti-clockwise a quarter turn. You will hear a hiss — that is the air escaping.
5. The moment the hiss stops and water starts to dribble out, close the valve again. Do not let much water out; a little escaping is normal.
6. Repeat for every radiator with a cold top, working from the ground floor up.

Check the pressure afterwards

Letting air and a small amount of water out drops the system pressure slightly. Check the boiler pressure gauge once you have finished all the radiators, and top up if it has fallen below the manufacturer’s minimum (usually marked on the gauge). If you find yourself needing to top it up every few weeks, that is not normal and is worth investigating — see our page on why a boiler keeps losing pressure.

If bleeding doesn’t fix it

A radiator that is still cold after bleeding, or one that was cold at the bottom rather than the top, has sludge in the system rather than air. That is what a power flush clears, and it is worth doing properly rather than repeatedly bleeding a radiator that was never going to respond to it. If several radiators across the house are affected, or the water that comes out when bleeding looks black rather than clear, that confirms it.

New radiators, radiators that need moving for a decorator, or a system that has never been balanced properly are all covered on the same page — see radiator installation and power flushing.