How to bleed a radiator
If your radiators are warm at the bottom but cold at the top, trapped air is making your heating work harder. Here's the simple fix.
Bleeding a radiator releases trapped air so hot water can fill the whole radiator. It takes minutes, costs nothing, and makes your heating more efficient. Here's how.
How to tell a radiator needs bleeding
Turn your heating on and feel each radiator. If one is warm at the bottom but cold at the top, or has cold patches, air is trapped inside and it needs bleeding.
Step by step
- Turn your heating off and let radiators cool (so you don't get scalded).
- Find the bleed valve — a small square nub, usually at the top corner.
- Hold a cloth or small container under the valve to catch drips.
- Use a radiator bleed key (cheap from any DIY shop) to turn the valve anticlockwise a quarter-turn.
- You'll hear a hiss as air escapes — wait until liquid starts to trickle out.
- Close the valve firmly (don't overtighten) as soon as water appears.
- Repeat for any other affected radiators.
Afterwards
Turn the heating back on and check your boiler pressure — bleeding can drop it slightly. If the gauge is below the normal band (usually around 1–1.5 bar), top it up using the filling loop. If radiators keep needing bleeding regularly, there may be an underlying issue worth getting checked.
Why it saves money
A radiator full of air can't heat properly, so your boiler runs longer to warm the room. Bleeding restores full output — more heat for the same gas.
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