When it's needed
If a radiator is cold at the top but warm at the bottom, that's air trapped at the top of the radiator stopping it from filling with hot water properly. Bleeding it lets that air out. See our no heating page if this is happening across several radiators at once, since that can point to a wider system issue rather than just one radiator needing air let out.
What you'll need
- A radiator bleed key (cheap, available from any hardware shop — some radiators take a flathead screwdriver instead)
- A cloth or small container to catch drips
Steps
- Turn your heating off and let the system cool — bleeding a hot radiator risks a scalding water spray.
- Find the bleed valve — a small square-ish fitting at the top corner of the radiator.
- Hold your cloth underneath, then turn the key slowly anti-clockwise, a quarter turn at a time.
- You'll hear a hissing sound as air escapes. Keep the key steady until the hissing stops and water starts to dribble out steadily.
- Close the valve promptly once you see water, turning clockwise until snug — not overtightened.
- Check your boiler's pressure gauge afterwards — bleeding radiators drops system pressure slightly, so you may need to top it up. See our pressure guide for how.
When to stop and call someone instead
If a radiator needs bleeding repeatedly — every few weeks rather than occasionally — that's not normal, and points to a system fault worth diagnosing properly rather than a job you should keep repeating. That's exactly the kind of pattern our problem finder can help you make sense of.