Blog · How-to
How to Bleed a Radiator (and Why Your Heating Feels Cold)
A radiator that is cold at the top but warm at the bottom almost always means trapped air. Bleeding it is a simple job most people can do themselves — here is exactly how, plus how to know when the problem is something bigger that needs our central heating team.
Why radiators trap air
As hot water circulates, small amounts of air and hydrogen build up inside your system and rise to the top of the radiators. That pocket of gas stops hot water reaching the top of the panel, so it feels cold up top and warm lower down.
A bit of trapped air is normal and easy to fix. But if radiators keep needing bleeding every few weeks, that points to a bigger issue — often corrosion or a small leak drawing air in — and is worth a proper boiler and heating service.
What you need
You only need two things, both cheap and available from any DIY shop:
- A radiator bleed key (or a flat-head screwdriver for newer valves)
- A cloth or small container to catch drips
How to bleed a radiator, step by step
Turn your heating off and let the radiators cool — bleeding a hot radiator risks scalding water spitting out. Once cool, find the square bleed valve at the top corner of the radiator.
Hold your cloth under the valve, fit the key, and turn it anti-clockwise a quarter to half turn. You will hear a hiss as air escapes. When water starts to trickle out steadily (no more hissing), close the valve firmly — do not over-tighten.
Repeat on every radiator, working from the one nearest the boiler to the furthest away, then from downstairs to upstairs.
Check the pressure afterwards
Bleeding releases pressure, so once you are done, check the gauge on your boiler. Most combi boilers should read between 1 and 1.5 bar when cold. If it has dropped below 1, top it up using the filling loop (your manual shows how).
If the pressure keeps falling after topping up, read our guide on why your boiler is losing pressure — that is a separate fault worth chasing down.
When to call an engineer
Bleeding is safe DIY. Call a professional if radiators are cold at the bottom (that usually means sludge, not air, and needs a power flush), if you keep having to bleed the same radiator, or if you smell gas or see a leak.
We cover homes across North Yorkshire and County Durham — get in touch for a free, fixed quote if your heating is not performing as it should.
Quick questions
Do I need to turn the heating off to bleed a radiator?
Yes. Bleed radiators with the heating off and cool. A hot, pressurised radiator can spit scalding water when you open the valve.
How often should I bleed my radiators?
Once a year, ideally at the start of the heating season, is plenty for most homes. If you find yourself doing it every few weeks, there is an underlying fault worth investigating.
Cold at the bottom instead of the top — is that air too?
No. Cold at the bottom usually means a build-up of sludge (corrosion) settling in the radiator. That needs a power flush or chemical clean rather than bleeding.
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Sources & further reading
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