What you'll need
- A radiator key, sold cheaply at any DIY shop. Some radiators use a flat-head screwdriver instead.
- An old cloth or towel.
Step by step
- Turn the heating off and wait until the radiators are cool. Hot water can scald.
- Find the bleed valve. It's a small square or slotted valve at one of the top corners of the radiator.
- Hold the cloth under the valve and turn the key slowly anticlockwise, about a quarter turn.
- Listen for a hiss. That's the air escaping.
- Close the valve as soon as water starts to come out instead of air.
- Repeat on any other radiators with cold spots.
- Check your boiler pressure. Bleeding radiators lowers it, so you may need to top it up. How to top up boiler pressure.
- Turn the heating back on and check the radiators warm up evenly.
Still not warming up?
- Cold at the bottom, warm at the top: that usually means sludge, not air. Bleeding won't help, and it's a job for an engineer.
- Needing to bleed often: air keeps getting in, which can point to a problem worth getting checked.
- Only one radiator cold: check its valves are open. The thermostatic valve on the side may be turned down or stuck.
Many plans that cover central heating include radiator problems, but sludge and the cleaning needed to fix it are often excluded. What boiler cover doesn't include.
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Last updated 30 October 2026