Bleeding a radiator is a simple DIY task that takes 5 minutes and can make a noticeable difference to your heating efficiency. Here's exactly how to do it.
What you'll need
- A radiator bleed key (available from any hardware shop, around £1)
- A small cloth or bowl to catch drips
When to bleed radiators
Your radiators need bleeding if they're warm at the bottom but cold at the top. This trapped air stops hot water from circulating to the top of the radiator, reducing its heat output.
Step-by-step instructions
- Turn on your heating and let it reach full temperature. Feel each radiator to identify which ones have cold spots at the top.
- Turn off your heating and wait 10 minutes for the system to cool slightly (hot water under pressure can scald).
- Locate the bleed valve — it's the small square nipple at the top corner of each radiator.
- Insert the bleed key and turn anti-clockwise by half a turn. Have your cloth ready.
- Listen for hissing — that's trapped air escaping. Keep the key in and wait.
- Close the valve as soon as water starts to dribble out (not a stream — just a drip). Tighten clockwise but don't overtighten.
- Check your boiler pressure — bleeding can cause it to drop. If it's below 1 bar, top up via the filling loop.
After bleeding
Turn your heating back on and check the bled radiators are now fully hot from top to bottom. If they still have cold spots, the problem may be sludge build-up rather than trapped air — in which case a power flush may be needed.
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