Bleeding radiators is one of the few central heating tasks homeowners can safely do themselves. If your radiators are cold at the top but warm at the bottom, trapped air is the likely cause — and bleeding fixes it in minutes.
What you need
- Radiator bleed key (£1–2 from any DIY store; some modern radiators use a flathead screwdriver)
- Old cloth or small bowl for water drips
Step by step
- Turn the heating on and let it reach full temperature — this makes it easier to identify which radiators need bleeding (cold tops).
- Turn the heating off and wait 10–15 minutes to reduce pressure slightly and avoid scalding water spray.
- Identify the bleed valve — a small square nut at the top end of the radiator (usually on one side).
- Place your cloth under the valve to catch any drips.
- Insert the bleed key and turn slowly anti-clockwise (about a quarter turn). You'll hear a hissing sound as air escapes.
- Hold the key in position until water starts to trickle out, then close immediately by turning clockwise. Don't over-tighten.
- Repeat for every radiator that was cold at the top, starting from downstairs and working up.
- Check your boiler pressure after bleeding — it will have dropped. Re-pressurise to 1–1.5 bar if needed.
When to call an engineer
If radiators need bleeding repeatedly, or if the water is very dark or black, you have a sludge problem. Book a power flush assessment.
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