How do I bleed a radiator?

A radiator cold at the top and warm at the bottom has air trapped in it, and bleeding takes about two minutes. A radiator cold at the bottom and warm at the top has sludge, and bleeding will not help that at all — it needs flushing. Check which you have before starting.

Safety

What you'll need

• Radiator bleed key
• Cloth and small container
• Towel for the floor (optional)

Step by step

1. Turn the heating on and feel every radiator to identify which are cold at the top.Cold at top = air. Cold at bottom = sludge, which bleeding won't fix.
2. Turn the heating off and let the system cool for an hour or two.Bleeding a hot system risks scalding and pushes hot water out under pressure.
3. Start with the radiator furthest from the boiler, on the lowest floor.Working systematically stops you having to repeat radiators as air redistributes.
4. Hold a cloth and container under the bleed valve at the top corner.The water that comes out is black and stains carpet permanently.
5. Turn the key anticlockwise a quarter to half turn until you hear hissing.A quarter turn is plenty. Do not remove the screw entirely.
6. Close the valve as soon as water flows steadily instead of air.Snug, not forced — the brass is soft and the seat is easily damaged.
7. Repeat on the other radiators, working towards the boiler and upwards.Upper floors last — air rises, so they collect what's released below.
8. Check the boiler pressure gauge and repressurise if it has dropped below the marked range.Typically around 1–1.5 bar cold, but check your boiler's own guidance.

Tips

• If radiators need bleeding repeatedly, air is getting in from somewhere — a small leak, a faulty valve, or corrosion producing hydrogen. That's worth investigating rather than bleeding monthly.
• Black water is normal. Clear water suggests the system was filled recently.
• Bleed keys cost pence and are always missing when needed. Buy two and tape one inside the boiler cupboard.
• Some modern radiators have a bleed valve that takes a flat screwdriver instead of a key.

Common mistakes

• Bleeding while the system is hot — Scalding risk, and the system is pressurised.
• Opening the valve too far — The screw can come out entirely, at which point you have a jet of dirty water and no way to stop it.
• Forgetting to check boiler pressure afterwards — Bleeding releases pressure. Too low and the boiler will lock out.
• Bleeding a radiator that's cold at the bottom — That's sludge, not air. It needs a power flush or a chemical clean.

Questions people ask