How to bleed a radiator, and what the cold spots mean

A radiator that is cold at the top has trapped air. Turn the heating off, hold a cloth under the bleed valve at the top corner, open it a quarter-turn with a bleed key until air stops hissing and water appears, then close it. Check the boiler pressure afterwards and top up if it has dropped. Cold at the bottom means sludge, not air, and bleeding will not fix it.

Which radiators need bleeding?

Ones that are hot at the bottom and cool at the top, or that gurgle. Air collects at the highest point, so upstairs radiators and the tallest radiator on the run are the usual suspects. Work with the heating OFF so the pump does not drag more air around the system while you bleed.

How do I bleed one?

Put a cloth under the bleed valve, the small square-headed vent at a top corner. Fit the bleed key and open it about a quarter of a turn. Air hisses out; when a steady dribble of water replaces the hiss, close the valve. Do the furthest radiator from the boiler first and work back. Bleeding lowers the system pressure, so check the boiler gauge when you finish and re-pressurise to about 1.2 bar if needed.

What if the radiator is cold at the BOTTOM?

That is sludge, magnetite settled at the bottom of the radiator, and no amount of bleeding moves it. The fixes are a chemical clean or a powerflush, and a magnetic filter to stop it recurring, which is heating engineer work. Persistent air in the system is also worth an engineer's look, because something is letting air in.

Sources

• HSE, Gas health and safety in the home (accessed 2026-09-05)
• GOV.UK, Find a Gas Safe engineer (accessed 2026-09-05)

This page is general information for property professionals, not legal advice. For a decision that matters, take advice on your specific facts.

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