How to bleed a radiator

Why a radiator is cold at the top, how to bleed it safely, and when a cold radiator means something a plumber needs to look at.

A radiator that is hot at the bottom and cold at the top has air trapped in it. Bleeding lets the air out so the whole panel fills with water again. It is a job most people can do, and it is worth doing before the heating season.

How to do it

• Turn the heating off and let the radiators cool, so you are not working with hot water.
• Put a cloth and a small container under the bleed valve, at the top corner of the radiator.
• Turn the valve anticlockwise a quarter turn with a bleed key until you hear air hissing.
• When water comes out in a steady dribble rather than spitting, close it again.
• Once all radiators are done, check the boiler pressure and top it up to the normal band if it has dropped.

When it is not just air

A radiator that is cold at the bottom, or one that needs bleeding again within days, is a different problem: usually sludge in the system or a balancing issue, not trapped air. That is a job for a heating engineer, and a power flush or a balance is what fixes it, not a bleed key.

Radiator fitting and bleeding is listed under plumbing on Halo Hive.

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Last reviewed 2026-09-28.