Cold at the top, warm at the bottom? Bleed that radiator

Trapped air stops a radiator heating properly. You need a radiator key, a cloth and five minutes.

Home · By the Natter Daily team · 1 min read · Tuesday 22 September 2026

When the heating goes on for the first time in autumn, feel each radiator. If the top is cold and the bottom is warm, there's air trapped inside and it needs bleeding.

How to bleed it

Turn the heating off and wait for the radiators to cool, so you don't get sprayed with hot water. Find the small valve at one top corner. Fit a radiator key, hold a cloth underneath and turn it a quarter turn anticlockwise. You'll hear air hiss out. When water starts to dribble, close it again.

Do each radiator that needs it, starting with the one furthest from the boiler on the ground floor.

Check the boiler after

Bleeding lets water out of the system, so the boiler pressure can drop. Look at the pressure gauge on the front of the boiler and check it against the range in the manual. The manual also shows how to top it up. If you're not sure, call a Gas Safe registered engineer.

Radiator keys cost a few pounds at any DIY shop. Keep one in the kitchen drawer, because you won't find it again otherwise.