How to bleed a radiator — step by step

If a radiator is cold at the top but warm at the bottom, it has air trapped inside. Air is lighter than water and collects at the highest point of the radiator, preventing hot water from filling that section. Bleeding releases the air.

Turn the heating on and allow the system to reach full temperature. Identify which radiators are cold at the top — these are the ones that need bleeding. Turn the heating off and allow 20–30 minutes for the pump to stop (on older systems).

Locate the bleed valve — a small square-head brass fitting at the top corner of the radiator, usually with a white plastic cap over it. Place a cloth underneath to catch any water.

Insert the bleed key into the square slot and turn anti-clockwise (about a quarter turn). You will hear a hissing sound as air escapes. Keep the key in position — do not fully unscrew the valve.

When the hissing stops and a steady trickle of water appears, close the valve clockwise until snug. Do not overtighten — the brass thread is soft.

Check the boiler pressure gauge — bleeding reduces system pressure. If it has dropped below 1 bar, repressurise via the filling loop (usually two valves under the boiler joined by a braided hose) until the gauge reads 1–1.5 bar. Repeat on all affected radiators.