How to bleed a radiator (properly)

If a radiator is hot at the bottom but cold at the top, there's air in it. Bleeding is safe, takes minutes, and needs one tool: a radiator bleed key (any DIY shop, about a pound — some modern valves take a flat screwdriver).

Step by step

• Turn the heating off and let radiators cool — bleeding hot radiators sprays scalding water.
• Find the bleed valve: the small square-in-a-round fitting at one top corner.
• Hold a cloth under it and turn the key anti-clockwise a quarter to half turn — you'll hear hissing.
• Wait until the hissing stops and water dribbles out steadily, then close the valve snugly (don't overtighten).
• Doing the whole house? Start with the radiator furthest from the boiler and work towards it, downstairs before upstairs.

The step everyone forgets

Bleeding releases pressure from a sealed system. Check the boiler's gauge afterwards — if it's dropped below about 1 bar, top it up via the filling loop (our boiler pressure guide covers how). Skip this and the boiler may lock out, and the 'fixed' radiator goes cold again for a different reason.