How to bleed a radiator (and when it won't help)

If a radiator is hot at the bottom and cold at the top, there is air in it, and you can fix that yourself in five minutes with a bleed key from any hardware shop.

The steps

Turn the heating off and let the radiators cool. Bleeding a hot radiator sprays hot water at you.

Start with the radiator furthest from the boiler, and work back. On a two-storey house, do upstairs first.

Put the key in the valve at the top corner and turn it slowly anticlockwise, no more than half a turn. You will hear hissing. Hold a cloth under it.

When water comes out instead of air, close it. That radiator is done.

When you have done them all, check the boiler pressure. Bleeding lets water out, so the pressure will have dropped. Top it back up to about 1.2 bar with the filling loop.

When bleeding will not help

Hot at the top and cold at the BOTTOM is the opposite problem: sludge, not air. Bleeding will do nothing. That radiator needs flushing, or the system does.

If every radiator is lukewarm, the problem is the boiler or the pump, not the air.

If you bleed a radiator and it fills with air again a week later, air is getting in from somewhere. That is worth an engineer's opinion, because it usually means a small leak or a failing expansion vessel.