How to remove trapped air from radiators

From time to time, air may become trapped in the radiators in your water-based central heating system, reducing the effectiveness of a radiator.  Look out for radiators that never seem to achieve the heat you would expect, as they may need bled.

Before you start

Make sure your central heating system is switched off (although it should have been on beforehand, as you need warm water in the radiators). If you bleed a radiator when the heating system is on, you may actually end up increasing the amount of air in the system.

You should also make sure you know how to adjust the pressure on your central heating system.  You may need to top-up the pressure after you have bled the radiators.

You will need a radiator bleed key and a large cloth.

Bleeding a radiator

1. Heat the radiator you wish to bleed.
2. Turn off your central heating.
3. Locate the bleed valve on the radiator. It is normally at one of the ends on the top of the radiator, or on the back.
4. Attach a radiator key to the bleed valve.
5. Wrap your cloth around the valve and key to soak up any water that dribbles out.
6. Turn the key a half-turn anti-clockwise to open the valve.  You should hear a hissing noise as air is expelled.
7. When water starts to wet the cloth, close the valve by turning the key half a turn clockwise.  Never tighten the valve any tighter than that.
8. After you have bled all the radiators that need done, check your central heating pressure and adjust it as necessary.  Make slight, gentle adjustments only.