What you need
- A radiator key (some modern valves take a flat-head screwdriver)
- A cloth or small towel
- A container to catch drips
The steps
- Turn the heating off and let the radiators cool. Bleeding with the pump running can pull more air in, and the water inside can be scalding.
- Find the bleed valve. It is the small square pin inside a round nut at one top corner of the radiator.
- Hold the cloth under the valve and fit the key.
- Turn anticlockwise about a quarter to half a turn. You will hear a hiss as the air escapes. Do not open it further than a full turn.
- Close the valve as soon as water appears in a steady dribble. That means the air is out.
- Repeat for any other affected radiators, working from the lowest floor upwards.
- Check the boiler pressure. Bleeding releases pressure from a sealed system, so the gauge may now read low. Top it back up to around 1.2 bar. Our guide to repressurising the boiler shows you how.
- Turn the heating back on and check the radiator now heats evenly to the top.
If the same radiator keeps needing bleeding
Air should not keep appearing in a sealed heating system. If you are bleeding the same radiator every few weeks, something is letting air in or generating gas inside the system, and that is often an early sign of corrosion and sludge build-up. Frequent bleeding, cold patches and dirty black water at the bleed valve together point towards a system that needs cleaning. See our guide to radiators cold at the bottom and our power flushing service.