How to bleed a radiator

Trapped air stops hot water filling the whole radiator, so the top stays cold. Bleeding lets the air out. It is a quick job and needs no special skill.

What you need

• A radiator key (or a flat screwdriver on some newer valves)
• A cloth
• A small container or old mug to catch drips

Steps

1. Turn the heating off and let the radiators cool. Bleeding a hot system can spray hot water.
2. Find the radiators that need it. Feel each one. Cold at the top and warm at the bottom means air. Cold at the bottom is usually sludge, which is a different job.
3. Find the bleed valve. A small square nut at one of the top corners, sometimes behind a plastic cover.
4. Put the cloth and container under the valve to protect the floor.
5. Open the valve a quarter to half a turn anticlockwise. You will hear hissing as the air escapes. Do not open it fully.
6. Wait for water. When a steady dribble of water replaces the hiss, the air is out.
7. Close the valve clockwise. Snug, not forced.
8. Repeat on the other radiators, starting downstairs and working up.
9. Check the boiler pressure. Letting air out drops the pressure. If the gauge is below about 1 bar, top it up using the filling loop as the boiler manual describes, then turn the heating back on.

If a radiator needs bleeding every few weeks, air is getting into the system somewhere and it is worth having a plumber look at it.