Step by step
Turn the heating off
Allow the radiator to cool. Locate the small bleed valve near the top corner.
Catch any water
Hold the cloth under the valve and place the bowl nearby.
Release the air
Turn the key gently anticlockwise, only enough to hear the air hiss. Do not remove the valve.
Close the valve
When water begins to appear, turn the key clockwise until it is snug. Avoid overtightening.
Check the system
If you have a sealed system, compare the boiler pressure with its manual and top up only as instructed. Then run the heating and check for leaks.
Know when to stop
When to stop and call a professional
Stop if the valve will not move, water will not stop, the pressure keeps dropping, or the radiator remains cold at the bottom. Ask a heating engineer to investigate.
General information for a UK home; always follow your product instructions and current local rules. Images are illustrative, not technical diagrams.