How to bleed a radiator - the 7 steps to follow once a year
To begin, turn your heating off and allow your radiators to cool. Always begin with the radiators on the lowest level and gradually move upstairs to the last radiator in the system. Open the thermostatic valve fully. Locate the air vent and open it, typically this is at the top of the radiator, on the opposite side to the thermostatic valve. With the radiator key, unscrew the air vent, also referred to as the bleed screw. Unscrew it gently, until you hear air whistling. After a while, the valve will spit water. Which will usually be brownish. Leave the valve open for a few seconds until the t...
Source: express.co.uk