Below is a basic guide to bleeding a domestic radiator:

1). Fitted to each radiator in the circuit there is a bleed valve on one of its top corners, this is a square shaped block in the centre of the fitting. In order to open this valve, there is something called a bleed key. The key is small and has the correct sized and shaped recess to accommodate the square valve spindle so it can be turned. Professionals such as South Kensington Plumbers carry all styles of bleed keys.


However if no key is there, they can be bought from most plumbers merchants.
Having turned off the boiler and isolated its circulating pump. Use the key to turn the block anticlockwise for a quarter of a turn. There should be no need to turn it any further than that, but just in case of opening too much have a container ready for any spills.


Once the air begins to escape you will hear a hissing sound.
Make sure that the radiator bleed key stays in place on the block so when the air stops and a dribble of water appears you will be able to immediately close off the valve. A South Kensington Plumber can carry out this task if you are not confident or unsure and will also guarantee the work.
If nothing is present when you bleed the radiator, check that the feed and expansion tank in the loft is empty. If it is still full, the valve has a blockage.


Shut the radiator down with the inlet and outlet valves on both bottom ends, then remove the screw from the middle of the bleed valve. With a piece of wire, Clear the blockage with a piece of wire or similar, and open up one of the radiator valves slightly to produce some water from the hole. Shut the radiator valve and refit the bleed valve screw.
Open up both radiator valves again and test again.