A radiator that has become air-locked will need to be bled using a radiator bleed key. The process is not difficult provided that you have the correct key and some confidence. The air inside the radiator may have entered the system following maintenance or after fitting a new radiator. If you are not confident you can call a professional tradesman like a Greenford Plumber who will carry out the work the work and also provide a guarantee.

Method:
Isolate the central heating pump by shutting the isolating valves fitted to both sides of the pumps pipework. There is no real need to isolate the boiler for this. Radiators are fitted with a bleed valve at of its top corners, you will see that it has a square-section or shank in the middle of a recessed round opening.
Professional services like Greenford Plumbers use these keys.

*If you do not have one, they can be purchased from any DIY shop or plumbers merchant.

Fit the key and then turn the square nut anticlockwise for no more quarter of one turn initially. There will probably be no need to turn it further, but, do have a small bowl on hand to catch any water, should you open it too much. As the air comes out you will hear the hiss and maybe some slight bubbling. Normally as the hiss stops, the first dribble of water will appear, so you can now close the valve tightly. Should there be no water evident, this might mean that the valve itself is blocked. Make sure that the feed and expansion tank in the loft is empty. If there is water in it, then close the inlet and outlet valve on each end of the radiator. Remove the screw from the middle of the bleed valve. Clear the hole with thin wire, and re-open one of the radiator valves slightly and eject some water from the hole. Shut the radiator valve and fit the screw back in the bleed valve. Open both radiator valves and try again.
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