As water is heated in a central heating system it will expand, and the expansion has the affect of releasing any trapped air that is present. Any air in a heating system will cause problems for the hot water to move around the system and its radiators. Corrosion in the pipework will release different gases. A good indicator is to feel a radiator. Hot at the bottom but cold at the top indicates an airlock and bleeding will be required. Locate the bleed valves, which are usually at the top corners of a radiator. Sometimes, they are located behind the back of the radiator and obscured, behind a plastic cover. The water that is moving around the system in the radiators is dank and black, therefore it is a good idea to have a cloth ready under the bleed valve when you begin. Heated towel rails for bathrooms usually have their bleed valve at the top corner. Panel radiators with two sides will usually have two bleed valves, one for each of the sides. East London Plumbers bleed systems after a repair or new installation.Place the bleed key on the square head of the valve and undo slowly anticlockwise. The valve only needs a couple of turns to begin releasing the air, so definitely do not remove the block. The air will hiss as it is released. An East London Plumber has the experience to bleed radiators in a central heating system. The water will then rise in the radiator following the air and will start to leak out, (use the cloth) The valve can be closed when you see water. There is a chance that if it is a sealed system it will need a slight re-fill back to 1bar working pressure. If gravity fed enjoy a cup of tea in front of a nice hot radiator.