When water is subject to heat it will firstly expand and the will release air trapped within it. Whilst any corrosion in the pipework will release different gases. The best way to see if this is happening will be to feel the radiator, if it is hot at the bottom but still cold at the top then it is definately filled with air and will need to be bled.So, find the bleed valves, which are nearly always near or at the top corners of the radiator, although now and again can be located behind the back of the radiator and hidden from view, behind a plastic cover. Bathroom heated towel rails normally have the bleed valve on the top corner. Double panel radiators will usually have two bleed valves, one for each of its panels. Putney Plumbers bleed systems after a repair or new fit.Required:1). Bleed Key2). Tissue paper.Radiator water is notoriously filthy, so the tissue will come in handy under the bleed valve once you start. Put the bleed key onto the square head of the valve and turn slowly anticlockwise to open the valve. Only turn the valve a few turns and don’t remove it. Ther should be a hissing sound as you turn as air escapes. A Putney Plumber has the experience to bleed radiators in a central heating system. As the water rises the top of the radiator the hiss will turn into wet spluttering. It is time to close the valve, but at the same time catch the dirty water in your tissue. If your system is sealed, it will need recharging, in other words back to working pressure (normally 1bar). If your system is gravity fed there is nothing else for you to do except have a cup of tea in front of a nice hot radiator.