A problem with a radiator in your home does not necessarily mean that you must call a plumber. It is quite possible that you can fix it yourself
A problem that often occurs with wet central heating systems is when the top of the radiator does not heat up as much as the bottom. This is a result of a build up of air pockets in the radiator, the main causes being corrosion in the system that releases gas or air entering the system when the water is topping up. The solution is to bleed the radiator. Bleeding a radiator is a straight forward task that may have to be done on occasion to keep your heating system up to maximum efficiency.
Firstly you will have to locate the air bleed valve which is usually found at the top, on one end of the radiator. You will then need to open this valve with a radiator key to release the air thus allowing the water to fill back up to the top and restore your radiator to its maximum heating potential. The central heating system should also be switched off while you carry out this task.
The radiator key is similar to one that you would use to wind up a clock. Once attached to the valve begin turning and you will hear the hissing sound of the air as it is released. You need to be aware that you need to close the valve as soon as the water starts to escape so it is a good idea to wrap a towel around the key to catch the excess water.
With the central heating system switched back on and your radiator topped up, it should heat up evenly and as a result your heating system will be more efficient.
If you consider that your problem is more serious and that your radiator is completely cold it is still worth trying to bleed it first as it may just be a case of the radiator having become completely full of air. It is not uncommon for an entire system to need bleeding. In this case you would bleed all the radiators and effectively flush the system and help clear it of any corrosion and rust. This will be achieved by continuing to drain the water until you notice clear water replacing the slightly dirty water that has built up in the system.