If you find that the top half of a radiator is not as hot as the bottom, the cause is most likely to be trapped air in the heating system. This can be easily remedied by bleeding the air from the radiator.
Tools required: Radiator bleed key and an old towel or cloth.
First of all, turn off the central heating system, you will need a radiator air bleeding key, which are available at DIY stores such as B & Q, Focus, etc and an old towel or absorbent cloth. Locate the air bleed valve at the top end of the radiator and hold the towel / cloth just beneath it.SAFETY POINT – Any water that comes out of the bleed valve may be very hot, so take care to avoid being scalded. Using the key turn the valve anticlockwise just enough to release the trapped air (a low hiss can usually be heard ) when water starts to dribble out of the valve close it tightly and wipe away any drips or runs of water from the radiator.
Switch the heating system on and check the radiator. If the radiator still feels cooler at the top repeat the procedure.