Radiators need bleeding (having the air taken out of them) if they are cool at the top and hot at the bottom. You will need a radiator air bleed key which is available from any DIY store. Locate the air bleed nut at the top of the radiator on the left or right hand side. Simply turn the key anti-clockwise to let the air out (turn slowly until you hear a hissing sound). When air stops coming out and water starts coming out turn the key back clockwise to close.
Tip – It is advisable to have a cloth or some kitchen paper ready to catch the water and protect your decorations and furnishings as the water may be discoloured. If the water is heavily discoloured this may indicate that there is sludge in your system and it may benefit from a powerflush.