There are often problems in a central heating system that you can easily fix yourself. This includes trapped air in radiators. Even in the best of central heating systems, the radiators sometimes need to be bled. If a radiator is cold at the top but warmer further down, the usual explanation is that air has become trapped and needs to be released. Releasing the air will relieve the pressure and enable the central heating system to fill normally.
Turn off the central heating to prevent more air from entering the system. Locate the bleed valve on the radiator, usually found at one end of the top or at the back. It will have a four-sided nut that the bleed key fits over, as well as a small outlet to allow water out of the radiator.
Fit the bleed key onto the nut on the bleed valve. Hold a cloth below the valve to catch water. Turn the bleed key slowly, counterclockwise, until you hear a hissing sound, indicating that air is being released.
Catch water with the cloth as soon as the hissing stops and water comes out instead of air. Turn the bleed key, clockwise, to tighten the valve.
Locate the filling loop next to the boiler. It looks like a stop tap in the middle of a metal pipe. One end of the pipe is connected to your boiler pipes and the other to a water supply.
Turn the tap slowly, counterclockwise, to allow more water into the system. Water refilling the system is indicated by a rushing sound in the pipes. Keep looking at the pressure gauge on the boiler as the pressure increases.
Close the tap when the pressure indicates one bar, by turning it clockwise, as far as it will go. The rushing noise will stop, and the pressure gauge needle will remain steady. Turn the heating back on.
Check the pressure gauge occasionally to make sure it hovers around one bar. Keep the radiator bleed key looped over the end of a radiator for easy access. Don't worry about over pressurising the system because most systems have a safety valve to release excess pressure automatically.
Always turn the heating off before bleeding radiators; otherwise, more air could enter the system. If all the radiators on the same floor seem to need bleeding, the source of the problem may lie elsewhere, so call a heating engineer.