The various valves on a radiator determine the way in which water flows through it and how energy efficient it is. The flow and return valves at the base of a radiator are adjusted to balance the radiators in the house and ensure they all give off the same level of heat. The bleed valve on the top is used to release the air that becomes trapped over time and allows water to fill the radiator. If the valve becomes damaged, it can affect the performance of the whole system and will need to be replaced.
Turn off the central heating and open all valves at the base of the radiators by turning them counterclockwise. Attach a hose to the drain cock on the lowest radiator with a jubilee clip and place the other end outside next to a drain. Open the drain cock with a wrench and allow the system to empty of water before detaching the hose and tightening it again.
Hold the body of the valve steady with the wrench and remove the outer nut with an adjustable spanner turned counterclockwise. Take the valve to a building supply store for a replacement. Test-fit it to the opening to check that it's the correct type.
Wind five turns of PTFE tape around the thread of the fitting in a clockwise direction with the head of the valve facing you. Screw in the valve by hand and tighten it up with the spanner but not so tight that it damages the thread. Remove any excess tape with a knife.
Close the new bleed valve by fitting the radiator bleed key into it and turning it clockwise. Close the valves at the base of all radiators by turning them clockwise.
Turn the central heating back on and open the radiator base valves to allow water to flow back into the system. Slowly open the new bleed valve with the bleed key until water comes out then close it.
Wind the PTFE tape onto the valve thread in the same direction you'll be screwing the valve into place to prevent tears in the fabric. When opening valves on radiators to drain the system, note the number of turns and repeat them in the opposite direction when closing them later on.
Always drain the system before replacing radiator valves or you could get very wet.