Flushing a central heating system can often be necessary task, particularly when the system is producing rust or sludge, a tell-tale sign that your system is operating inefficiently. Flushing may also be needed if a new system or radiator has been installed.
Turn off all the power that controls the heating system. Any gas or electricity connected to the boiler, as well as control switches on the boiler, should also be turned off. Locate the feed/expansion tank, also known as the header tank, in the loft. It will be the smaller of the two tanks. The tank's water supply must either be turned off, or the ball valve must be blocked. Make sure that there is no water in the ball. A leaky ball can make the heating system less efficient, and would need to be replaced. Stop water from entering the tank during draining by laying a piece of wood across the tank, lifting the valve onto it and tying it to the wood so it will remain shut off.
Check the system to see if it is fully flushed once water has stopped running from the hose. Air in the system may have prevented some water from draining. If this is the case, loosen the ball cock in the header tank so about 6 inches of water can come in. This water should drain from the hose in a few seconds. If an air lock is stopping the water from draining, connect the hose's other end to a cold tap and shoot water into the blocked radiator.