It will be necessary to drain the central heating system if you intend to flush and clean your heating system, change radiators, the pump or the pipework. You should only attempt to drain the system if it is a standard vented one. Sealed systems with combi boilers work under pressure making the refill trickier. For this reason it should be left to the specialists.
The first step is to turn off all the power that manipulates the heating system. Turn off any gas or electricity to the boiler, making sure it is out. Any control switches that are on the boiler, flick to the off position.
Next, locate the feed/expansion tank in the loft. Also referred to as the header tank, it is the smaller of the two tanks. The water supply to the tank can either be turned off, or you must obstruct the ball valve. The valve is opened and closed by a ballcock attached to an arm. When the water rises, the ballcock on the arm is lifted, blocking off the valve. When the water is used and exits the tank, the ballcock drops and releases the valve allowing in replacement water. Check to ensure that there is no water in the ball. If it has a leak that allows in water it will impede its performance. Replacement balls can be purchased from your local DIY store for less than a loaf of bread. To prevent water from entering the tank when you are draining the system, lay a piece of wood across the tank, lift the valve up onto it and tie it to the wood to keep the valve shut off.
Locate the drain cock, which is usually found at the lowest point of the pipework, and connect a hose to it secured by a jubilee clip. Run the hose outside but not onto roads or pavements where it could cause an accident or nuisance. You may need a pair of pliers to open up the drain valve.
To speed up the flushing process, open the bleed valves on all the radiators. Begin with the highest level and work your way down once the water has dropped. If you open the lower radiators too soon water will push out the valves rather than suck in the air.
Once the water has stopped draining from the hose, check the system has fully flushed before attempting to refill. There may be some air in the system that has prevented the water draining. To check this, loosen the ballcock in the header tank so as to allow for about six inches of water to fill in. After a few seconds you should see this drain from the hose. If you have an air lock hindering this, simply connect the other end of the hose to a cold tap and shoot some water into the blocked radiator.
Refilling the heating system
Shut all the bleed valves and the drain cock that has been opened, and then release the ballcock in the feed/expansion tank to allow it to refill. Once it is full, bleed the lower level radiators and work your way upward. This should complete the system refill.
Check through the system to make sure all the valves are securely shut as necessary, and then turn on or relight your boiler. As the air in the system expands there may be a knocking noise. To stabilise the system, you should bleed the radiators again and check the heating system throughout for leaks.