There are several reasons why you migh need to drain a central heating system, from repairing a leak to replacing radiators. It is a fairly simple process if you follow some basic rules. This guide is based on the most common type of system, an Open-vented System.
Turn off the boiler at the control panel and leave it to settle for a few minutes. Now switch off the main electricity supply to the heating system, which is usually found on a spur near the programmer panel. If your boiler uses solid fuel, make sure the fire is out and let the boiler cool down completely before continuing.
Turn off the water supply to the feed and expansion cistern (usually in the loft) using the stoptap located on the branch pipe leading to the cistern from the rising main. If there is no stoptap you can stop the cistern refilling by placing a length of wood across the top of the cistern and tying the ball valve to the wood in the raised position.
You now need to attach a length of hosepipe to the drain valve outlet. The drain valve is usually to be found near the bottom of the boiler, but may be located further along the pipework of the system and some systems may have more than one drain point. feed the end of the hose outside and towards a drain. You should now locate all of the air vents on your central heating system. These will be on each radiator, as well as on the primary flow near the hot water boiler (if you have a fully pumped system). There may also be some other vent points in your system, but these are the main ones.
Open the drain valve with a spanner or Isle of Man key (a three pronged tool with different shaped sockets) by turning anti-clockwise and the system will start draining along the hose and safely into the drain outside. With this done, visit each of the air venting points, starting at the top of the house, and open it. Doing this will help the system drain more quickly.
Refilling the System
When you are ready to refill the system, close all of the drain valves and turn on the stop tap to the header tank (or untie the ball valve if this is how you stopped it refilling). Starting at the bottom of the house, start to open air vents on radiators until water starts to leak out. Do this for each air vent downstairs before repeating with all the radiators on the upper floor.
Check that the ball valve on the cistern is at the correct height (the ball should be just floating and the tank about two thirds full to allow expansion of the water). Switch the electricity to the system back on and re-light the boiler pilot light. Turn on the system at the programmer panel and turn up the thermostat. As the system heats up you will need to vent more air from the radiators. Do a final check for leaks around the valves, etc, and you are finished.