Before you begin, you must switch off the boiler, disconnect it from the power supply and turn off the gas or oil supply. (Or, in the case of a solid fuel boiler, ensure that the fire is out.)
You should also turn off the water supply to the feed and expansion tank. If there is no valve for this, or you can’t find it, place a piece of wood across the top of the cistern and tie up the ball cock with a piece of string.
Draining the Water
Once you have located the drain cock (usually at the lowest point in the pipe work), connect a hose to the drain cock and use a jubilee clip to secure it. Place the other end of the hose outside the house so that it discharges into a drain.
Using a pair of pliers, open the drain cock and allow the water to begin emptying. As it does so, open the radiator bleed valves on the upstairs radiators to allow air in to replace the water. If you don’t allow air in, the water won’t come out. In some systems, there may be additional vent points, so check for these as well.
As the level of water in the system drops below the height of the downstairs radiators, you can open the radiator bleed valves in the lower radiators. But beware: if the level hasn’t dropped far enough, water will be pushed out of the valves, rather than air sucked in.
When no more water is coming out of the hose, the system should be empty. However, it is always possible that some water has been trapped by an airlock, so take care when you remove the old radiator.
Refilling the System
First, you need to close the drain cock and all the radiator bleed valves which you opened to help drain the system. Only then should you turn the water supply to the cistern back on and/or release the ball cock. (Make sure that there is enough room for expansion as the system heats up. If necessary, adjust the ball cock.) The water will fill the pipes and radiators. You will be able to tell when the system is more-or-less full when the cistern starts to refill.
Once the ball cock closes off the water supply to the cistern, you can first open the downstairs radiator bleed valves, before doing the same with the upstairs ones.
Check the whole system, especially the newly-installed radiator, for leaks before you reconnect the power and gas (or oil) supply. Now, you can re-light the pilot light (following the manufacturer’s instructions) and fire up the boiler, remembering to turn up the thermostat, if necessary.
As the system heats up, you will need to open each radiator bleed valve again to drive air out of the system. (You will probably have to repeat this action several times over the next day or two until all the air is expelled.)
Once the system is up to temperature, inspect thoroughly for leaks again. This is very important because a connection that is watertight when cold, may expand and leak when it has heated up.