The unfortunate thing regarding our home fitted systems such as our central heating is that you can almost guarantee that it will at some stage go wrong (and always at the wrong time). So if you are ever unlucky enough to experience a fault on your central heating such as a pipe blockage, this will definately mean that you will have to carry out a full drain of the system to carry out a repair to a pipe. You could of course call a professional like a Holloway Plumber who is experienced. However if you have are going to have a go, there are some points below:1). The power for the whole system must be isolated. The pump usually has a permanent live electrical feed. Check the supply is off with a tester.2). Always allow the pipework to cool enough so that you can begin.3). Isolate the main supply to the expansion tank.4). Place a wooden batten across the top of the tank with a piece of string tied in the middle, and use this to tie the float valve in the up position so that the inlet valve remains shut throughout. This will help when you de-isolate the water later. 5). Connect a rubber garden type hose to the outlet on the tank and run it to away to a drain. Open the valve on the drain cock, with the right sized spanner. Holloway Plumbers drain systems.6). The water should now drain away, and you can bleed the radiators to drain the water. 7). Find and repair the blockage and then, close the drain cock and all the radiator bleed valves. Remove the hose and temporary water stop in the loft. Re-fill the system carefully.8). When charged, bleeds the radiators in the house, starting upstairs, working down and finally bleeding the pump. Check for leaks.