Amateur advice, so beware.
Do not be tempted to use the pressure relief valve to drain the boiler; you may get debris in the valve which will cause problems.
The way I have done this in the past is to turn off the electrical power, let the system cool and then drain boiler, radiators and all by using the drain cock of the heating system, usually to be found at the lowest point. Failing that, use the lowest radiator's bleed valve.
When you come to refill the system, use the built-in filling loop (blue knob underneath the boiler), and use it carefully: people say that it's a bit fragile. Pressurise to 1.0 bar.
Jonathan, October 2008
