Sealed System
First thing to do is to switch off the boiler,
and locate a drain off point normally found at the lowest point in the system under a radiator. Attach a hose to the drain off (good idea to secure hose with a jubilee clip and have a roller tray to catch any drips) and then open valve with an adjustable spanner.
Radiator valve with a drain off point.
You will need to allow air in to aid draining so you should undo a bleed valve a little on an upstairs radiator. When water has stopped you can work on the system.
Refill the system see faq 6 and add / top up inhibitor see faq 28
Bleed radiators starting with the ground floor ones first see faq 16
Vented System
As above but firstly you should tie up the ball valve to a plank of wood placed across the cistern to cut off the water supply. To refill, simply untie the ball valve. Don’t forget to add inhibitor and bleed rads.
Please note that each drain off point only drains that particular “zone” of pipework so you should check for other drain off points for a complete drain down.