Assuming this is a 2 story building with pump on upper floor this is no big deal as you only nee to drain the upstairs portion of the system.
would recommend that before you start you check that the expansion tank does not have any slime floating on top of water in it. if it does remove it first or it will go into your system when you refill, also get some anti bacterial treatment for when you refill.
having checked done that;
- switch off cold water supply to house, or if that needs to be kept on put something across top of expanion tank that can be used to tie the ball cock up to (stop system refilling untill job done).
- connect (push on) piece of garden hose to drain valve on one of downstairs radiator, open drain valve, having placed other end of hose somewhere that your happy to have water drain to.
- you will get some water out but after a while flow will reduce, at this point open all the bleed valves on the upstairs radiators, you will hear the air being sucked in as the water falls. wait untill flow stops (all water from upstairs and the one rad downstairs you are draining from has gone). at this point open the air valve alongside the pump, this will allow the water in that part of the system to fall out too.
- when job is done, close all valves, bleeders etc and turn water on or release expansion tank ball cock.
- when water stops being taken up by system go round and bleed all upstairs rads plus one used to drain system downstairs plus the air valv along side pump.
may as well change both isolating valves whilst you have the chance as they are usually less than about £3 each.
