Draining the system to change a 2 port valve

[scorps1212]

Hi all. Im new here. My Dad came across a problem with his heating the other day. When he turns the room stat, it clicks but does not bring the heating on. I went up into his airing cupboard where his 2 port valve is. When i manually move the lever to heating, it stays there but does not work the valve. A plumber told me that the valve needs replacing.
To drain the system to change the valve, am i right in thinking i've got to tie the ball valve up in the loft on the tank and drain the system from a downstairs radiator?
If not, can someone please tell me how to drain the system to change a 2 port valve.
Thanks in advance.

 

[petit_pablo]

Thats should do it. Remember to refill with inhibitor added

 

[scorps1212]

Nice one. Ill have to get some inhibitor then.