Power max 155x System Draining

[Mikebh53]

Hi I need to change some rad valves am I right in thinking that if I turn the boiler off at the main switch then tie the ball cock up in the header tank I can then drain the system from the downstairs drain point. The rad is upstairs that needs the new valves so hopefully won't need every last drop out. Then add the inhibitor to the header tank then let it refill and bleed rads?

Boiler does not have a pressure gauge.

Thanks
Mike