Those of us of a certain age will remember from early childhood that to get milk from a tin it is necessary to make two holes so air gets in as milk comes out. Working on closed heating systems, once pressure is released, this principle has numerous applications:- Radiators can be drained by fixing hose to remote drain off and then opening air vent at radiator, Rad valves can be changed one at a time without drain down, inhibitor can be added by siphon and the replacement of air faulty air valves etc. is a doddle.