this is the type of old radiator valves we have on our old system, It looks pretty obvious but am i correct in thinking that these valves only shut off ONE of the connections to the radiator, and hence we cant remove a radiator without draining the system down.........Also does anyone recall what sort of era these valves were commonly used, as we are trying to figure out just how old the system is, and how long the boiler had been installed before it "retired" last year