hot water and steam systems and their controls vary widely. some are
partial shutoffs on a loop, some are full shutoffs, see GRAINGER.
also great info and routine maintenance at:
http://home.howstuffworks.com/how-to....htm/printable
see more about one-pipe steam and 2-pipe steam:
http://en.wikipedia.org/wiki/Hydronics

Ray wrote:
Our building plumber told me that the on-off valves on steam radiators
should be either turned on or off. He said they are not like a water-faucet
which can regulate the stream, but are like light switches which are either
on or off.

Is this accurate?