Thanks to all for the replies. I suspected that what you described is the way to do it, but it's much better to have people who really know tell you.
As for the issues of common property, a common boiler for the entire
35-unit building, and others' legal rights, well, I don't mean to sound snotty any more than you meant to sound scolding, but I'm not an idiot. I've read my condo bi-laws, I know the legalities of my own situation and, bottom line, each unit has its own boiler and separate hot water heater.
I only wish there was a valve at each baseboard unit. But as it stands, I'm about to go get reacquinted with my boiler and my plumbing tools... Thanks again, everyone!
--Bonnie.