A little history is necessary:

In 1995 our 80-year-old six-unit apartment building replaced our
furnace-boiler. From the start steam came out of the chimney. That seemed to
grow worse until 2004, when the steam got really bad and soon the boiler
broke down.

We replaced the boiler -- and there was no steam at all. The new one was
much more efficient.

But this morning when the temperature dipped down into the single digits, I
saw steam coming out of the chimney for the first time. It wasn't a great
deal, but enough to cause a little alarm.

I'd love to hear from anyone who has had experience in this sort of thing.