Water loss in one-pipe steam system

Bob W._3

You could flood the boiler to see if it is leading above the water line. Mine was.

chris_69

Water loss in one-pipe steam system

I've got a one-pipe steam system; 100 year old radiators; in 1996 the system had a new Burnham conventional gas-fired boiler installed, all new vents, a couple of valves replaced, and a loop added to circulate condensate through a heat exchanger for a radiant floor.

This year the system has begun to lose water. About a half inch in the sight glass per week. That's not much but it has never lost water in the past.

All the radiator and main vents snap open and shut nicely. There's no audible hissing anywhere once the vents are closed (but the system runs at such low pressure that a leak would be hard to hear anyhow). All the piping is visible except for a few tiny runs through walls. I can't see or hear a leak anywhere.

Should I suspect the boiler itself? How would I know if there were a leak inside, above the water line? Our water is not very reactive here, but there is copper piping in the condensate loop -- could that be speeding up electrolytic damage?

I've got a great plumber / hydronic mechanic but I'd like to have some idea what's up before I call him in to go on a fishing expedition that my not end up resulting in a job if there's nothing to be found.