new vacuum at boiler (single-pipe steam)

David Laidlaw

I just went down to the basement and noticed that the water in the boiler sight glass was acting strange. I opened the purge valve at the bottom of the glass and it sucked in air for quite a white before settling down. It was a little scary -- in six years I've never tried purging and had it suck air in. I don't understand where the vacuume came from. I recently installed brand new vent-rite vents on 14 radiators as well as a new main vent in the basement. They all are venting air out -- I've checked each one. I haven't checked that all of them are letting air back into the cooling system, but I have noticed that many of them are (including the main). I've also looked for sags in the mains, but there don't seem to be any. This doesn't make sense to me -- how could the system have developed so much vacuum?

I should mention that we have replaced a number of radiators on the system (downsizing to reflect closed windows in the bedrooms...) as well as extending the system into a newly renovated area of the building, so there may be a change that has caused a problem. However, it has been working for a few weeks without this problem.