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Main vent "breathing" and water hammer

whitwo Member Posts: 50
I've been doing some maintenance on a single pipe steam system and have run into a situation that I'm not sure is expected or not. I was fairly confident that the main vent was the original and was in needed of repair - but also probably not sized for a gas boiler (picture attached). I replaced it with a Gorton No.1 and noticed immediate improvement (less hissing from rad vents, etc). However, I was left with a much louder water hammer than before. My theory on this was that there was a good chance the main return no longer had enough slope. Greater venting + not enough slope = greater water hammer (plus I believe the house has settled in this area).

The thing I'm not sure about is that the main vent tends to "breath" - by "breath" I mean that I hear and feel steam getting pushed through the vent, but it also seems to suck air back in, almost like it exhales slowly and then inhales suddenly. Is this normal?

I noticed it because my system actually has 2 loops - 1 that serves the back of the house and 1 that serves the front. Before doing anything else I decided to replace the other main vent (took some hunting as it is behind a wall in the crawlspace) with a Gorton No.1 as well. This vent doesn't seem to make the same "breathing" noise (or it's possible I just haven't been able to hear it yet).

Is this normal behavior? Or is there a problem on the rear loop where something is causing the steam to retreat and the vent to inhale (vacuum, leak, near boiler piping, etc)? I did have a pro come out before I replaced the 2nd vent. He said the rear main didn't have as much slope as he would like, but seemed to think it was enough. But it was a warmer day and the pipes didn't water hammer like they do when it gets colder (I'm in NC - so cold is relative and there aren't many people that know steam systems around here).

As always - help and wisdom greatly appreciated.


  • Jamie Hall
    Jamie Hall Member Posts: 23,270
    Somewhere in that line that "breathes" there is a sag which is holding some water. Maybe not that much, but some... may be hard to find.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 15,510
    What @Jamie Hall said is correct. Water laying in a pipe makes some of the steam condense and when that happens the volume change fro steam to water is 1700 times. Find the sagging pipe and the problem will be fixed. Breathing at the vent is steam condensing
  • whitwo
    whitwo Member Posts: 50
    Ok, that makes sense. Thanks!