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Vacuum ONLY vent for Main????


I'm wondering if there is a such thing as a "vacuum only" vent for steam mains?

I have PLENTY of venting in my system, but due to the way my house is radiated,
(see ad nauseum details here:
my system pressurizes and then cycles a bit more than is desired. Because of this, I get a some hissing from radiators around the house. And because the main vents don't really get a chance to cool down, they aren't doing their fair share of the "inhaling" into the system.

SO, I was wondering if there is a vent I could put on the main that ONLY opens when there is any sort of vacuum in the system... to act as the "inhaler" until the mains can open. My thought being that if this vent could be the path of least resistance it would take that burden off of the radiator vents, and also make for a quieter system.

I know it's treating the symptom, not the problem (see link above), but it's not feasible at this point to move radiators around in our house to even out the heat.

Thoughts? Random observations?



  • Mark N
    Mark N Member Posts: 1,115
    Get a vacuum breaker, I think it is a Hoffman #62
  • Fred
    Fred Member Posts: 8,542
    It is the Hoffman #62 but I'm not sure if you build enough vacuum to pull it open. I guess you have to try it and see what happens
  • Shalom
    Shalom Member Posts: 165
    edited February 2015
    Spec says "Adjustable from 1⁄4" - 20" (8-508mm) Hg vacuum - factory set at 2” (51mm) Hg vacuum".

    edit: the spec is here. It says to replace it every 15 years.
  • cnjamros
    cnjamros Member Posts: 76
    @Shalom: thanks.

    How much vacuum does a 1-pipe system generate? I have no frame of reference.
  • Fred
    Fred Member Posts: 8,542
    Most don't generate any to speak of. It is driven by each individual set of circumstances.
  • cnjamros
    cnjamros Member Posts: 76
    As it turns out, the vacuum breaker wasn't necessary.

    I put Vent Rite #1s on the problem radiator in the master bath, as well as the larger rad in the bedroom that hisses occasionally. I sat with them for a heating cycle and adjusted just below where they started to make noise.

    My system seems to run at about 2.4 oz for most of its cycle. When it starts to go above that, it continues to rise well past 8oz, or until the VaporStat shuts it down. So, I adjusted my Stat for 3oz with a 2oz differential.

    With those changes, I'm now finally running a nice, quiet system with even heating in all my rooms. Finally. What a thrill!

    Thanks again for all the assistance.