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what's this valve?

Hi all,
Just a homeowner here. I have steam radiator heating from our boiler. In the basement, this valve has begun to hiss. Can someone tell me what valve this is? It looks different than the valves I've replaced at the radiators themselves. Thanks!


  • bburd
    bburd Member Posts: 917
    edited January 2022
    That is a main vent. You may have more than one. It helps to minimize fuel use and balance the system by allowing air to vent quickly from the steam pipes at the start of each heating cycle.

    It is not unusual for the main vent to hiss  at the start of a heating cycle, but it should close when steam reaches it and should not release steam.

    Many older main vents are undersized for oil or gas fueled systems. Operation can sometimes be improved by putting on a larger one.

    Does your system heat quickly, evenly and quietly?

  • BobC
    BobC Member Posts: 5,478
    That might be a ventrite #35, it's a very good main air vent but probably to small to do it's job. Do you have more than one steam main? What is the approximate length and diameter of each steam main?.

    In general you want to vent the radiators slowly and the steam main fast. With the above info we can tell you what kind of main vent you should have.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • neilc
    neilc Member Posts: 2,703
    at the boiler,
    what are you seeing for pressure when the vent is hissing?
    has the pigtail been serviced ?
    what are your Pressuretrol settings?
    picture of the Ptrol, and general distant shot of the boiler
    known to beat dead horses