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Air Vent on return line always hissing

joe143 Member Posts: 22
Hello Folks, 
The air valve on the return line is always hissing..is this normal or should it stop and some point..? Looks like the ones used on the radiators but a little larger. it's about 20ft from the steam boiler.
Thanks in advance for your inputs..


  • Ironman
    Ironman Member Posts: 7,224
    Is steam or water coming out of it? If not, then it’s doing its job.

    It may or may not be sized correctly.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Jamie Hall
    Jamie Hall Member Posts: 22,162
    Need a LOT more information on the system. If this is a two pipe/dry return system. the only vents on the system should be main vents (but not always) and a main vent on the return. So... is this a two pipe/dry return system?

    If it is, it is quite normal for the main vent on the return to always be open. That it is hissing suggests very much that you are plagued by some combination of excessive pressure and a much too small vent.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • neilc
    neilc Member Posts: 2,522
    post a picture of said vent, and a distant shot showing us where it is,
    might as well post a picture or 2 at the boiler,
    show the controls, sightglass,
    and a more distant shot or 2, floor to ceiling, so we see all the piping above and around the boiler,
    has the pigtail been serviced? are you blowing down the low water cutoff? is the water dirty?
    known to beat dead horses
  • joe143
    joe143 Member Posts: 22
    Thanks All!