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Can someone explain the dropaway pressure on Vents

acl10 Member Posts: 349
Can someone explain the dropaway pressure on Vents. Also how do you use different psi numbers on valves. What determines which psi to use?


  • Jamie Hall
    Jamie Hall Member Posts: 23,276
    Dropaway pressure would refer to the internal pressure in the vent at which it can reopen, having closed for whatever reason. For instance, if the vent closes on steam, it will stay closed above the dropaway pressure, even if it cools. Some types of traps do this, too.

    Pressure rating (and use rating) on valves is determined by the design of the valve body and seat. You should not use valves rated at less than twice the pressure of any safety valve on the system. In my view anyway. Using valves rated at a much higher pressure, though, will just cost you money.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Mark N
    Mark N Member Posts: 1,115
    Buy "The Lost Art of Steam Heat" sold here in the Store. That and many more questions will be answered.
    JUGHNE Member Posts: 11,060
    Jamie, then could you expound on how say ball valves could be marked as approved for working steam and maybe just condensate use.
    Sometimes I see "steam trim" on specs or WSP.
  • Topazy
    Topazy Member Posts: 3
    Right now my pressuretrol is set for a cut-out of 1.5 psig and I am thinking I could lower that even further but the Heat-Timer website doesn't have the technical info save for some comparison venting charts. I may get a vaporstat but not sure how the wiring will be with the pressuretrol.

    In anycase all the vents are vari-valves except for two vent-rite no. 1's on two rads just above the boiler which with the vari-valves got waaaay too hot and waaaaay too quick hence the reason I switched them even though I had them on the closed setting. This worked very well. It would seem the drop away pressure for these vari-valves would be much lower than other adjustable vents such as the hoffman 1A, dole vari-vent, and vent-rite no.1.