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Two-pipe steam system - main vent question

Does anyone know if a two-pipe steam system could have more than one main vent?

Comments

  • Jamie HallJamie Hall Posts: 11,458Member
    The short answer is yes. The long answer is... it depends on how the system is built. Some two pipe systems are constructed with crossover traps at the ends of the steam mains going into the dry returns. These also will, almost invariably, have drips from both the end of the steam mains and the dry returns going into corresponding wet returns (which MUST be we at that location -- a problem with some not well thought out boiler replacements). Other two pipe systems do not have crossover traps, in which case they must be vented -- so if there are two or more mains, there will be two or more main vent locations.

    To go back to the systems with crossover traps. All of the main venting in these systems should be in one location, where the dry returns connect before they drop to the wet return and boiler In many vapour systems, the only venting which is "allowed" is at that location; any vents elsewhere will defeat some or all of the operation of the system.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • SteamCTSteamCT Posts: 6Member
    Very helpful, thanks. I am going to be replacing the (very old) main vent on my system, which will hopefully fix the frequent cycling issue I'm having. If not, I will give some further thought to the crossover trap concept. Thanks!
  • clammyclammy Posts: 2,356Member
    edited December 5
    Remenber to chk the type of vent it is weather it’s just a air vent or a vapor vacuumed vent and replace w the correct one . Even though most say just put a standard air vent I find that if it has a vacuumed vent and the boiler is sized correctly ( not oversized ) and pipe up to snuf that replacing w another vacuume vent has yielded great outcomes for myself but it not for ever job and I think I would be looking for a cross over trap Peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating
  • SteamCTSteamCT Posts: 6Member
    I was going to use a Hoffman 401432 - Air Vent for Steam, No. 76, which is described as "...steam main vent, vacuum valve, float type...". Do you know if this would be considered a vapor vacuumed vent?
  • Jamie HallJamie Hall Posts: 11,458Member
    SteamCT said:

    I was going to use a Hoffman 401432 - Air Vent for Steam, No. 76, which is described as "...steam main vent, vacuum valve, float type...". Do you know if this would be considered a vapor vacuumed vent?

    That is exactly what it is.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • PMJPMJ Posts: 866Member
    edited December 5
    If you are going to buy a vacuum vent .... 2 pipe natural vacuum systems require only one check valve of some sort on the dry return. Nothing at all is required on the main. Cold start air in the system will easily leave through the radiators faster than you can heat the piping up anyway. You will be shocked how little air there is to vent on warm starts if your system will hold any vacuum at all.

    I highly recommend vacuum operation. PM me if you are interested to know more. I have run a 2 pipe system this way many years.
  • SteamCTSteamCT Posts: 6Member
    Thanks all, I installed the new Hoffman 76 main vent and the system is performing MUCH better.
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