Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

vacuum steam system

boberg Member Posts: 14
I'm a plumbing supv for a city agency in NYC. I have been working in a 20 story building that has a problem with two steam risers. It appears the the only way tenant get steam above the 13th fl is by opening up the zone valve more than 50%, causing the rest of the building to overheat. Because I'm new to this problem{ had not witnessed the problem in the winter} I'm making a visual inspection without test conditions . The steam risers in question is being piped in as a one pipe system,that is no F&T traps at end of line and pitch back to the steam main. Also the steam main and condense header are buried in a trough and jumps up 6ft before breaking across to pick up the 2 risers. It hard to say if the steam main is pitch back to the boilers,they run the lenth of the basement. According to boiler tech there is no banging what so ever. The other spit main has a F&T trap at the end of the run. What I feel saves these tenant in this apt line is that it has ideal vacuum(21 inches) and the condense is being draw back thru the lower floors. We are going to repipe the overhead header pitching to the new F&T traps. What baffles me is what the lack of water hammering signifies, does the high steam pressure and great vacuum capable of pushing the 6ft water column up the risers and back down the return risers ? or is the condense in the steam main pitch back to the boilers without it effecting steam output ? And what about the lack of water hammering? Would it be wise to put another F&T trap at the base of the 6ft jump up or is the steam pressure designed to push the condense to the F&T traps before going up the risers? So many question, I wait with baited breathe. Would appreciate any input,will continue next winter. Even us city workers earn our keep


  • Bob_19
    Bob_19 Member Posts: 94
    I'm already confused

    and the post just put me over the edge. Only kidding.

    But seriously, is this a 2 pipe system? being vacuum return I'm assuming that's what it is.
    Does this happen during winter heating season?
    What pressure are you running at? do you change running pressures during this time of year.
    You mention zone valves, I'm thinking unbalanced vacuum problems due to these valves. All the returns will need to be in vacuum to properly vent these cond lines. Also with a valve you'll need to remove any back-up return water after the valve closes and that steam condenses.
    Also if you need to open the valves to gain more heat, quantity and momentum come in to play here, possibly a faulty valve not opening to the correct aperature.
    Is the lack of water hammer in the affected zone only? or overall? I would open the affected zone return and gauge for vacuum and check amount of return, could be blocked.
    Have all the traps been checked?

    How much makeup water is needed? does it seem excessive, if so blocked or slow returns can affect steam distribution and makeup water rates.
  • Is this a Vari-Vac system

    Are the supply mains in vacuum as well?

    Are the returns above 160°F when this room needs heat?

    I'm getting the picture in my mind that the mains are a Vari-Vac system. The trouble I have with this is the one pipe radiators. How does the air get out, if this system has a vacuum pump on it? There can't be air vents on radiators with a vacuum on them, can there?

    So many questions.


  • boberg
    boberg Member Posts: 14

  • boberg
    boberg Member Posts: 14
    sorry for the confusion

    The heating system I was dicussing was a Vari-Vac system. A 20 story bldg. with 1 zone valve with a micro switch that activated vacuum pumps which eliminated the air and draws the steam around. I never stated it was a 1 pipe system, what I did say was the header in question feed 2 steam risers that was back pitch to the steam supply because there was no F&T trap at the end of the run. The steam supply was in a trough with the condense return that jump up 6ft to feed the steam header. I doubt the steam supply is pitch back to the boilers and there is no F&T trap at the base of the vertical pipe. I assume that the 8 to 10 pdsof steam and the good vacuum pushes the condense around to the return risers . Because I have not witness the system work in the heating season I'm working blindly. Would a F&T trap at the base of the jump up make the system work more efficiently or is the F&T trap at the end of the run sufficient? Does pushing the condense around create wet steam ? .creating uneven heating . Any comment would be deeply appreciated. To be continue next heating season.
  • Steamhead
    Steamhead Member Posts: 16,719
    Vari-Vac Info

    Bob, you're in good hands. Noel and I both have much Vari-Vac information, and this type of system is still being made. So whatever the problems, they can be fixed.

    I wonder about the 8-10 pounds of steam pressure. Is the boiler used for anything other than heating? If not, the pressure is probably too high.

    To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Bob_19
    Bob_19 Member Posts: 94

    anytime you have a change of direction, especially vertical you should have some means of removing any condensate from the low point that will collect there, low loads will cause this steam to mingle to long and condense.
    But, the lack of hammer is confusing, you would think that at that turn up 6' you should have cond sitting there, something is causing the cond to drain back on the off cycle, or it is being carried in small enough amounts up the riser to avoid hammering by the vacuum to the trap.
  • boberg
    boberg Member Posts: 14
    lack of hammering

    is only hearsay, can't testify yet, could the high pressure push the condense into the steam riser and down the return risers? The high pressure also takes care of 1k gallon dhw tanks with steam coils. Spence steam regulators are used which needs about 3to 5 pds to work properly

  • Bob_19
    Bob_19 Member Posts: 94

    steam with momentum will hit that condensate and hammer at a turn/bend or restriction. But yes it could easily push it along to the return to be dropped out, but also the steam will probably carry a fair amount of moisture through the system abit. Wet steam poses it's own unique problems, dryer the better.
    You should also have a trap at the outlet of a automatic valve to remove any condensate that will most definitly collect there when the valve closes.

    Also vacuum systems behave differently, operating under a vacumm some of that condensate could possibly flash to vapor if it's trapped somewhere on the distribution side. Temp stays up but pressure is removed lowering the boiling point of the condensate, this could effectivly lessen hammering.

    About the loop that needs more pressure to heat up, is that your thought? increased pressure is pushing condensate, vacating the lines to allow steam to enter.
    If it was doing this, I would check the return vacuum on that loop to be sure there are no restrictions and it is purging properly. A isolation test would confirm this if possible on that loop, test from each end to see if you achieve vacumm.
    One thought that comes to mind is if you have good vacumm at your pumps this only tells you have no leaks overall,

  • Mad Dog
    Mad Dog Member Posts: 2,595
    before you touch anything, Bob, fire that puppy up..............

    I know, I don't care if it's summer, let 'em open the win dows. Rather they be a little hot now, than you try to tackle this when it gets cold. I wouldn't make ANT assumptions until you can run the system a bit. Mad Dog

    To Learn More About This Contractor, Click Here to Visit Their Ad in "Find A Contractor"
This discussion has been closed.