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Steam pressures with Vari-Vac

Hey folks,

Looking a several larger buildings in NYC...

Typical building is around 400,000 sq ft and has ~400 units. Two 300 HP Cleaver Brooks feeding steam to a Vari-Vac controlled distribution system with one or two main valves, communicating with the Vari-Fac, supplying the building. Apartment terminal units are PTACs with variable speed blowers. Operating pressures at the boilers are higher than I typically see for a 2 pipe steam system...around 7 PSI is typical. Vari-Vac DCC1000 control panel.

Two questions floating around my head:

1. Can the boiler pressure be dialed down on this type of system? 7 PSI seems really high, but I'm not quite grasping the relationship between the varying load at the PTACs (due to variable speed blowers), the Vari-Vac valve/vacuum modulation requirements, and probably 3 other things I don't know I'm not grasping.

2. Can I nix the vacuum system and install orifice plates?

The goal here is to recommend effective energy conservation opportunities. Thanks a ton!


  • Dave in QCADave in QCA Member Posts: 1,660
    sounds way too high!

    Usually, the upper limit on these systems is 2-3 psi. 

    For quite a bit of information on these systems, go to the Resources menu item, then "older steam systems" then look for vari-vac.  There are several publications located there.  Might also try looking at the Dunham Section, and also go to google and search for varivac, or marshal engineered products.

    Orifice plates will not replicate the performance of the Vari Vac.  The varivac system is designed to operate at sub atmospheric pressures in moderated temperatures, which means the vapor will be much cooler than the temperature of steam, which will provide better comfort levels in the units and less piping losses in the building.

    Best advice is to study and learn the proper operation of these units, then tune and repair them as necessary so that they operate the way they were designed to.
    Dave in Quad Cities, America
    Weil-McLain 680 with Riello 2-stage burner, December 2012. Firing rate=375MBH Low, 690MBH Hi.
    System = Early Dunham 2-pipe Vacuo-Vapor (inlet and outlet both at bottom of radiators) Traps are Dunham #2 rebuilt w. Barnes-Jones Cage Units, Dunham-Bush 1E, Mepco 1E, and Armstrong TS-2. All valves haveTunstall orifices sized at 8 oz.
    Current connected load EDR= 1,259 sq ft, Original system EDR = 2,100 sq ft Vaporstat, 13 oz cutout, 4 oz cutin - Temp. control Tekmar 279.
  • Long Beach EdLong Beach Ed Member Posts: 686

    Vari-vac is a great system and somewhat complex. 

    Dave is right.  You have quite a bit of learning to be conversant with the Vari-vac and its operation.  It's a very economic system to run and gives great comfort to its occupants.  But it has the be kept in good repair and operated properly. 

    Many NYC municipal housing projects were built with the Vari-vac system.

    And no, you can't abandon it an install orifice plates.  It would defeat the entire advantage of the system and under heat the building. 

    Sure it's running at 7 lbs?  That's too high. Sounds like perhaps the vacuum pump is turned off or failed and some knucklehead's trying to compensate. 
  • Definitely have been seeing 7psi....or higher

    Operating pressures on these systems (I have seen about 10 in the past 6 months) have all been around the 7psi mark. One was as high as 9psi, another lower, at around 5psi. Most at either 6psi-7psi.

    These visits have been in the summer, where the boilers were making steam for DHW production. The super on site confirmed that he operates the system at 7 psi all winter. Also, the Vari-Vacs, on average, are pulling around 6" of vacuum.

    Tons to learn about Vari-Vac....thats why i'm here :)
  • Long Beach EdLong Beach Ed Member Posts: 686

    The Dunham-Bush manual for the Vari-Vac Sub-Atmospheric Heating System states that it operates between 2 lbs of pressure and 15" of vacuum, with the 2 lbs being on a very cold day with steam temperatures of about 218 degrees. 
  • Sounds like the systems I'm looking might be....

    Way off.

    Thanks for the context. This is exactly what I was looking for.
This discussion has been closed.


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