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Questions about Vacuum System

Chris_83
Chris_83 Member Posts: 24
What controls the amount of vacuum in a vacuum / vapor system, and can (should) it be adjusted?  Are there guidelines as to the optimum min/max amount of vacuum similar to the min/max steam pressure? 



Previously discussed my home's system at <a href="http://www.heatinghelp.com/forum-thread/145189/Vapor-vacuum-questions#p1292367">http://www.heatinghelp.com/forum-thread/142756/Requesting-Help-to-Understand-and-Maintain-2-pipe-system</a> and <a href="http://www.heatinghelp.com/forum-thread/145189/Vapor-vacuum-questions#p1292367.%C2%A0">http://www.heatinghelp.com/forum-thread/145189/Vapor-vacuum-questions#p1292367. </a>



Thanks again to all of the helpful people here!

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 15,334
    Keep in mind

    that there are two groups of systems which are intended to have a vacuum in them, at least at times.



    The first group is actually older.  It is best exemplified by the Paul and Webster systems, but there were others.  In these, the vacuum is deliberately created -- sometimes by a vacuum pump, sometimes by a steam powered ejector (I've always liked the elegance of those contraptions!).  For those, it is certainly reasonable to enquire about how deep a vacuum one wants, and in fact by modulating the depth of the vacuum one can, in effect, modulate the heating in just the same way -- and with a good tight system, over much the same range -- as an hydronic system.



    The other group is some vapour systems, which have a lot of different names but all operate on the notion of very very low pressure steam.  In those, the vacuum is produced naturally as the heat source is dialed down; in the good old days the dialling down was done by closing a damper on the coal fire.  In the systems -- such as the Hoffman equipped -- which had vents which held closed against a vacuum (most modern vents open on a vacuum, like a vacuum breaker) a remarkably deep vacuum could be obtained and held, but it wasn't controlled.  Note that other vapour systems vented directly to the atmosphere, sometimes without a vent at all -- just an open pipe.  In those, obviously, no vacuum was produced.  Systems of any name with modern vents are also, effectively, open to the atmosphere when they cool off, as the vents open when subject to a vacuum.



    So... it depends on what type of system you are talking about.



    Not sure that that helps any...
    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
  • Chris_83
    Chris_83 Member Posts: 24
    System Type

    Thanks, Jamie.  I just realized that I had inserted the same link twice in my post above so I changed the first link.  I have a two-pipe system and there are two vents located together in the steam return line: a Hoffman Main Vent Vacuum Valve #76 and  an American Radiator Co. Ideal Vacuum Vent #822.  There is a vacuum in the system after a heating cycle finishes. 



    It has been recommended that I add a low-pressure gauge (0-3psi), but I am wondering if I need to purchase one that will read vacuum too.  There is already a 30-0-30 gauge on the boiler but obviously it is not very accurate. 



    Can a 0-3psi gauge be damaged by vacuum?  Is there any reason to accurately measure how much vacuum is in the system?
  • Jamie Hall
    Jamie Hall Member Posts: 15,334
    Sounds as though...

    Yours may have been, once upon a golden age, a Hoffman equipped.  That #76 vent is getting hard to find -- hang on to it!  The American Radiator vent isn't exactly common either....



    What you don't need at this point is any more venting!  This is one of those occasions when the comment that you can't over vent a main is wrong (there's an exception to every rule...).



    Whether to add a low pressure gauge.  Hmm.  I never have, on my Hoffman Equipped system -- but I have an old mercury vapourstat with a clear cover.  It's not calibrated, but as a 0 to 8 ounce gauge it can't be beat.  If I didn't have one I would add the gauge; not hard to do.  Will a 0 to 3 gauge be damaged by a vacuum?  Shouldn't be.  On the other hand, it won't say anything useful, either.  I'd probably get one, if I got a gauge at all, which did read the vacuum as well -- but it is more a matter of curiousity and bragging rights than anything else.



    I have to admit that in my less sane moments I contemplate taking the Gorton and non-vacuum Hoffman vent of of my system, and putting a couple or three #76s on.  But I really don't think it would increase the efficiency much... just kind of cool.



    By the way -- you have to keep that 30 - 0 - 30 gauge, even though it never says anything, to keep the insurance and code compliance folks happy.
    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
  • Chris_83
    Chris_83 Member Posts: 24
    Next Steps

    Thanks - I think I am starting to "get it".



    Please let me know if this is correct:



    I shouldn't worry about system venting; just keep the vents that are already installed.  There is no way to adjust how much vacuum the system will have.  As long as it has vacuum after a heating cycle, it's probably ok.



    When I install a  vaporstat, I won't really need a 0-3psi gauge since I'll have the ability to make fairly accurate adjustments for cut-out/cut-in steam pressure.  I should use system performance rather than pressure (psi) to optimize the vaporstat adjustments.





     
  • Jamie Hall
    Jamie Hall Member Posts: 15,334
    Bingo

    Sounds like a plan!
    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
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,422
    Vacuum vents

    The Hoffman 76 is a slower vent than the 75, so in the shoulder season where the time between firings exceedes the time the system can hold a vacuum, the response time will be slower on a call for heat.

    If you had a big building, you might benefit from a vacuum pump.--NBC
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