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Steam vents missing

Hi all, long time Lurker, first time poster.

First, just thought I should mention that I own and have read lost art, pocket full of steam problems, and we got steam heat several times. I have only worked on a few of them and am a total newbie to steam, so be gentle...

I was looking at a two pipe system today, and noticed that the last guy that was there had the pressure set to 6 and diff to 2. It is a three story 53 unit apartment building with 4"( I think) mains. I set it to . 5 and 1. Not sure if that is right, but this guy also removed the main vents on the return. He didnt plug it, he just left it open. So i am not to sure. Don't trust his judgement.

My question is, what will happen with no vent, just a hole? I mean, that is a big vent right?

This system has a feed pump with a tank.


  • Jamie Hall
    Jamie Hall Member Posts: 23,448
    Say what?

    If you are fortunate.  And if all the traps are working.  And if the vents (or where the vents used to be!) are on the dry returns, not the mains.

    You can, in principle, get away with just an open hole.  In theory.  And I might add, sort of in practice -- a surprising number of vapour systems, particularly, simply had an open pipe which went into the chimney (these often had a large radiator, called a condensor, on that line, though, as well).

    Murphy, however, is alive and well and fiddles with steam systems when one isn't looking.

    However.  If a trap fails open (which does happen) that open hole is going to be hit with steam, unless the radiator with the bad trap is happens to be big enough and the inlet valve is set so that it will condense all the steam that gets to it -- which is unlikely.

    Therefore -- step one is to figure out what size vent or vents you need for each location, and get them on their -- before you have a basement full of steam.

    Unless this system has crossover traps from the steam mains to the dry returns, you need vents on the steam mains.

    Glad you got the pressure reset, anyway!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Dave in QCA
    Dave in QCA Member Posts: 1,786
    Well..... perhaps a different slant on it

    Jamie, I'm not disagreeing completely.  I would have left he vents in the return lines because they certainly do add a measure of protection in the event a radiator trap or crossover trap fails open.   At any rate, they opening should NOT be plugged.

    If the system is in good condition and all of the traps are working, it is probably OK to leave the return line vented through an opening. 

    Here's why I say this.  In looking back through the various vapor systems literature, I note that Hoffman did in fact use a vent on the return piping.  But keep in mind, they also used a device called an Equalizing Loop.  This device allowed the return lines to be pressurized with steam if the pressure in the boiler got higher than 10 oz.  This is how they got the water to return to the boiler when pressures got too high for a gravity return. So, it was critical in normal operation for the return line to have a vent that sensed steam and closed.

    Other systems such as Dunham and Marsh used a device called an Air Eliminator.  This device had a float that would close the vent if condensate backed up, and usually an air check to establish a self induced vacuum, but if steam made it through the return piping, it would simply be blowing out the vent into the boiler room.  These systems also used a return trap to return condensate to the boiler when boiler pressures got too high for gravity return.

    Also, way too many of our old systems have been ransacked and refitted with needless condensate pump units.  In these cases, the vent on the condensate tank becomes the functioning vent for the return piping.  It never has a vent or other restrictive device on it, and by code and the manufacturers warnings, it is prohibited.

    But, if it were me, I would put the vent back in unless it was faulty.  If it is a Hoffman 75 or a Gorton #2, which have floats, it will protect against back up condensate leaking out as well as a steam escaping into the boiler room if a trap would happen to fail.
    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.
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