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Understanding Main Vent Calculation Theory

mattop Member Posts: 3
I need some help properly understanding how to calculate vent capacity for steam mains in a one pipe system. I’m working with a system in a 5 unit apartment building with one Hoffman 75 at the end of two 75 ft 2” mains. Obviously that is inadequate. I’ve read the chapter in Dan’s book, and read Gerry Gill’s write up, but I guess I don’t understand the purpose of timing how long it takes steam to reach the end of the main with the vent removed, or what the capacity of the main pipe is if my vent capacity is limited by the 3/4” tapping that my vent array is going to come off of, which is going to be 9.5cfm.

What am I missing? All I need is 3 Big Mouths in each main, right? I know I’m missing something because if that were always the answer then these calculations and charts wouldn’t be needed. Then I’ve also read several times that one Gorton #2 per 20ft of main is a standard of such.

I’ve searched the forum and reread all this stuff but I guess my thick head isn’t getting something.

Thanks in advance.


  • adasilva
    adasilva Member Posts: 144
    edited January 2019
    The theory behind timing the air within the pipe without a vent in place is to see how long it takes to get steam to that point. From there you can make a proper selection of what size vent or vents you will need to install to allow the same amount of air to vent out within that time frame. One big mouth is good for about 50' of 2" piping.
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    If you have an accurate low pressure, (0-3 psi) gauge, you can add vents on each return, until you see a backpressure of less than 2 ounces, as the air is being pushed out.
    The aim is to balance the resistance of the air escaping from the mains against the aggregate resistance of the air being pushed out of the rads. Keeping the main venting capacity generous causes the mains to be filled first, and then subsequently and simultaneously the rads. This is why starting with slow radiator vents is so important.
    If you have an old fashioned soaker hose, with many holes spouting an equal amount of water each, and you make one hole much, much bigger, then the watering becomes unbalanced, as too much water flows out of the bigger hole.—NBC
  • Jamie Hall
    Jamie Hall Member Posts: 22,947
    The problem with venting theory is that if one actually starts getting into the physics and engineering and all it gets remarkably complicated. I expect that your thick head is doing just fine.

    The beauty of the timing approach is that it will give you the best you can do -- a nice wide open pipe. There is simply no point in putting on more vents, and vents cost money. The beauty of the rules of thumb -- a BigMouth for 50 feet, or a Gorton #2 for 20 feet -- is that they require no though at all, only a tape measure.

    The problem -- if it is a problem -- is that both approaches are conservative. That is, they will always give you at least as much venting as you really need -- and likely a good bit more. As I say, this isn't really a problem, since over venting a main won't hurt your heating at all. Won't help, either, but won't hurt. It may hurt your budget, since vents aren't free...

    The timing method, I might add, has one real advantage: it allows for the insulation of your mains, as an insulated main requires faster venting than an uninsulated one, all else being equal. Which is not an excuse for not insulating your mains!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • mattop
    mattop Member Posts: 3
    edited January 2019
    Thanks for your help so far. It’s starting to get clearer. Perhaps this is a better way to put it to get to the bottom of it all.

    If I have 70 foot mains, and rule of thumb says 2 Big Mouths are adequate, am I wasting money to put on 3, since that would be maximizing the 3/4” vent tapping? Essentially, is the rule of thumb saying that those two Big Mouths are going to vent that 70’ main as fast as the open pipe will, but not max out the 3/4” tapping’s ability to vent? Does that make sense? 3 Big Mouths will max out the vent tapping, but the system isn’t going to vent any faster?
  • Jamie Hall
    Jamie Hall Member Posts: 22,947
    Sounds like you've pretty well got it... point of diminishing returns, and all that.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ethicalpaul
    ethicalpaul Member Posts: 5,618
    But if you time it first with nothing in the tapping, you might find that with one big mouth on there, it's no slower than the open pipe, so you can save $$
    NJ Steam Homeowner. See my sight glass boiler videos: https://bit.ly/3sZW1el
  • KC_Jones
    KC_Jones Member Posts: 5,707
    Putting random numbers on it strictly for understanding purposes.

    Let's say you use 2 big mouth vents and your main fills in X amount of time. Now say you add the third and it vents in X-5 seconds. Is the cost of that vent worth the 5 seconds venting time?

    Now if the third vents it in X-1 minute that would be a different story. Every second saved on venting is a second of burner run time saved, calculate fuel savings then compare to the vent cost. That is the final answer to the question.

    This is how you utilize the open pipe theory, the open pipe is your baseline. You put on 2 big mouth vents, time it and compare to the open pipe. Then you know the benefit of adding the extra venting, roughly speaking of course.

    In addition, boiler output versus system size and venting capacity influence it as well. If the boiler is over sized and has a greater steam production you should be able to take advantage of higher venting capacity as there is more steam to move the air, versus a properly sized boiler.
    2014 Weil Mclain EG-40
    EcoSteam ES-20 Advanced Boiler Control
    Boiler pictures updated 2/21/15
  • mattop
    mattop Member Posts: 3
    Thanks a lot for all of your help. I appreciate it very much. I ordered some Big Mouths and will put them on the system sometime next week.