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Venting Mains

cdmcdm Member Posts: 61
I have two 2" mains, 30' and 36' long.  I am venting each with a 3/4" Hoffman #75 vent.  The 30' main volume is .690 cu.ft. - 36' is .828 cu.ft..  The Hoffmans are suppose to vent .96 cu.ft/min at 3 oz. steam pressure.  Is a #75 Hoffman enough for these mains or should I increase venting - If so - how much?

cd mulford


  • MTCMTC Member Posts: 181
    I personally aim

    to vent a main in 1 minute at 1 oz. Your Hoffmans will vent .5CFM at 1oz. so you're a little undervented.

    You also want to try to keep some balance, so you want slightly higher venting on the longer main.

    I'd probably put both of your hoffmans (assuming they're working well) on the shorter main, and a Gorton #2 on the longer one.
  • NY_HammerNY_Hammer Member Posts: 65
    Is overventing the main could be a problem?

    there such thing as too much venting of the mains?
  • RodRod Posts: 2,067
    Maximum Venting

    Hi- You might want to take a look at Gerry Gill's website

    Take a look at "Maximizing Your Venting"  in the "Steam Heating" section of the website

    Gerry is an very experienced Cleveland steam pro who is the co author of a very useful book of Steam System venting. It's available in the Shop section of this website. Here's a link to it:

    - Rod
  • MTCMTC Member Posts: 181
    2nd Rod's suggestion

    About Gerry's site/book. I have read most of his site, and have his venting capacity chart, its extremely helpful. You can also get most of the same info that's in his booklet in Dan Holohan's Greening Steam book. I refer to it often.

    There is no such thing as too much main venting, in that it can't ever hurt your system. There is such a thing as too much if its way beyond what you need, in that you're wasting money.

    There's 2 ways to waste your money (that I can think of) in this manner. The first is putting more venting on a pipe opening than the opening can handle. For example, a 3/4" pipe can handle up to 4 Gorton 2s and 1 Gorton 1. Any more, and you're using your money for toilet paper. It will do nothing. A 1/2" pipe will handle 2 Gorton 2s.

    The other way is to set it up so that you have enough venting to vent out all the mains in 3 seconds. It just isn't gonna ever happen that fast. It won't hurt anything but your wallet, but why waste the money. That's the reason for the 1 min at 1 oz method. its easy to calculate the evacuation in 1 minute since everything is rated in CF/minute, and 1 oz is a good benchmark b/c we have data for it, and the early stage of steaming is ideally under nearly no pressure until the mains fill.

    If you have 1 pipe, overventing the rads is a real issue. But mains can be vented as fast as you want to pull money out of your wallet to pay for it.

    Vent your mains fast, and your radiators slowly but completely.
  • cdmcdm Member Posts: 61
    Venting Mains

    Thanks - I will try your suggestion.
  • cdmcdm Member Posts: 61
    Venting Mains

    I just ordered "Maximizing Your Venting" - Sounds like a good reference.

    cd mulford
  • cdmcdm Member Posts: 61
    Venting Mains

    Rod - Your second suggestion makes sense - I don't think doubling my venting will be too much.

    cd mulford
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