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Help Sizing Steam Main Vent

Hi Everyone. Recently purchased a two family home built in 1920. I have two old snowman boilers (1 for each unit). I believe they were originally coal fired converted to natural gas burners at some point.

Currently looking to change the main air vents for each system. Both have a Hoffman 4A installed currently but unsure of their age. Based on the Vent Capacity Chart I have 1.077 cubic feet of air in the 1st Floor unit steam main and 1.368 cubic feet of air in the 2nd Floor unit steam main.

So looking at the chart I think that a single Gorton #2 or a couple of Gorton #1s would do the trick. I guess where I'm a little confused is the three columns with the 1, 2, 3 oz of CFM. I'm not sure I quite understand what that means and which column I should be referencing.

Also curious to get everyone's thoughts on if I should add another main vent. The boilers have two steam pipes coming off the top. One goes around the perimeter of the house (currently have the Hoffman main vents). The other goes straight to the back of the house to feed two further away radiators. Those don't have a main vent. Is it worth adding a main vent or just sizing the radiator vents a little larger to include the short run of pipe before the branch off to each.

Thanks in advance!


  • Jamie Hall
    Jamie Hall Member Posts: 23,323
    Probably a Gorton #2 on the longer main, and I'd be much inclined to put a #1 on that short main. That way the radiator vents only have to vent the riser and the radiator, and can control the heating better.

    I wouldn't try to play a numbers game with the three columns. They are very useful for comparing relative venting speed, but what the columns tell you is how much air that vent will pass at that pressure differential. Simply saying well, I want so and so many cubic feet of air to leave in such and such a time is all very fine, but steam in a cool or cold main doesn't actually move the way air would. That said, it's almost impossible to overvent a main.

    But it does bring up one other point -- those mains are insulated, aren't they? They need to be -- that makes more difference than the vent size!
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 15,540

    How long is the main that doesn't have a vent? And what size pipe?
  • DoubleBoiler
    DoubleBoiler Member Posts: 3
    @Jamie Hall Thanks for your response, possibility of overventing was a thought that popped into my head glad to hear I don't need to worry about it. I'll plan on using a Gorton #2 for that. I believe what I have is a counterflow system where the main runs all the way around the basement then comes back to the boiler above the water level and drops down into a return to the boiler. The current vent is right at the elbow down to the return. Based on my research here I'll elevate it up a bit so it's not right at the elbow getting hammered by water/steam.

    And yes at some point someone removed all of the asbestos insulation and replaced with fiberglass so thankful for that. Needs so minor repairing in areas which is on my to do list.

    @EBEBRATT-Ed That second "main" is 1-1/2" pipe. For the 1st Floor boiler it is 9' long not including the branches to the 2 radiators it feeds. For the 2nd Floor boiler it actually only feeds a single radiator and it is 5'5" horizontal then about 10'4" vertically up to the second floor where it goes right to that radiator.