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what is a main vent?

Brian O'Hagan
Brian O'Hagan Member Posts: 7
Hi I'm a new home owner (older house). I have a one pipe steam system and have been using this site to learn about how to maintain it.

At the start of the winter I replaced the vents on each of my radiators b/c they either spit water or were glogged with paint.

I was recently remined about the main vent but i am not sure what that is or where to find it on my system.

can anybody help me find it?



  • kevin
    kevin Member Posts: 420
    Boy have you come to...

    the right place... The main vent on a steam heating system is a very fast air venting device that allows the air to get out the piping and get the steam to the rads asap. It is usually/properly found at the end of the dry return piping about 12" before it drops into the wet return and about 6" abouve the horizontal pipe.Its 3/4" in diameter.Not 1/8 " as in radiator vents....Thats the short. There are a lot better qualified people here that will do a better job of it than I.kpc
  • Brian O'Hagan
    Brian O'Hagan Member Posts: 7


    Thanks for the info but now I need to know if it's possible that I don't have a main vent. I looked along all the steam pipes that come into and out of my furnace and cannot find a main vent on any of them. I'm in a condo where everybody has their own furnace so I looked at my neighbor's furnace and found the main vent right where you said it would be.

    My unit looks like it has 2 dry return pipes that come together just before they "turn into" the wet return.

    Is there something I'm missing or another place to check?

  • Steamhead
    Steamhead Member Posts: 16,715
    Kevin, you pretty much covered it

    The only thing I would add is that it may be located at the end of the steam main before it drops into a wet (located below the boiler waterline) return, or at the point where a steam main turns into a dry (above the waterline) return. It is always located after the takeoff to the last radiator.

    In the Coal Boiler Era, many systems were installed without main vents. The fire in one of these boilers burned all day, and once the air was out it stayed out- so what if it took an hour to vent it? So you may not find main vents on your system- but they can be added by drilling and tapping holes in the pipe. This is a job for a pro.

    Nowadays, with oil or gas boilers cycling on and off, we have to get the air out of the main quickly. This should take about a minute, after which steam will be available to all the radiators at the same time. The system will be balanced and run much more efficiently.

    We size main vents by measuring the length and diameter of the main. This tells us how much air we need to vent. If you measure your mains, we can tell you what you need.

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    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Brian O'Hagan
    Brian O'Hagan Member Posts: 7

    Looks like Steamhead posted a response while I was writing my last question. I think he covered it for me and the answer is I need to have someone come and do some work for me.

    thanks for all the help
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