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Main vents

I have only one main vent in my system, as far as I can tell.  It is on the second floor, near the top of a pipe that goes through the ceiling, but terminates below the roof. Seems to be there mainly to be a main vent.  There is a #D maid-o-mist air valve on it.  Should this one have a #1?

Also, while investigating where all the new steam lines in the basement run, I discovered a place under the stairs where the main steam line runs to, a condensate return line drops down, and there's a nipple where there was briefly an air vent, while the system was being installed.  But then, when the stairs were put in, they took the vent off and plugged the nipple.

My question is, should there really be an air vent there?  If so, should I find a way to run 1/8" pipe from the nipple, a little sideways, then up through the stair tread to reach open air and install an air vent?


  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,575
    do i need main vents?

    yes you need the most venting on the mains you can install to lower the back-pressure of venting to an ounce or so. when you have the main venting generous enough to allow the air to escape without having to be forced out by the gas company, then replace your radiator vents with hoffman 40's-a nice slow vent.this will get the steam arriving at all radiators on a floor at the same time. whereas without a main vent, the steam first fills the closest radiator, and then the next, etc. you can install the new main vent on the vertical portion of the dry return, but keep it up as high as possible. you could come over with a 3/4 in menorah beyond the steps. even though your water looks clean, you may still need to skim the boiler, if it was not done before.

    when you install an accurate low-pressure gauge on the same pigtail as the pressuretrol, you can see whether your pressuretrol is actually working as it should.

    i suggest you get the steam books from the shop here so you will get a clearer idea of why you have water-hammer.--nbc 
  • Rod
    Rod Posts: 2,067
    Main Vents

    Hi- First of all we need to determine what type of steam system you have. One pipe or two pipe.   If only 1 pipe is connected to each radiator then you have a one pipe system. If there are 2 pipes connected to each radiator,you have a two pipe system. Let us know the type and we can then tell you where you need mains vents.

    - Rod
  • type

    Single-pipe system.  The house is 102 years old.
  • Rod
    Rod Posts: 2,067
    1 Pipe Vents

    Hi- Attached is a diagram of a typical one pipe steam system. The diagram shows a single main. Each main should have a main vent either after the last radiator or on the return (see diagram)  Sometimes other venting can be added to "help out" but all single pipe systems need a mai vent in the above location. If you can tell us the size of the mains and their length we can calculate the amount of venting you need on each main.

    - Rod
  • that pesky second vent

    Thanks for the diagram!  I will try to document my entire steam system on Thursday and post a PDF here.  (Oh, goody, this will give me a chance to explore the piping tools in Vectorworks!)

    I can say the following at this point:

    1) My main runs the length of my house hidden above the basement ceiling, then turns and goes upward through the first floor (good candidate for getting some insulation there) and through a large closet/small bedroom on the second floor, then is capped above the ceiling there.  I have a D-size air vent aboiut 10" below the ceiiling.  This is the high point of the system.

    2) The dry return comes off the main mid-way along the length of the house, coming laterally from the main to under some stairs, then dropping down to the floor.  This is where there was briefly a vent, but now is none.  According to the diagram, it seems like there should really be a vent there!

    I'll take pictures Thursday, too.  Maybe even video and can post a link here.
  • Rod
    Rod Posts: 2,067
    System Diagram

    Be sure to include the direction the mains are sloped and the size of the pipes. To determine pipe size, measure the circumference and use the attached chart
  • gerry gill
    gerry gill Member Posts: 3,070
    A 'd' vent

    vents at basically the same rate as a Gorton #1, so i see no advantage in changing that..least not in changing to a #1..you didn't say if your having any problems with steam distribution tho..are you?
    Serving Cleveland's eastern suburbs from Cleveland Heights down to Cuyahoga Falls.

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