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Main vent size?

RonNY Member Posts: 4
Hi everyone,

This will be our third winter with steam heat, and I'm just starting to get serious. 1920's house, we bought a new Weil-McLain boiler when we moved in, but I made two critical errors: replaced all of the radiator vents with Varivalves (half of them are no longer working), and did NOT replace the main vents. Of course, the company that installed the boiler was no help!

Anyway, I'm trying to figure out the correct sizing for replacement main vents. My long main and short main both have the same setup at the end: an elbow with a little port on the side, leading to a nipple with the vent at the top. The vents are the same size. There are a couple of pictures attached.

The vents are stamped SLATER NO. 14 AIR VALVE but I couldn't find any info on these online.

Ok, so now the questions: what is the best replacement for these? Can you tell from the picture if a Gorton #1 (3/8") or #2 will fit the coupler at the top of the nipple? Honestly, pipe sizing has always confused me -- I took it apart, but I'm not sure what to measure to know what size pipe this is!

Another stupid question: Lowe's has main vents (Durst A883, 3/4" x 1/2" "quick vent") for under $10. Compared to $55 for the Gorton #2... if I put one of these on the end of each main, would that really be so terrible? :-) How important is it to have a larger vent on the long main, vs. the same vent on each?

Sorry for all of the newbie questions... I'm sure there will be more. Thanks for your help!



  • BobC
    BobC Member Posts: 5,473
    edited November 2011
    Do you feel lucky?

    How long is each main and whet diameter is the pipe? We need that information to calculate the volume of the pipe and determine the required venting.

    I have never heard of a Durst 883 vent but I have heard of USAV 884 radiator vents and they are not good vents, you could try them but watch them carefully. Also without published data it would be hard to say if they have the required venting capacity.

    When replacing the radiator vents your best off sticking with Gorton, Ventrite, Hoffman, or Maid O Mist - they cost more but they have proven track records.

    As to the pipe size your dealing with just measure the pipe circumference and divide it by 3.14 and that will give you the pipe OD. The chart attached will let you convert OD to the pipe size


    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • cheap main vents

    every steam system needs big main vents. only you can decide whether to use something cheap, whose performance and longevity is unknown, or a well-known product, whose name is mentioned in virtually every post on this site.

    in addition to those considerations, will you want to reinstall something which has failed on a sunday evening, when the store is closed?

    as a compromise, why not install 1 of each on each line, so which ever one fails will be backed up. i would choose the gorton #2.

    while it is still warmish, you can turn off the boiler at the switch, and remove the vent and take it to lowes, and have the size identified, and purchase the cheapie to get you by until the gorton arrives.--nbc
  • Rod
    Rod Posts: 2,067
    edited November 2011
    Pipe Chart

    Hi Ron-

    I've attached chart showing the circumference of standard pipe sizes.

    Use a strip of paper and a pen and wrap the stripo around the pipe you

    want to measure and make a mark where the end of the strip touches the

    paper. Use a ruler to measure the marked paper and then check the chart

    to figure out the corresponding pipe size.

    Let us know the size and length of your mains and we can tell you the size/number  of vents you need. 

    Buying vents- You get what you pay for.

    Edit:- After I posted this I see Bob has added a pipe sizing chart. Either one will work fine.

    - Rod
  • Agreed

    IMHO, being cheap on main vents is false economy.
  • RonNY
    RonNY Member Posts: 4
    No shortcuts this time...

    Ok, the sizing charts were very helpful, so here we go. The existing vents have 1/4" fittings. I assume I can simply increase from 1/4" to the size of the vent fitting? I may need a shorter nipple (vertical clearance is tight, especially on the other vent, not pictured) but this shouldn't be a big deal. 1/4" x 1/2" and 1/4" x 3/8" black couplings seem to be readily available.

    As for the mains themselves, they're 2" pipe all the way around. From the header, the short main is 45 linear feet of pipe, and the long main is 55 linear feet. I guess the long main isn't all that much longer, so it may be that the same size vent will do for both, but I'll leave it to the experts to figure out!

    (And just so you don't think I'm too cheap... the Lowe's vents have the advantage of being available today; my 2-year-old's bedroom is on the long main, which gets steam MUCH more slowly than the short main, probably due to a vent that looks like it's been partly painted shut. But once the sizing is confirmed, I will take your advice and order the Gortons at once!)
  • Rod
    Rod Posts: 2,067
    edited November 2011
    Main Vent Sizing

    Hi-  Here are the calculations for main vent sizing.  Using Gerry Gill’s venting chart for a reference, a 2 inch pipe holds .023 cubic feet of air per lineal foot. That would mean:

     the 45 foot main contains 1.035 cu.ft. of air

     the 55 foot main contains 1.265 cu. ft. of air

    At one ounce pressure:

     the Gorton # 1 vents @ .330 cu.ft. of air  per minute

     the Gorton #2 vents @ 1.100 cu. ft. of air per minute

    Venting time of 45 ft. main using Gorton #1 = 3.13 minutes.

                                                 Using a Gorton #2 = 0.94 minutes

    Venting time for 55 ft main using Gorton #1 = 3.83 minutes

                                                 Using a Gorton #2 = 1.15 Minutes

    On main venting times 3 minutes is considered as good time for venting a main. Using Gorton # 2s might be a little over kill. Perhaps a pair of Gorton #1s on each main would be about right.  Use a tee and street elbows and mount them on the existing pipe.

    For a good article on venting go to this website. Gerry Gill is a Cleveland steam pro, who wrote the book on venting.


    Here’s some more info I dug out on the Durst Valves


    On your radiators you might want to use Maid -O- Mist # 4 0r 5 s or Hoffman 1A s (adjustable vents)   The Vari-Vents can cause problems.  With main vents you can’t over vent but with radiators vents  you can. "Vent you mains quickly and your radiators slowly".

    I might also suggest you take a look in the Shop section of this website as there are a lot of good books on steam heating available there. The Best one to start out with is "We Got Steam Heat! "  Here is a link to it.


    This book is written for homeowners new to steam heating and details what maintenance you can do yourself and what is best left to the pro. This book pays for itself over and over very quickly!

    - Rod
  • Steamhead
    Steamhead Member Posts: 16,733
    I'd go with the Gorton #2 vents

    we always try to vent the mains in a minute or less, so all the rads will have steam available to them at the same time. if nothing else, this will shave a couple minutes off each firing cycle, saving that much fuel. 
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • RonNY
    RonNY Member Posts: 4
    New parts are on the way

    Many thanks to all... you are truly lifesavers! I ordered Gorton #2s for both mains, Maid-O-Mist vents in moderate sizes for most of the house, and Hoffman 1As for a couple of "problem" radiators. The Vari-valves are going in the trash.

    I found an especially informative thread on the importance of not over-venting your radiators:


    Also found some info on the Gill site about the problems with radiator covers, which we do have in some rooms. Overall, this has been quite an education already -- I can't believe we've been suffering with uneven heat and probably wasting money all of this time! Thanks again for all of your help and advice!
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