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  • Curiosity question

    Out of curiosity, disregarding code for a moment, why does a vent stack have to be as large as it is? Why wouldn't a 1 1/2 " pipe vent as well as a 3" for example. Just some thing I have always wondered about. Thanks, Jim

  • #2
    Re: Curiosity question

    I can't answer your question. But I sure am thankful that the pipe runs full size to the roof, because every time I add a bath to an attic renovation I can tap right into that line, rather than having to open up a bunch of walls on the lower floors to run a new waste line. I definitely couldn't do that if the pipe size had been reduced on its way up.

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    • #3
      Re: Curiosity question

      A 1-1/2" pipe wouldn't give enough air flow to properly serve the fixtures as a 3" pipe would.
      Proud To Be Union!!

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      • #4
        Re: Curiosity question

        a long time ago some one said it was to be bigger so in the winter the moisture will freeze at the top closing the pipe so by using a bigger pipe this will not close it off .

        just what some one said ?
        i think they called it Hoarfrost
        deposit of ice crystals on objects exposed to the free air, such as grass blades, tree branches, or leaves. It is formed by direct condensation of water vapour to ice at temperatures below freezing and occurs when air is brought to its frost point by cooling. Hoarfrost is formed by a process analogous to that by which dew is formed on similar objects
        Last edited by HVAC HAWK; 01-22-2008, 09:42 PM.
        Charlie

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        • #5
          Re: Curiosity question

          But just 'moisture' freezing and expanding filling a 1-1/2" pipe is pretty insane.
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          • #6
            Re: Curiosity question

            The main reason every house has to have a 3" vent(I'm quoting the Canadian building code) is to vent the building drain/sewer. Aaron a 2" vent can actually vent quite a few fixtures, but doesn't supply enough open air to vent the main sewer. This is in theory of course, it was the explaination I was always given.
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            • #7
              Re: Curiosity question

              yes but in the north with temp below 30 * with no Sunny days for a week or so it is possible
              Charlie

              My seek the peek fundraiser page
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              • #8
                Re: Curiosity question

                Big Pipe- I agree with you about the number of fixtures that can be vented on a 2" vent. I was only using his example of an 1-1/2" pipe.
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                • #9
                  Re: Curiosity question

                  the code states that the cross sectional area of all vents has to equal the size of the building drain. typically 4'' on a residential home.

                  so rule of thumb is 4-2'' vent lines is equal to 1- 4'' building drain line.

                  in the old days, i think up to 1958 or so, we use to have to vent the main stack off a toilet full 4''. this is great when i need to snake a main from the roof. nowadays it's typically a 2'' line on the roof. and a combination of other 1.5'' and 2'' vents, still equalling 4'' in cross sectional area.

                  rick.
                  phoebe it is

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                  • #10
                    Re: Curiosity question

                    Thanks for all the replys. I agree that a 4" is nice if you have to snake from the roof, and I agree with the freezing discussion (had my 4" vent freeze shut more than once) and I understand that there are code requirements.(We all know that some code requirements are still there because of tradition etc.) but I still want to know why a system wouldn't work with a smaller vent. Aaron, you say it wouldn't work as well as a larger one--I'm just asking why?

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                    • #11
                      Re: Curiosity question

                      Imagine a slug of water in a 4" inch drain moving down (like a soup can shaped cylinder of water). As that soup can moves down, the space above it that it previously occupied has to be replaced with air, or a vacuum will occur. So a "slug" of air of the same volume has to come down and fill that space. That 4" slug of air couldn't come through a 1.5" opening fast enough, which would create a partial vacuum, slowing the water.

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                      • #12
                        Re: Curiosity question

                        Originally posted by PLUMBER RICK View Post
                        the code states that the cross sectional area of all vents has to equal the size of the building drain. typically 4'' on a residential home.
                        In Phoenix, we use the UPC and the code states that the total cross sectional area of the vents must be equal to or larger than the REQUIRED building drain...
                        If you have a 4" drain but only require a 3" (by fixture units etc), a vent system equal to the cross section of the 3" pipe would be sufficient...
                        This really comes into effect in commercial buildings... We commonly install 6" building drains for future fixture loads but do not have to have that much venting... Only enough for what would be the minimum size of drain if the building was plumbed for just the current fixtures...

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                        • #13
                          Re: Curiosity question

                          Originally posted by Aaron91 View Post
                          A 1-1/2" pipe wouldn't give enough air flow to properly serve the fixtures as a 3" pipe would.
                          Exactly and if you look at some sizing charts you can learn why

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                          • #14
                            Re: Curiosity question

                            I disagree (at lest in theory) with the amount of air that can enter a smaller hole and keep up with a larger amount of liquid movement,

                            and the example I will use is a gasoline Jerry can, you start to pour the fuel out of the can it will gulp or vacuum and not flow, but pop off the 1/8" vent hole in the top rear of the jug, and it will flow a 1" or even without the nozzle a 2" flow out of the can with out much problem.

                            on a few of my old books it shows a recommend expansion top on any thing under 3" expecily in cold climates,

                            and it make sense to me, you have "warm" water vapor rising out of the vent on occasion and if it is good and cold there is very real possibility for it to frost up in a given time, the example was given about hoarfrost, earlier. but I can see in below 0" weather how it could happen easily and through a cold attic.

                            and in a lot of the early plumbing I think there was a lot more wet venting that occurred, and less venting expecily in retrofit plumbing that took place when indoor plumbing was being added to many houses, As in our area there are many older houses that have the main vent runs on the exterior of the building and in some buildings there may be only one vent or possibly two total vents. One where the drain leaves the building, a 3" or 4" depending on where what was used as the main, and usually a 1 1/2" at the kitchen sink and if there is any other plumbing it is tied in some where in the middle of the two vents.
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                            • #15
                              Re: Curiosity question

                              Originally posted by BHD View Post
                              I disagree (at lest in theory) with the amount of air that can enter a smaller hole and keep up with a larger amount of liquid movement,

                              and the example I will use is a gasoline Jerry can, you start to pour the fuel out of the can it will gulp or vacuum and not flow, but pop off the 1/8" vent hole in the top rear of the jug, and it will flow a 1" or even without the nozzle a 2" flow out of the can with out much problem.
                              Wow, talk about thinking outside the box. I love questions like this. Really makes you think why we do things the way we do.
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