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  • #16
    Re: Curiosity question

    I'd like to see what (if any) air pressure difference there is between the outside air and the air in the jerry can as it pours. Seems to me there would have to be a lower pressure in the can even with the 1/8" vent open.

    We vent like we do to keep an equal pressure on both sides of P-traps, to prevent siphonage.
    Brent

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    • #17
      Re: Curiosity question

      I am not saying that the more flow that there is not a difference of air pressure, but air can flow faster and equalize faster than water or fluids, was my point,

      not saying that having a full area is not better but I would guess one could down size and not see much difference, in actual practice, (not that I am recommending it or suggesting it).

      It is jsut like draining out the wand on my power washer, If I put it tip down the unit will not drain through the small orifice on the high pressure tip, but if I put the wand up with the tip in the air it will drain out fine, as air will flow in the pin hole size orifice easier than the liquids can pass through it. yes it would drain much faster if I would remove the tip all together,
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      • #18
        Re: Curiosity question

        BHD, Exactly why I asked the question!! I am not advocating anything other than what code calls for, but just trying to figure out why. It sure would have made framing easier with a 2" vent stack rather than a 4". BAPlumber, I will bet you a beer that if you take a 2"
        trap connected to a 2" drain and vent it with a 3/4" pipe, you will never siphon out the trap.

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        • #19
          Re: Curiosity question

          I think it has to do with allowing sufficient air flow to ensure nearly equal pressures on both sides of the traps...
          In the gas can example, you are forming a partial vacuum in the can which causes the air coming in to race (even "whistle")... In a plumbing system this differential in pressure could cause the traps to be siphoned or blown depending on which direction the gradient is in... In the past (memory fades...LOL) I remember hearing the figure 1" WC pressure differential allowed when designing systems... (that is a very low pressure difference - about .04psi)... Even a slight restriction to air flow (undersized venting for example) could allow this pressure differential to occur.... This would reduce to 1/2 the depth of the common 2" trap seal...

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          • #20
            Re: Curiosity question

            in trade school we tried an esperement after a shop class opf dwv roghins was done. could a 1/2"copper pipe vent a full bathroom group all flushed or drain at once the answer is yes. the velocity of incoming air in the 1/2" was large and actually loud but it worked multiple time. not a realistic install for many reasons but puts perspective on things
            how is it that so many answers are in the instructions

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            • #21
              Re: Curiosity question

              Hey proplumb I did that same thing with 3/4"
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              • #22
                Re: Curiosity question

                The hissing sound coming from the 1/8" hole as you pour in the gas can example tells all.
                Picture a laundry sink in the basement thats seldom used, you have an undersized vtr.
                Every time you flush the toilet the trap weir in the basement laundry sink rocks, a little fluid is lost each time.
                Here in the north we have to keep the vtr full sized, same as the main to prevent frost buildup & closure.

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                • #23
                  Re: Curiosity question

                  Originally posted by DuckButter View Post
                  The hissing sound coming from the 1/8" hole as you pour in the gas can example tells all.
                  Picture a laundry sink in the basement thats seldom used, you have an undersized vtr.
                  Every time you flush the toilet the trap weir in the basement laundry sink rocks, a little fluid is lost each time.
                  Here in the north we have to keep the vtr full sized, same as the main to prevent frost buildup & closure.
                  When spec'd on a job out here,for the frost/snow the required fixture vent/vents were increased to two pipe sizes at or close to the roof for their penetration.
                  Last edited by drtyhands; 01-29-2008, 12:55 AM.

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                  • #24
                    Re: Curiosity question

                    i NY many years a go when we had a 2" vent going out we had to increase it to 3" before it went out side
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