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Thread: In-Line Vent vs. Air Admittance Valve?

  1. #1
    DIY Junior Member dzd9fy's Avatar
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    Default In-Line Vent vs. Air Admittance Valve?

    What is the differnece between these two, and what is the recommended use for an S trap in a basement lav? I am looking at the Oatey 3901 In-Line and the Oatey 39016 AAV if that helps..... Thanks!!

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    In the Trades Wally Hays's Avatar
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    If you have a S trap there is no point in venting it anyway.
    Perception is 3/4 of reality

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    Plumber jimbo's Avatar
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    The 3901 is just an air admittance valve, el cheaop version. Make sure if you put on ANY aav that it is installed vertically.

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    In the Trades Wally Hays's Avatar
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    Where would you put a vent on an S trap that would make any difference. If you vent the lateral that it dumps into it still has no effect on the S trap siphoning. If you put it at the only place that would prevent siphioning, which would be the crown of the trap, then you have an illegal crown vent.
    Perception is 3/4 of reality

  5. #5
    Moderator & Master Plumber hj's Avatar
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    They are essentially the same, regardless of what they call them. You CANNOT vent a true "S" trap. You can only use an AAV to keep a "P" trap from becoming an "S" trap.

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    In the Trades Gary Swart's Avatar
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    S straps are illegal under every code, the reason being, they are not vented.

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    DIY Junior Member dzd9fy's Avatar
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    Ok, thanks for the clarification on the differences between the two.

    I am a bit confused on the comments that since it is an S trap it cannot be used, so looking for some insight here. Given the trap is where it is, has worked well for the past 10 years we have been here and guessing since the house was built [1950], what is the suggestion since this is burried under the cement?

    Also, how and why would one use this valve vertically on a P trap? That makes absolutely no sense to me....

  8. #8
    Plumber jimbo's Avatar
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    S traps used to be used, but are very prone to the trap siphoning dry, so are not allowed today. To install the aav, you could possilby santee off the vertical, turn up with an elbow, and put the aac there. Being below the flood rim, this could be problematic. Maybe a wye and a 45 to take if vertical? I would be interested in an inspectors take on this,

  9. #9
    In the Trades Wally Hays's Avatar
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    The only way to do it and be compliant would be to come vertical out of the floor into a sanitary tee with the aav on top of the tee and a P trap into the fixture. As long as the S trap remains there is no code compliant way to vent it.
    Perception is 3/4 of reality

  10. #10
    DIY Junior Member dzd9fy's Avatar
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    ok..... so should I just leave well enough alone? It has been working fine, I was just thinking this would be something that would be a good improvement to a less than ideal, and now not permitted use.

    Thanks guys!

  11. #11
    In the Trades Wally Hays's Avatar
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    Yep, let it be. If you notice any odor, run a bit of water in the sink. called priming the trap in the old days.
    Perception is 3/4 of reality

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