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I know that ideally, an AAV (Air admittance valve) would not leak air or water out. But, should it be able to leak water under certain circumstances (such as running the garbage disposal) while still being considered "working"?

Consider a house has just a kitchen sink and dishwasher connected to a stack. Everything else was on a different stack. The kitchen stack was vented to the roof, however it was determined that there was a leak in the pipe to the roof. A plumber disconnected the vent, and placed an AAV under the sink.

The issue here is that if the sink is full of water, and the garbage disposal runs for more than 5 seconds, a decent amount of water leaks out of the AAV.

AAV leak plumbing diagram Rev2

AAV under sink

I am reading that AAV's are supposed to be a "one way valve". See diagram below. Air can come in, but the seal should close and not let pressure out...yet water is getting out.

AAV diagram (Image from Amazon.com)

The plumber who installed the AAV insists that the drain under the basement floor must be to blame, and should be "snaked"/cleaned out. Perhaps it is clogged under the basement, but I'm still wondering if an AAV should leak. I'm thinking that it may help to raise the AAV as much as possible. It's obviously below the flood level of the sink. Maybe it could be routed around the sink, just under the counter top, as seen in the following crude photoshop:

AAV hypothetical reroute

Is the plumber free to blame the basement drain, or is this AAV dysfunctional?

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    If water is reaching an AAV then it is not installed properly Or you have blockage somewhere, water should never reach an AAV.
    – Alaska Man
    Dec 17, 2018 at 19:23

2 Answers 2

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I don't know much about AAVs (or venting in general). But I do know that disposals push a LOT of water (and everything else!) very hard & very fast. If there is a clog in the line then stuff will come out elsewhere - e.g., in my case I have had output from a 2nd sink. So with a low-position AAV, I would not be at all surprised to see the disposal + clogged line overwhelm the AAV and end up with a mess.

Based on a little searching and my own experience (with disposals & dishwashers, but not with AAVs) is that:

  • You do likely have a clog in the line, which will need to get snaked or otherwise cleared out, and until that is resolved the AAV is simply not going to stand up to the disposal.
  • The AAV, by design, just can't keep up with the disposal if the waste line is not running totally free. A regular vent would handle a clog better. However, the AAV should be fine when the line is reasonably clear.

You should also make sure your dishwasher has either an air gap or a high hose loop. Your diagram doesn't show either one. Without an air gap or high hose loop, the disposal discharge backup could go into your dishwasher as well as your AAV.

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  • Indeed, there doesn't appear to be an airgap or high hose loop. I guess there could be a high hose loop hidden behind the side wall of the under-sink cabinet, I'll have to confirm. Now that you mention air gaps...Could that alone prevent the AAV from leaking when the disposal runs? It seems like it to me, but perhaps I misunderstand how they work. This person from another DIY SE quesiton had water spewing from their air-gap, so I'm assuming it could help: diy.stackexchange.com/questions/27547/…
    – Bort
    Dec 3, 2018 at 14:42
  • I have not had an AAV leak in the past but I usually create a loop and position the valve at the absolute top of the space.
    – Ed Beal
    Dec 3, 2018 at 14:47
  • @EdBeal - Can you elaborate on "loop"? I'm not sure the exact shape you are implying. As I understand, an AAV itself should not have a trap, as they are for air only, and water would impede their efficiency.
    – Bort
    Dec 3, 2018 at 14:53
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    I was taught to create a loop of pipe above the trap level and put the AAV at the top of the loop. In this case the loop would be almost 2' wide and ~12" above the trap with the valve on a t at the top of the loop this provides some additional air space for the valve, if something has back flowed into the valve it may not be closing like it should and may need to be cleaned, you are correct the valve should be 1 way.
    – Ed Beal
    Dec 3, 2018 at 15:08
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    The loop is used under island sinks where you cannot place the AAV above the sink level or if no vent because of wall material. If I have space and can place the AAV above the highest fixture no loop is needed.
    – Ed Beal
    Dec 3, 2018 at 16:10
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According to the manufacturer of the AAV: "It should not leak, even with a garbage disposal running."

I also asked "What if the drain is clogged?". They kind of hemmed and hawed and said "Hmm, it shouldn't really matter."

I will increase the height of the AAV to mitigate leaking, as well as cleaning out the drain.

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