Unique Plumbing Install

Hey Folks-

Anyone have an idea of why they might have done this? I don’t know if they were going to put an AAV and decided not to or what. Would this configuration create any problems?



No, that’s right. That loop will help prevent siphoning in a situation where an AAV isn’t allowed and vent is needed. I can’t explain how or why it works, but I’ve seen drawings/diagrams showing that same exact thing.

The principle (assumption) is that the amount of air contained within the “loop” is sufficient to drain a full bowl of water. I have seen plumbers increase the pipe size on the loop to accomodate ‘both’ bowls of water being drained at the same time. As the water moves through the drain, it draws the air with it, and ‘theoretically’ creates a vacume break. I’m not 100% sure I agree, but I have yet to see a problem with this setup, if done properly.

Typical Island Sink Drain configuration…:smiley:

Not typical at all sir! Look again.


It’s missing the required cleanout on the “vent side”, but it’s close enough that I would not make an issue out of it, at least not on an old ABS installation like that. I never get to see the lateral connection, because it’s always under concrete.

The pipe in the loop is upsized, not to provide a sink’s worth of air volume, but to ensure that the stream of water coming from the smaller fixture drain cannot completely fill the pipe and create a siphon. This ensures that air can circulate through the loop with the water to prevent a vacuum for forming. If the loop is not a larger diameter than the fixture drain, it’s not configured properly. You should be able to drain any volume of water through this arrangement without siphoning.

Thanks the picture Michael…I’ve never seen that before. Is that a better install than having an AAV? From your picture, why not just install an AAV instead of do the whole loop?

The AAV is a mechanical device and subject to failure and needs periodic replacement, so I would say yes.

In that illustration, with a CO installed at the left end of the horizontal leg of the vent pipe, that should be the only arrangement allowed.

I have that same setup on my kitchen island, it’s worked fine for 20yrs.