What is this? Found this under a kitchen sink at an island today and I have never seen this before. Instead of an air admittance valve, they had a loop in the piping above the drain line.
Is this correct?
This is a botched attempt to vent the sink trap with a loop. It’s wrong. They forgot the vent.
They could either install an AAV or route that top loop to an actual vent.
They made an attempt, but didn’t understand the assignment. It should look like this:
Learn more about it at this link:
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It looks like a sorta loop vent using oversized pipes, which would be considered wet venting. “Wet venting involves combining the venting of fixtures with the drain lines. This can be a space-saving solution, but it requires careful design and adherence to plumbing codes.” If the vent stack piping was the same size as the sink drain, I would think that it would not work well under full water flow (make gurgling noises trying to get air). Below is the picture of a typical loop drain. However, since the vent stack piping is larger diameter than the sink drain, likely works pretty well (I would test under full flow).
If it works fine, I might put this in the “it wasn’t done to code, but…” category. Remember that we are not code inspectors.