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OK, thanks for people's help on my last question, now I've confused myself again.

I came across this picture, similar to the last question, but I don't think it's a cleanout. It looks like a vent to me. I'm referring to the part in the red border but I've included the rest of the picture for reference.

Am I correct in saying this is a wet/dry vent to equalize the P-trap from the closet? If so, I haven't seen many installations like this, why is it done this way? (more specifically, I thought vents were usually downstream of the flow...?)

EDIT Well, I've answered the easy part of it - if it is indeed a vent, then anything after the P-trap in the closet is technically "downstream of the flow".

enter image description here

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would not pass in australia also its incorrect to use a square junction on its back in both pipelines – user15053 Sep 12 '13 at 10:21
up vote 3 down vote accepted

Yes, it's a vent for a toilet. If it were an in-floor shower, you might see a P-trap and vent, but the toilet has the trap included in the fixture.

The reason you see the vent here and not other locations is that a sink will be vented starting in the wall rather than inside the floor, and the toilet is often after the sink, making the line from the sink to the toilet a wet vent for the toilet. In the pictured situation, the toilet is at the end of the line, and without a wet vent it needs a dedicated dry vent.

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Spectacular, thanks! – Ben Jun 5 '13 at 0:04

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