For the longest time I have always called out “kitchen island” drains not vening vented. Just fo the simple fact if the sink is not near the wall I cant see the vent pipe when I am in the attic area. So, today I hade a upset builder over me calling this out. I am pretty sure I have seen this in the IRC but cant find it now. Can someone give me a little help on this please.
there are ways to vent an island sink…did You take any pictures ?
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Greg, this thread might help you rethink your “simple fact” theory. Do you have any pics of the sink in question?
https://forum.nachi.org/t/plumbing-question-for-you-guys/225103
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Just attached the pic to my origional post.
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I will usually see an AAV installed behind the sink, I understand that some jurisdictions may not allow these types of vents.
Did it drain properly?
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Based on that pic, it appears like the proper call would be not properly vented, plus the drain configuration forms a “S” trap.
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Yes, it did drain properly, no sounds etc.
there is no vent in that picture but that doesn’t mean it is not vented…
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I am not seeing the “S” trap.
@kleonard Thanks for that link, I was unaware. I have not seen this oversized drain/vent combo system in the wild yet. It may prevent some of those pesky builder phone calls. Nice one!
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No, it’s not your standard “S” trap, but without proper venting, it could possible siphon the same. Btw, I don’t call out “S” traps as a defect per say, but for informational purposes only. From my reading and study on them, they can function properly as long as there is a continual use with proper venting to prevent syphoning.
So, in the OP’s pic, I feel if the vertical drain was extended higher with an (allowed) AAV, it would be a better configuration. Of course I don’t explain this to a customer, I would just note that “sink ventilation was not visible. The sink was draining properly at the time of inspection…” and leave it at that.
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I agree 100%. For all we know it’s also a unvented S-trap.
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So, this is where I get confused. My understanding is that s-trap does not have the waste arm extension (and is unvented). I see this as a potentially unvented p-trap 
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This exact configuration the OP has illustrated was considered a S trap in my union plumbing school in Chicago. The class was taught by a state inspector. I’m going with what I was taught. 
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I agree. I have always understood an s-trap to be whenever the waste arm is less than twice the diameter of the pipe.
So this example would require at least 3" of an extension arm after the trap.
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Or called a “wet vent”
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Here we go, this says 2.5 times the diameter, not sure if that is IPC though
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Same here and I think your formula is more accurate than this:
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I would not call that an S trap, but that just may be me…I didn’t pull out a tape measure and check the length of the trap arm (wall tube )…if it looked close enough it was better than most of the crap I ran into under sinks…
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2018 IPC study guide