Island Sink Venting?

[greyboy39]

Is this confirguration ever okay for an island sink install. Capped 3 inch downpipe under the sink and no vent connection anywhere along the run to the basement. I figured it needs an AAV or bow vent, but any light shed on known allowances would be appreciated.

[Jim Katen]

I remember reading an article in either JLC or Fine Homebuilding about 20 years ago. It was called "three ways to vent an island sink" or something like that. The method in your picture was one of the ways. The idea is that the 3" pipe is so much larger than the trap arm, that it will be impossible for this fixture to ever create a slug of water that will completely fill the 3" pipe, so it will never push air ahead of it or siphon air from behind it. Of course, fixtures upstream of this could push a slug of water that could increase pressure in this pipe and cause air to "burp" through this trap -- essentially the same thing that would happen with an AAV. 

[greyboy39]

The 3inch pipe is dedicated for this island sink only. No connection to the mains/vent was observed and therefore was incorrect according to the local AHJ. If a connection to a vent was present the configuration would have been allowed like you said Jim as a result of the size of the 3 inch downpipe. Thanks for the info.

[Bill Kibbel]

Probably works just fine though.