Plumbing vent under kitchen sink

I came across a kitchen sink drain that vented out beneath the sink rather then going to the outdoors. There appeared to be some type of vent cap present. Doesent this need to vent to the exterior? Thanks!

Sounds like an AAV. Google it

Yeah, especially when older home plumbing systems are renovated since they dont have plumbing vents they install air admittance valves. It’s in the residential plumbing course.

A basic understanding of the device is needed here. It does not “vent out” otherwise it is faulty. It allows air “In” for the proper operation of the DWV system. It is an Air Admittance Valve. Sometimes called a “Studor Vent” (Studor is a brand name, like Xerox). See these when there is an obvious reason there is not a standard vent pipe “through the roof” i.e. sink is in an island or peninsula countertop, bank of windows above the sink. Depending on your local AHJ you may even see one inside an attic.

One good way to learn about numerous and unique home construction items is visit the local home centers and look at all the stuff they are using to build homes with today, especially plumbing, electrical, etc. It gives you a chance to handle and familiarize yourself with the millions of components and ask questions.

Did it look something like this?

Or how about this?

hi, yeah that looks familiar for islands

My issue is there’s a tie in to the stack behind my kitchen sink, but there’s also an AAV. Anybody have any ideas why someone would have an AAV AND a vent to the roof?