This is a home owner special. Basement bathroom sink.
I am aware of the S-trap. My question is in regards to the vent connection below the trap. The horizontal line goes through the back of the cabinet and wall. On the other side of the wall the vent goes vertical straight up to the floor joists and then horizontal to the vent pipe.
The sink seemed to drain fine during the inspection however it is my understanding that air should be introduced above the trap for proper drainage.
Any expert verbiage or code ref. would be helpful.
Thank you in advance!