I want to add a wash basin in my basement. I have 2 PVC pipes running verticaalyin my basement into the cement. One is 2 inches and the other is 3.5 inches. The only one that I can hear anything draining into is the 3.5 inch. The 2 inch pipe runs to the bathroom on the first floor. I have not heard anything drain down this pipe. Can I assume that it is connected to the vent stack in the bathroom?