mystery pipe in the basement

[lamo3/13]

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?

[finnegan]

Do you have a bathroom already in your basement? If so, the 2" pipe could be a vent pipe for the shower or toilet. The 3.5" pipe is probably a 3" or 4" trade size. If you have no fixtures in your basement, then the 2" pipe is probably not a vent, but you never know.

[Winslow]

If it was a vent then you should also hear water running through it as it is tied into the drainage system.

[lamo3/13]

I opened the cleanout on the pipe and in the floor drain and they are connected. The furnace and and water softener drain into the floor drain. My next question is it possible to add a sink that drains into the vent stack for the floor drain and if so can I add it directly or should I bow vent it? I guess I'm cofused on why a vent pipe would have a clean out.

Thanks

[Cass]

The vent has a C/O because they can clog and not work right. BTW just FYI the vent is 1 1/2" and the other is 3". The pipe size is based on the inside dia., you are measuring the outside.