We had our house built in 2009 (we live in Connecticut) and thought ahead to get rough-in plumbing in the basement for a future full bath. Since it's below sewer line we had them install an ejector pump in the corner of the basement. The standard piping is present (photo 1) - a 3" toilet, a 2" pipe under the slab for the shower drain, and a 2" pipe for the vanity.
Based on my understanding, the vanity drain pipe will wet vent the others, and I'll need to extend that pipe to the ceiling and connect it to the main vent stack.
After talking to the plumbing company that installed everything, they said if there wasn't a dedicated vent they could tie into a waste drain and that would be ok. After searching various sites I don't think they can do that.
I looked through the pictures we took while the house was being built and unfortunately we didn't have the foresight to take pictures of the basement ceiling. I could go through and remove all the insulation but that would be messy and I'm sure I'd end up not finding anything.
I did, however, find this picture:
This is the first floor kitchen, the master bath is behind. As you can see the bathroom sinks drain into the right-side pipe (2"), and there's another pipe to the left (1.5"). Looking at these pipes in the basement, the left-hand side connects to the pipe connecting to the bathtub with a T fitting. The other drain pipes that connect to a "main" line are using wyes.
I'm fairly confident the left hand pipe is only a vent, not a drain. Since this pipe connects above the 6" requirement, can I use this as a vent for the basement bathroom? I would need to add another T fitting in the basement ceiling, but that's easy enough to do.
Although I'm 97% sure it's only a vent (based on its location and other pictures we took), how do I figure out if it actually IS? Is there any way?