My questions has three parts. The purpose of this is that I want to install a shower in the basement.

Background: I recently bought a 90 year old home. The basement has recently been redone (new concrete floor and completely dry--thank god). It already has a toilet installed, resting directly on the concrete floor. About ten feet away about a foot from the wall, and right next to the water meter (not sure if any of this is important) is a square hole (looks like it was cut into the recent concrete). In that hole is a 4" capped pipe (the cap looks to be brass with a square center section to loosen it--but the pipe looks very old) at 45 degrees pointing away from the wall. My uncle said this was a "clear out" or "clean out"--not sure. I have a floor drain on the other side of the basement.

First question--I was wondering if I can drain my shower through this capped drain? I am wondering if this is a drain (I can't imagine what else it could be). I have planned the shower about 3 feet from this drain, so I wouldnt have to break up the floor (I plan on simply building a 8 inch platform)

Second question--My air conditioner has a pipe that runs across the floor to the floor drain to drain the water from condensation, but it is right next to this "drain"... I also plan on installing a sink close by... is there a way to drain (if that is what it is) all through this one capped drain?

Third question--This drain has no vent, and I know this will be required. Can I vent this through the wall as I would a dryer, or should I simply use an AAV on both the sink and shower? Is it all right to have these AAV's right behind the wall? (my local code allows AAVs)

Lastly... I know I said three questions, should I plan on any backup through the sewer? What should I install other than the basic trap to stop this (or am I over analyzing this)

Hey Tommy,

First question--I was wondering if I can drain my shower through this capped drain? I am wondering if this is a drain (I can't imagine what else it could be). I have planned the shower about 3 feet from this drain, so I wouldn't have to break up the floor (I plan on simply building a 8 inch platform)

Your uncle was correct. This is a 4" cleanout.(not a drain) and the reason it's at a 45 degree angle is so a sewer tape, (we used flat sewer tapes in the old days) wouldn't have to negotiate a right angle turn to enter the main. Sewer tapes aren't as flexible as snakes.
To answer your question, NO, you may not connect a shower to it. If you did you would be leaving a direct connection between the open sewer and your house allowing sewer gas to vent inside. Sewer gas is dangerous to your familys health and the methane content makes it explosive.

Second question--My air conditioner has a pipe that runs across the floor to the floor drain to drain the water from condensation, but it is right next to this "drain"... I also plan on installing a sink close by... is there a way to drain (if that is what it is) all through this one capped drain?

You're batting a thousand today aren't ya.? Plumbing codes forbid draining, storm water run-off, sump discharge, AC condensate or water softener discharge into the house sewer system. The reason being that the utilities company has to clean and process ALL the liquid that the sewer sends it. The above is considered "clean water" and may be discharged outside. I guess if you wanted to you could break up the floor and reduce down to 2" and come up in a wall to pick up a sink, but you would also be required to vent the sink, or a shower, if you did.

Third question--This drain has no vent, and I know this will be required. Can I vent this through the wall as I would a dryer, or should I simply use an AAV on both the sink and shower? Is it all right to have these AAV's right behind the wall? (my local code allows AAVs)

You may vent out the wall providing it's not under a window and away from doorways. If you're going to install a AAV in a wall it's a wise idea to place a access panel in front in case the AAV goes bad, and they do, and has to be replaced.

Lastly. Should I plan on any backup through the sewer? What should I install other than the basic trap to stop this (or am I over analyzing this)

Unless you're on the low end of the city sewer a backflow preventer isn't necessary. If by "basic trap" you meaning a running trap in the sewer main, that's not needed either. A trap will not prevent a backup. A traps function is to provide a water seal so sewer gas doesn't enter your house.

In short, of you wish to add fixtures in your basement I would tie into the sewer main and work from there. I would leave the clean out where it is. If you have a 90 year old sewer pipe running from your house to the street then, if it's original, it's Terra-Cotta/ Clay and you're going to need that clean out sooner or later. Good luck, Tom