Bathroom Rough-In- Mystery Vent

flyboy

A few years back, I bought a house where the plumbing for a first floor bathroom had been roughed-in but no fixtures were installed. The room was framed but no drywall was hung (walls still open). Oddly the room was partially tiled (nice tile, no walls). Access from beneath the first floor is really difficult (no basement, bottom of house is covered in insulation and sheathing).

I am finally getting around to thinking about finishing off the bathroom and I am trying to figure out what I have got. The sink and toilet connections are pretty clear but there is a second vent connection which joins the same vent stack that serves the sink. I can not associate this extra vent with any drain.

Tying to figure out what is going on, I opened up a small hole in the subfloor near where the mystery vent goes into the floor. The attached diagram shows (in very crude form) what I found. The mystery vent joins the drain line just upstream from a threaded plug. The plug looks like a cleanout but joins the drain line at right angles (T-connection).

All the other plumbing for the house vents into the main stack which is located on the other side of the room.

Is this threaded plug a rough in for a future tub or shower (or just a very inconveniently located cleanout) ? What is the function of the extra vent ?

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flyboy

My long and wordy question (and crummy drawing) did not get any bites.

Perhaps someone can answer some more general questions on venting.

1. Why would a vent stack have more vent pipes attached to than drains draining into it ?
2. Are there situations where a vent is located (i.e. joins the wet part of the drain line) upstream from the drain it serves ?

TheEplumber

The mystery vent may be for a future tub or shower- cut a san tee into it for the tub's trap arm. what size are the pipes in the drawing?
If that's a bathroom, wheres the WC?

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flyboy

Not at home right now, but my best guess is 2" PVC for the drain line under the floor and 1.5" PVC for everything else. Will get exact measurements this weekend.

The WC rough in is on the other side of the bathroom from the sink/secondary stack, very near the main stack. The main stack serves everything in the house except the bathroom sink/mystery vent/mystery threaded plug. The the secondary stack joins the main stack near the roofline.

My assumption is that the WC connects to the main stack. Somewhere under the floor the main stack drains and the secondary stack drains must merge to join the septic pipe that leaves the house. The hole I opened up in the floor is not big enough for me to see any of this.

TheEplumber

The mystery plug is probably a test tee. We use them to section off an area of new piping to perform leak tests.
I'm betting the installer piped through the tub or shower because he wasn't sure what he was going to install- covering his butt.

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flyboy

So I took another look. The pipes are all 2" PCV.

I also found where the 2" line shown in the diagram joins the main 3" line. It is about 8 inches downstream from the threaded plug shown the diagram.

Anyone want to hazard a guess as to weather I could plumb a shower trap into the threaded plug ?

flyboy

Any thoughts on this ?

TheEplumber

You can not use any tee for drainage when it's installed horizontal- only in vertical.
Why not put a tee above the 90 on the mystery vent- thats where it belongs if you're installing a tub or shower.

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flyboy

Thanks Eplumber

Sounds like I have some work ahead of me if I want a shower.

Just to clarify, you are talking about adding a vertical T junction just above where the secondary stack (the red pipe in the diagram) goes horizontal under the floor ?

Is it generally true that a a fixture branch drain line must join a vertical stack ? My house has no basement so the first floor pluming goes horizontal (i.e sloped with a small angle relative to the horizontal) almost immediately after passing thru the first floor.

I probably need to get a plumber out to look at this but I can see that it is going to be hard to get enough vertical height under the floor to allow the shower drain to join the stack before it takes a 90 and goes horizontal.

TheEplumber

Above the 90 in the purple mystery line
no, a branch can and often does wye into another horizontal line. Vents on the other hand, need to come off the top of the line. That's why the tee for the shower needs to go above the 90.
You have 1 more option for the shower-
Forget about the santee. Where the horizontal purple line goes vertical- replace that 90 with a combination wye and 45(combo) installed on it's back. This will allow you to continue the horizontal line up to another 5ft for a shower line. You can put a 90 in that 5ft piece(trap arm) in order to hit the shower location and still be properly vented. This might help eliminate height issues

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flyboy

Thanks Eplumber

Got it. This makes sense.