To vent, or not to vent?

[DangerDIY]

Plumbing pros-

I am starting a complete remodel of my basement bathroom. My home has cast iron pipe and already had a shower, toilet, and sink in the basement. When I demo'ed the existing bathroom, I noticed there was no vent present for any of those fixtures. I need to know if I need to place a vent or not. There is no stack nearby, although the main 4" sewer line is approx 5' from the furthest fixture. Everything drained fine prior to demo...no gurgling. So I'm confused. I understand the theory of a vent and don't understand how these fixtures were draining like they were.

I attached a photo of the drains exposed under the slab. In the bottom right you can see the main 4" line that goes through the wall and out to the public sewer. It has a 4" tee which runs to the toilet, then reduces to a 2" shower drain. The toilet is a tee off of that 4" line. There was a 1.5" sink drain plumbed into the toilet drain as well...if you look close you can see the blue rag stuffed where that was plumbed in to.



Essentially I am going to keep the drains in similar order, with the shower in the furthest corner of the picture, then a toilet, and then a sink. Should I consider an air-aspirating valve kit in the sink drain?

It's a lot of questions. I appreciate the professional advice. Thanks guys!

Joe

[jimbo]

Are you sure that there is not a vent rising in the wall, behind the sink?

Fixtures do not need a vent to drain. They are self venting. The issue then is the traps will siphon, and you can have odors.

You really do need a vent, and if truly there is not one, a solution not well-loved but acceptable under most codes, is to use an air admittance valve, also called "studor vent"

[DangerDIY]

Jimbo-

I'm 100% sure there is no vent present for these fixtures. There is a vent on a branch stack/drain for the 1st floor bathroom approx 15 ft away. Is it possible that using that as the vent was acceptable in the 1950's when this was constructed?

My plan right now is to use an AAV at the sink.

Thanks for the reply.
Joe

[Terry]

With a 4" line, it may allow enough open space above the flow line to allow venting.

"May"

Proper venting "assures" that the traps won't siphon.
With everything open, now is the time to bring it up to current practices. If you use a studor or an AAV, I like to run 2" to them.

[DangerDIY]

Terry-

I was imagining that the reason a 4" line was run to these fixtures was to allow a natural vent with the open space in that pipe...like you said. Thanks for the advice about a 2" line to the AAV...I'd already purchased a 4"x1.5" Tee for the sink, but I'll upgrade to a 4"x2" Tee.

Thanks again!
Joe