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Shower Vent - Before or After P-trap?

D

ddawg16

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Jul 11, 2008
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21,005
Location
S. California
Does it realy matter what side of the p-trap the vent is on? If I recall, it is normally done on the downside of the p-trap.....but I'm trying to finish the plumbing on my new shower.....it's not going to be easy to run the vent on the downside....but upside of p-trap works out clean.

????????
 
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larry_g

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Apr 28, 2007
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16,879
Location
oregon
I'm thinking outloud here but the vent is to provide a vacuum break that prevents the flowing water from sucking the water out of the P-trap. Venting above the P-trap will not prevent this.

I'm not a plumber

lg
no neat sig line
 
K

kbs2244

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Joined
Nov 11, 2006
Messages
14,065
Downside.
You are not venting anything if on the shower side.
That is already open to the room through the drain.
 
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Bib Overalls

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Dec 4, 2006
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3,318
Location
Jonesboro, Arkansas
Friend of mine bought a commercial building for his business. It was previously owned by a HVAC company. The original half bath had been extended with a "step up" shower. The drain ran directly to a hole punched in the stack and sealed up with silicone. No trap. The odor was awful. I was able to fix it. But you have to ask "What were they thinking."

The drain can flow directly down into the trap. Depending on the length of the run and where it goes you may not need a dedicated vent.
 
F

Frank The Plumber

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Joined
Feb 19, 2011
Messages
2,644
Location
Chicago.
Drain- P Trap- vent.

Vent must be within 24" of the p trap to be efficient.

Drain must go down directly into p trap to prevent having a running trap.

You do not want a running trap.

Vent should be pitched up 45 degrees on run minimum.

The dynamic of this is to introduce air into the piping to break the siphonage of the trap.
If you do not break the siphon you will not get a trap seal.
No trap seal means you get to smell everyones poo. All the time. YAY!
 
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