Inadvertent S trap?

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DIY-er

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Think I know the answer, but in my DIY brilliance, I think I may have created an S trap or at least an improperly vented trap...

So, ripped out a shower that had been done by a very shoddy contractor hired by previous homeowner. One of the many ugly things done was a very rough conversion of a tub to a shower leaving the 1 1/2 tub drain in place with the overflow capped and buried in the tile mud.

A little investigation proved that the 1 1/2 drain led to a 2" trap and then into the stack. While buried back in the wall/floor pretty well, I was able to remove the old trap and plumb up to my new drain with all 2" pipe as it should be.

Issue is, I moved the trap from being down in the wall/floor to right under the drain... My very crude illustrations of before and after are attached. Do I need to start over?

Picture2.gif
 

Jadnashua

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The vent needs to come off of the trap arm before it turns down. The idea is so that air can move in and out...when you have it like shown on the right, the pipe could be full of water, and the air can't move. The idea is to break the siphon that may be caused by waste moving in the pipe so that the trap can stay full. FWIW, an S-trap can drain TOO well, and end up not leaving enough water in the trap to seal things.
 

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Great, thought so, but appreciate the confirmation, Jim and Mike.

I'll get back in the wall/floor to see if I can get the 2" PVC trap to fit in the proper place (old work was copper and reducer was soldered into the trap, so I cut it out downstream of the trap where I could get on a fernco). Will be a little tight, but should work.

The elbows and pipe going up to the drain are OK as shown on the left? (will do with 2" vs. the 1 1/2 that was there previously)
 

Jadnashua

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The disadvantage of having a long pipe to the trap is that it can accumulate soap scum, and other crud and start to smell on its own. So, the closer the trap is to the grate, that possibility is minimized. There is a code requirement on the maximum fall to the trap, but I do not remember that number. But, it doesn't have to be a straight drop.
 

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Cool. Makes sense. Assume that the downsides of an S trap outweigh the downsides of a long drop?
 

Jadnashua

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You get some scouring whenever water is running, so it may not be a big issue. But, a siphoned S-trap will likely smell, regardless.
 

WJcandee

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The vent needs to come off of the trap arm before it turns down. The idea is so that air can move in and out...when you have it like shown on the right, the pipe could be full of water, and the air can't move.

Yeah. DIY-er...one way the rule is phrased is that the weir of the p-trap has to be lower than the top of the opening of the fitting that connects it to its vent. Apply that and you'll always know the answer.
 

WJcandee

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There is a code requirement on the maximum fall to the trap, but I do not remember that number.

As short as practicable distance from fixture outlet to trap weir; tailpiece can't exceed 24".

Can you extend that horizontal piece of your current setup (the trap arm) over and tie it into the vent with a san-t? That would vent the p-trap properly, although I don't know if there's another reason that there is a vent on the line below that you would then turn into not-a-vent.
 
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WJcandee

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The "crown weir" in Reach4's diagram is also called the "trap weir" in some materials. The "trap arm" is also in some materials called the "fixture arm". I have seen a million definitions/explanations, but I just think of it as the point (height) at which any more water added to the trap or bowl would start to spill over into the rest of the DWV system. (In a toilet, it's right at the base of the top bend of the trapway; in a p-trap, it's right where Reach's diagram shows.)
 

hj

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quote;he idea is so that air can move in and out...when you have it like shown on the right, the pipe could be full of water, and the air can't move.

Whether the pipe is full of water or not, is immaterial to the location of the vent. Even if the pipe were NOT full of water it would still siphon the trap.
 

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Thanks all. I think I get it, terminology aside...

Will try to get the trap in the same location as before, despite the downside of a relatively long "fixture arm". The vent is shared with a tub (back to back bathrooms), so can't just tie in higher. Is the 24" tailpiece limit vertical, horizontal, or combined? If combined, I'll be right around that...

Tom
 

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OK, finally back to this project. Opened back up the wall to get back to my work.

Referring back to the original picture on the left, there are two traps at the same level - the two trap arms plus the vent come together on one side of a fitting (never seen one before) and the other side goes to another horizontal pipe that goes to the stack. Picture attached is a top view of that fitting - you can see the copper trap going to the tub in the next room, the vent coming up in the middle, and the fernco/pvc connecting to my lovely S trap.
trap picture.jpeg

Without destroying a joist, I can't fit in an elbow directly below the shower drain to direct the waste to a trap in the original location.

So, what I'm wondering is what if I leave the new trap where it is, adding a vent connection at the proper level, leaving the waste to drop down to the level of the other trap, which is also vented? Does that make any sense??

Thanks,
Tom
 
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