Sufficient Venting for Conversion from Half Bath??

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Lisa Childers

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I’m converting a half bath to a full bath. Built in the 1940’s, in a basement with a concrete floor and original 4” terra cotta clay drain pipe. Building out new studded walls in place of plaster on block and changing the location of everything meant of course digging up the floor, tearing out the old clay and replacing with PVC. There is a 2” vent stack going through the roof and its location didn’t interfere with any other designs so I decided to continue using it. The original design connected the vent to the bottom of a 90 degree elbow below the commode flange (which is no longer there due to necessary demo work).

We went from the incoming 4” clay directly to a 3” PVC with both the sink drain and the shower drain dropping down to 2”. The vent stack is now going to connect to the 3” main line instead of at the 90 below the commode.

I’ve attached a very simple diagram of the design.

I want to know of any issues anyone sees with this setup. (notwithstanding the lack of scale and proper fittings) Will it provide sufficient venting to the new system? I can't imagine changing the vent from the 90 degree elbow to about 6" further would make any difference. And it supplied sufficient venting for the original design of commode, sink, wall urinal and floor drain. The only difference was they all connected back towards the door in the blue shaded circle area through 4” wye branches.

Comments please? And thanks in advance.

new bath.jpg
 

wwhitney

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Where you have the dry vent takeoff shown under the floor suffices for the WC and the shower, but not the lav (except see the PS below). The vent takeoff would need to be on the top half of the horizontal pipe's cross section, and it would need to rise vertically (stay at least 45 degree above horizontal) until at least 6" above the flood rim of the WC. I.e. no horizontal takeoff or vent section until at least 6" above the WC flood rim.

However, the lav trap itself requires such a vent connection, and it must occur within one trap diameter (1-1/4" or 1-1/2") of the outlet of the trap. And if you provide such a dry vent connection, then with the fixture ordering you show, that will suffice to wet vent the WC and the shower.

So I suggest you just move your vent connection to occur on the lav trap arm, before it has fallen one trap diameter. The vent takeoff still need to rise vertically until at least 6" above the lav flood rim. If you like, you can make the dry vent and the vented lav drain 2" (the IPC, in use in WV, only requires 1-1/2" for each of those).

Cheers, Wayne

P.S. It is possible that if your lav drain is 2" starting at a location that would be a san-tee if you were dry venting the lav normally, but is instead an elbow, the lav in your drawing would be considered vented by the IPC as a Combination Waste and Vent. But that allowance is not as commonly used, so while I think your drawing would comply, I don't have enough confidence to suggest it. Certainly if relocating the dry vent connection to the lav trap arm is not hard, it is the far more conventional solution. If it would be very difficult for some reason, it may be worth looking into the Combination Waste and Vent.
 

Lisa Childers

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"...the lav trap itself requires such a vent connection...." I had a sneaky feeling that was the case and was the exact reason I posted this.

The existing vent pipe is there. It's not moving without a lot demo. Adding a new one at the sink really isn't an option, floor above, then 70 year old concrete asphalt tar and gravel roof. REALLY DON'T want to go down that road. That's the stack I have to use. Since the walls were originally plaster over block, I am building out new studded walls to run new electric and water lines through, as shown in the attached. And the vent stack conveniently tucks away inside the new wall and occurs exactly where the side wall for the shower needs to be.

So.... new vertical vent at the sink... not possible but... I could add a studor vent.
Otherwise....
The sink vent, keeping at 2", will have to run up, then over through the new stud wall around the corner and tee into the existing stack. I can also keep the sink drain line at 3" from the main over to the wall, (instead of dropping down immediately to 2") then switch to 2" at the base plate up to the p-trap as well.

So Studor Vent or... run through all the studs around the corner to the existing stack.

San tee and up (about 42" right?) to an elbow transition to horizontal. Then another elbow at the corner. Keep these at long sweeps right? Then the tee at the stack and that should do the trick for the sink. So the commode / shower will work as shown.

Stud walls.jpg

P.S. no inspection here, only wanting it work right.
 

wwhitney

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First just wanted to confirm that the stack marked "vent" is vent only with no drainage coming down from above.

Then I'd say you have 3 options:

1) Dry vent taken off at the lav rises in the wall to at least 6" above the lav flood rim (but could rise much higher), then goes to the right through the wall framing (or floor framing if wood floor), then crosses behind the WC (in the floor framing or in a soffit) to reach your vent location. This makes the vent connection below the slab optional. For elbows in the dry vent once you are above the flood rim, use whatever is convenient or fits best--vent 90, quarter bend, or LT90.

2) AAV at the lav, this makes the vent connection below the slab optional.

3) Research Combination Waste and Vent, I think your OP diagram may be OK as long as you use a 2 x 1-1/2 elbow behind the lav.

As to upsizing the underfloor portion of the lav drain to 3", the only benefit to that would be to provide a cleanout under the lav, so the 3" would rise to above the slab to a 3x3x2 cleanout tee (tee with 3x2 bushing in the top entry). Which is a good idea if you don't have any other cleanout.

Cheers, Wayne
 

Lisa Childers

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yes that is correct, strictly plumbing vent.

"Dry vent taken off at the lav rises in the wall to at least 6" above the lav flood rim (but could rise much higher)" yep.
"through the wall framing (or floor framing if wood floor)" basement concrete slab
"vent 90" hmmm interesting... that's a new one on me. I'll need to look into those.
"AAV at the lav, this makes the vent connection below the slab optional."
"Research Combination Waste and Vent" yep and that's great, except for getting the slope dead on. The main pipe is approx 4' away from the sink and probably 10-12" down from floor level (maybe more, it was very thick concrete).

So I'll need to dig out another 6-8 inches over at the base of the wall down to the same level as the main drain. Not really a problem. Probably easier and more expedient than running pipe all the way around the wall though. And definitely easier than running a new dry stack at the sink. Good thing I didn't give the demo hammer back to the person I borrowed it from yet. haha

Studor vents are easy. So if it turns out the slope isn't dead on and it gives problems down the road, studor could be added on or maybe put on from the start. And then combined with a Combination Waste and Vent, that might not be exactly dead on, that sink should drain just fine.

And yes... "2 x 1-1/2 elbow behind the lav" absolutely.

This discussion has been great. Thanks
 
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