Inlet Small

Inlet Small

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I want to add another water closet, back to back with an existing one. Will the existing 3" drain/vent work?

I am planning replacing the existing 3" tee with a double wye (3"). UPC say I can't use a double sanitary tee unless the two inlets are two pipe sizes smaller than the barrel (3"). UPC also says that I can use a common vent for both if both inlets are at the same level, and each will not readily drain into the other. That is why I chose a double wye. Is this a correct choice (to satisfy UPC)?

Also, is the existing 3" drain/vent adequate to handle two water closets? (Each water closet is 3 gal, and each 4" closet flange is locate 15" from the 3" drain/vent, centerline to centerline...)

The use of the double wye is the only way you can do it correctly. You will also need to install 2 street 45 degree elbows into each end of the wye or you can purchase the wye with them already in place. It's called a double combo. The existing 3" drain and vent will also work if you don't have other fixtures connected the same drain above on next floor. This is a common procedure in many homes.