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I am remodeling my own 67 year old house basement and putting in a full bath. The 4" waste line runs a few inches below the concrete floor and passes by the proposed toilet. Previously I replaced the 4" CI waste line and stack vent with PVC.  I have room for a 4x4x3 Y and an immediate 90 up to the toilet floor flange (3" sweep radius) without losing my 12" back wall clearance. The branch of the Y is only 2ft down stream from the 4" waste stack vent. Without any horizontal run my only option for a vent connection is to use a side or back outlet 90 (Charlotte 300S or 303) below the toilet flange.  Even at that, cross drains low in the back wall require this vent pipe to run up between back wall and toilet bowl to get past the cross drains then back into the wall below the toilet tank where I can put an AAV in the wall above the toilet bowl.  I would think the branch port could possibly clog rendering it useless. Considering there is virtually no 3" horizontal run and considering the 4" main stack is only 2ft back from the stack is additional venting required? It would be great if you could direct me to a code vent exception for this condition.

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