Double Combo vs Double Tee
Hi All,
The drains of my two storey home drain to one point at my basement ceiling, and then vertically to the basement floor via a 3" DWV pipe to a 4" sanitary tee that horizontally provides 1) a clean-out and 2) drain to the septic out the foundation wall. The center-line of the 4" sanitary is 9" off the floor.
I wish to add a basement toilet and sink within 3 feet of the drain, and tap into the drain. I've found a wall-mount rear-drain toilet with a 3" drain whose center-line can be 10" to 14" above the floor. (Geberit pressure assist toilet with in wall tank and carrier). So, I reckon I should be safe with the toilets drain slope to the main drain line mentioned above; eighth inch of slope per horizontal foot. However, this would require replacement of the drain's sanitary tee with a double connection.
To make this work, I'll need to replace the existing 4" sanitary tee with a horizontally mounted double connection; using either a double sanitary tee, double combo wye with eighth bend or double wye. I'd run the existing house 3" drain to one side and the basement's new toilet/sink drain to the other side. I'd run the 3" drains against the wall and come away from the wall to connect to the double connection -- I hope you can visualize this ugly looking thing from a top view; basically, both 3" lines run along the wall and then come way from it far enough to connect into each side of the double connection that comes away from the wall. Therefore, a double wye or combo wye would require more "looping" away from the wall to make the connection than a double sanitary would require.
Two questions:
1) Should I be concerned with going with a double sanitary tee, rather than a double wye or double combo wye? I'd prefer the smaller foot-print of the double sanitary tee, but I'm worried that the flow of the 1st & 2nd floor drainage coming into the tee on one side will affect the new toilet/sink drainage (e.g. traps) on the tee's opposite side.
2) I'm certain I'll need to vent this configuration. Yes? I'm thinking my best option would be an AAR.
Thank you for you thoughts,
David