Here is my current dilemma. The only workable location for my stack vent which went in over the weekend puts it very close to my bath P-trap. I am proposing to use a double tee to tie in my bath drain and laundry drain to the shared stack vent and drain pipe. Unfortunately in this close proximity it looks like the p-trap will form an S-vent as it joins the double tee. I know I can't have an S-vent due to the problems with siphoning. I have rea that the horisintal run needs to be at least double the distance of the diameter so 3 inches for the 1.5 inch diameter bath drain. Could I get away with an O-trap in this location to gain the distance I need?
I am also tying into a 2 inch double tee bith the 1.5 inch bath trap with an adapter. Is this going to create any conflicts?
The plumbing code does say twice the diameter, but it doesn't specify where the measurements should be taken from. Open to interpretation. If you put the fittings hub to hub, what's the length of piping between them?
I'll try to post pictures tomorrow as it will fit together. That may give a better idea of my concern. I'm mostly concerned with the bath trap siphoning. I may have to do it with two separate single tees after all. It just seemed very efficient to attach both drain lines to the double tee with the stack vent equally between them.