After months of intermittent stink in our new house, we finally discovered a untrapped, uncapped 1-1/2-inch drain in the ceiling above our mechanical room. (Behind drywall) It is a 2 x 10 joist bay. It was obviously intended as the discharge for the water softener. At the time we added the softener, the installer ran the discharge line up to the kitchen island and drained into a tapped fitting in the 2" PVC sink drain. (Illegal, I know) Now that we found the proper drain, I had a plumber install a trap and air gap for the softener. But when the softener backwashes, the air gap overflows and spills water all over the floor. Our first solution was to tie in roughly 18" of 2" PVC along with a 2" trap to create more space for the water volume/rush. Same problem. The air gap overflows. There is no room for a taller stand pipe because of the floor above. So now, we have the trap capped with a threaded fitting into the top of the cap. There is a check valve in line on the softener discharge. I know this is not ideal, but I don't have many options at this point. The entire basement is finished. Has anyone else come across this problem and what was the solution? Thanks.