The drain for my dishwasher is connected in an unusual way. The following shows what you see when you open the doors below the dual sink: Here is what's behind the garbage disposal. The dishwasher is behind the wood at the left of the drawing and its drain is shown at the bottom: Behind the drywall the two drains (marked "to Sewer) lead to a Y and then to the clean-out. I think, that is. And I assume the dishwasher drain shown at the left of the image is configured as an "air gap". I don't know what's behind the drywall, but I do know there is only one 1.5-inch pipe running from somewhere below the Clean-Out to the main sewer (because I can see if it I remove the basement ceiling tiles). Here is my problem: The top of the air gap is just below the underside of the counter top. If the waste pipe from below the Clean-Out clogs up, waste water can and will rise up above the level of the Clean-Out and eventually come out of the two "Open" parts of the "air gap" depicted on the top left of the drawing. Twice now that has happened and the water seeped into the basement ceiling and stained it. I am thinking about installing an air gap that would empty into the sink rather into its own dedicated pipe. However, I don't know what to do with the dishwasher drain to the sewer other than installing a cap at the wall. Also, anyone know the purpose of the dishwasher connection to a separate waste pipe rather than to the sink? Other than esthetics, that is? Any suggestions for me?