I recently moved my kitchen sink and dishwasher from along a wall to an island in the middle of the kitchen. I ran a 1 1/2″ drain from the island to the 4″ main drain under the existing sink, a distance of about 10 ft. Someone has told me I will need a “loop vent” for proper drainage. If this is true how do you do it and exactly what is it.
They were correct in that you need a loop vent. They should also have told you that a 2″ pipe is the very minimum that will work for a loop vent. A loop vent is an inverted “U” that rises as high as possible in the sink cabinet, but definitely higher than the bottom of the sink. Both legs of the “U” tie into the horizontal drain line, but the one designated as the vent, also has a tee connection that goes to a normal verical vent line. Some jurisdictions will allow the line off the vent to be eliminated if the horizontal pipe is 3″ or larger.