In our new kitchen we have a dishwasher that is not adjacent to the kitchen sink. So our contractor ran a new feed and a new dedicated drainpipe to the dishwasher. the dishwasher drainpipe eventually connects to the vertical drain stack further downstream in the lower level. At first everything appeared to be fine. Until I decided to fill up the kitchen sink about halfway with water and then pulled the plug. After a few second I heard water splashing downstairs, the water spewing out of the top of the open drain pipe where the dishwasher drains into. How can this be fixed? Our contractor suggests there is a clog further down the line which is making the water follow the "path of least resistance". I think something else is at play here. (Like maybe the horizontal drainpipe for the dishwasher is too low?) Here is my attempt at a diagram. The relative heights of the pipes may not be accurate. In actuality I think the pipe where the arrow is pointing is lower.