basically your pressurizing the rooms with that supply upstairs and with the vents closed in the basement.the unit is sucking air(must be a return opening to the unit down there)if you crack the basement with a little supply feed it will relieve the basement door ...if your not putting a supply air into a space you will have minimal return the air handler is looking to return air from the easiest location..a grill installed on the basement door will give it return and you actually will cool the basement with that return....might want to put the cat liitter box in a positive spot:whistling2: instead of on the return once you open the upstairs to the basement.