I have just the opposite problem, my central air cools my first floor VERY well, but the second floor you can barely feel it at night, and I also was told that I should have returns in each room upstairs, that just seemed to invasive not to mention expensive to do. I have one small return upstairs and it's in the hallway..not much help out there. My solution, I turn off my central a/c at night and use window units in the upstairs bedrooms..I'm not sure what the energy usage difference is, but it doesn't seem to be much...I'm very comfortable with that solution.