We live in a 1,800 Sq ft split level home built in the late 60s. Essentially three floors with 6 stairs between each level. Had central air installed last year. Freezing on lower level, hot upstairs. (they are essentially "stacked" on top of each other on a cement slab.) Temperate on the middle level (go figure, right?); middle level is over basement with attic above. When I hold a peice of paper against the cold air returns on the middle level, it gets sucked right to the grill. When I do this on the upper and lower level, nothing happens. I am assuming that this lack of cold air return is what is causing the temp variation (about 10 degrees). Any thoughts on this? Much appreciated. Thanks.