My son bought a rowhouse in Baltimore that has been completely gutted. His plan is to have a finished basement and add a third floor master suite and roof deck above the current 2nd floor. When discussing HVAC, he would like a duct system so that AC registers would be near the ceiling and heat registers would be near the floor. Is this worth pursuing in terms of efficiency and comfort? How is this type of system set up? Would it be more efficient to put the AC on the roof and use a high velocity air handler in a closet in the new roof addition? This would seem to take advantage of gravity to distibute cool air down through the building. Would a single 90% furnace in the basement with traditional ductwork handle all 4 levels? In a high velocity system, how are the cold air returns tied in? Any recommendations or guidance would be greatly appreciated. Also are any of the professionals on this site that are versed in these types of systems located in the Baltimore area?