In this area it is quite common to have a package unit for the downstairs and a split system for the upstairs. A package unit has the entire system built into one piece of equipment and the ductwork attaches to that piece of equipment. There will be what looks like a big metal box connecting the equipment to the house. This box is called the rain shield and its purpose is to protect the ductwork from the weather as well as preventing animals from entering the crawl space through the duct penetration. A split system is a system that has essentially been split in half. The condenser is outside, and two refrigerant lines connect it to the indoor coil, usually located in the attic or an upstairs closet. The ductwork is connected to the indoor unit. By moving the indoor unit upstairs, closer to the area it is responsible for conditioning, the air in the duct experiences much less friction loss which makes the system more efficient.