The units are sold either as split-system or packaged units. In the split-system units, the metal cabinet that is installed outside the home houses the compressor and condenser, while an inside cabinet houses an evaporator. A packaged unit contains the evaporator, compressor and condenser all in the same metal cabinet that is installed outside. Packaged units contain heating elements and eliminate the need to have a separate furnace, but are more expensive. Split-systems are the ones most commonly found in residential construction.