Air-source models are the most popular type of heat pump in the United States. They provide efficient heating and cooling without the need for costly fossil fuels. Homeowners and businesses can choose from split systems with separate indoor and outdoor components or packaged models where all of the equipment is housed in one self-contained outdoor unit. Ductless mini-split and multi-split heat pumps are also popular as are two-stage and variable-speed models, especially for consumers who are interested in achieving maximum efficiency.