Heat pumps can be used in almost any region of the continental United States but are most effective in regions that do not experience prolonged periods of sub-freezing weather. They can be paired with a gas heater as a backup in cold regions for greater energy efficiency.
Air-source heat pumps, the most common type, work by exchanging heat between the outside air and the air inside a building. In cooling mode, the heat pump transfers heat from the indoor air to the air outside. In heating mode, this heat transfer is reversed. In moderate climates, using a heat pump is more energy-efficient and economical than using a furnace and an air conditioner.
Because the heat pump cannot extract much heat from cold air, most heat pumps have a backup heating system of electrical resistance coils to supply extra heat when the outdoor air temperature falls below 40 degrees. This method of heating is not very efficient compared to a gas furnace and is why most heat pumps are best suited to areas with mild winters. Some modern heat pumps are equipped with reverse cycle chillers which improve efficiency in cold temperatures. Geothermal heat pumps are another alternative as they work by exchanging heat between indoor air and the relatively stable temperature of the top 10 feet of the earth's surface.