Three types of heat pumps are typically available for residences: (1) air-to-air, (2) water source, and (3) ground source. Heat pumps collect heat from the air, water, or ground outside your home and concentrate it for use inside. Heat pumps operate in reverse to cool your home by collecting the heat inside your house and effectively pumping it outside.
Heat pumps have both heating and cooling ratings-both in terms of capacity and efficiency. Capacity ratings are generally in British thermal units (BTU) per hour or tons (one ton equals 12,000 BTU/hr). Heating efficiency for air-source heat pumps is indicated by the heating season performance factor (HSPF). The HSPF tells you the ratio of the seasonal heating output in BTUs divided by the seasonal power consumption in Watt-hours. A heat pump can supply 2 to 3 times as much heat as it consumes in electricity because it moves energy from outside to inside (or vice versa). Heat pump efficiency varies with outdoor temperature. The performance of an air source heat pump in heating mode decreases with the drop in outside air temperature. The actual seasonal efficiency (as opposed to the rating) is, therefore, higher in a mild climate than in a severe cold climate.
In the cooling mode, a heat pump operates exactly like a central air conditioner. The seasonal energy efficiency ratio (SEER) is analogous to the HSPF but tells you the seasonal cooling performance.