A heat pump is an HVAC system that transfers heat from one location to another using a refrigeration cycle. In heating mode, it extracts heat from an outside source (air, ground, or water) and moves it indoors. In cooling mode, it reverses direction to remove heat from the indoors. Heat pumps are the most energy-efficient heating and cooling technology available, delivering 2 to 5 units of heat energy for each unit of electrical energy consumed.
Air-source heat pumps (ASHPs) transfer heat between the indoor air and the outdoor air. Modern cold-climate ASHPs maintain full heating capacity down to minus 15 degrees Celsius (5 degrees Fahrenheit) and operate down to minus 30 degrees Celsius (minus 20 degrees Fahrenheit). The efficiency is measured by HSPF (heating seasonal performance factor) and SEER (seasonal energy efficiency ratio). Minimum efficiency standards in the US are 8.2 HSPF and 14 SEER.
Ground-source (geothermal) heat pumps transfer heat between the indoor air and the ground. The ground maintains a stable temperature of 10 to 16 degrees Celsius (50 to 60 degrees Fahrenheit) below the frost line, providing higher efficiency than air-source systems in extreme climates. The closed-loop system circulates a water-antifreeze solution through buried pipes in horizontal trenches or vertical boreholes. Geothermal systems have a higher installation cost but lower operating cost and longer service life.
Water-source heat pumps transfer heat to or from a body of water such as a pond, lake, or well. They are the most efficient type when a suitable water source is available.
Heat pump efficiency is expressed as the coefficient of performance (COP). A COP of 3.0 means the heat pump delivers 3 units of heat for each unit of electricity consumed. Geothermal heat pumps typically have COP ratings of 3.5 to 5.0. Cold-climate air-source heat pumps have COP ratings of 2.0 to 3.5 at their rated winter temperature.
Heat pumps can replace furnaces, boilers, and air conditioners in most climates. Ductless mini-split systems mount an indoor unit on the wall and connect to an outdoor compressor, providing zone heating and cooling without ductwork. Ducted systems use existing forced-air ductwork. Heat pump Solar Water Heating extract heat from the surrounding air and are 2 to 3 times more efficient than conventional electric water heaters.