Modern air-to-water heat pumps are an emerging renewable energy heat source. They can also provide cooling. When combined with a hydronic distribution system, these heat pumps provide unsurpassed year-round comfort. They can also serve ancillary loads such as domestic water heating and pool heating. The efficiency of modern “low ambient” air-to-water can approach that of geothermal heat pumps on a seasonal basis, and usually at an installed cost that’s less than half that associated with geothermal heat pumps. This course introduces the technology used in modern air-to-water heat pumps and shows how to build complete systems around them.