UA Service, Training, and Recognition (STAR) Practice Test

Air-to-air heat pumps move heat from the outdoors into the home, so the amount of heat they can deliver depends on how much heat is available in the outside air. As outdoor air gets colder, there’s less heat energy in it to extract, and the refrigerant cycle can’t move heat as efficiently. This means the system’s heating capacity drops when it’s very cold. The inside-outside temperature difference is larger, and the refrigerant pressures at the outdoor coil fall, making it harder to transfer heat. In very cold conditions, many systems also use some supplemental electric heat, which highlights that the heat pump alone is no longer meeting the full load. So, the heating capacity decreases as outdoor air gets colder.