At what temperature is a heat pump useless?

A heat pump is most effective when the outdoor temperature is between 40-45°F. When the outdoor temperature drops below 40°F, the heat pump becomes less efficient and may not be able to keep up with the demand for heat. At temperatures below freezing (32°F), the heat pump becomes useless as it is unable to extract heat from the outside air. In this case, an auxiliary heating system, such as a furnace, is needed to supplement the heat pump.