Across cold regions in North America, Northern Europe, and East Asia, millions of households now rely on air-source heat pumps as their primary winter heating source. The idea that a machine can extract heat from freezing outdoor air often seems counterintuitive, but the process relies on standard thermodynamic principles rather than combustion. Even when the outside air feels bitterly cold, it contains thermal energy until reaching absolute zero, which is minus 273.15 degrees Celsius.
Extracting heat from sub-zero air
An air-source heat pump operates like a refrigerator in reverse. Instead of burning gas, oil, or wood to create heat, it moves heat from outside into the home. The key to this process is the closed loop of refrigerant circulating through the unit. Modern refrigerants such as R-410A, R-32, and R-290 have extremely low boiling points, turning from liquid to gas between minus 40 degrees Celsius and minus 50 degrees Celsius at atmospheric pressure.
When outdoor air passes over the external coil, the liquid refrigerant inside evaporates because the outdoor air is warmer than the refrigerant itself. Once the refrigerant absorbs this heat, it travels into the compressor located in the outdoor unit. Compressing the gas increases its pressure and raises its temperature significantly, typically above 45 degrees Celsius. The hot gas is then piped to an indoor heat exchanger, releasing warmth into the living space as it condenses back into a liquid. An expansion valve then drops the liquid refrigerant pressure, chilling it so it can repeat the cycle.
Engineering adaptations for cold climates
Older heat pump designs lost substantial capacity below freezing, but newer units engineered for cold climates use specific mechanical adaptations to maintain output down to minus 25 degrees Celsius or colder.
Variable-speed inverter compressors adjust their motor speed continuously rather than cycling strictly on or off. In extreme cold, these compressors speed up to pump a higher volume of refrigerant, offsetting the reduced density of cold gas. Manufacturers also use vapor injection, which diverts a portion of high-pressure refrigerant back into the compressor to cool internal components while boosting compression efficiency. This maintains usable heat output at sub-zero temperatures without burning out the compressor motor.
Another physical challenge in sub-zero operation is frost accumulation. Moisture in the outside air freezes onto the cold outdoor coils, reducing airflow and insulating the coil from the air. Modern units manage this through automated defrost cycles. The system briefly reverses the refrigerant flow, directing heat back to the outdoor coil to melt the frost over a few minutes before resuming normal heating mode.
Efficiency drops and physical limits
While a heat pump can continue operating in deep freezes, its efficiency declines as the gap between outdoor and indoor temperatures widens. Heating efficiency is measured by the coefficient of performance (COP), which compares the thermal energy produced to the electrical energy consumed. Electric resistance heaters have a COP of 1.0, meaning one unit of electricity yields one unit of heat.
At mild temperatures around 8 degrees Celsius, an efficient heat pump often achieves a COP between 3.5 and 4.5. At minus 15 degrees Celsius, that efficiency generally drops to between 1.5 and 2.5. Although the unit still delivers more than twice the heat per kilowatt-hour compared to baseboard electric heaters, its total heating capacity decreases at the exact moment the building loses heat fastest.
Every building and system combination has a balance point: the outdoor temperature at which the heat pump output matches the home heat loss. Below that point, supplemental heat is necessary. In colder climates, installations typically include auxiliary electric resistance elements inside the air handler, or they retain a backup fossil-fuel furnace in a dual-fuel configuration to cover extreme cold snaps.
Next developments in cold-weather heating
Research programs, including the U.S. Department of Energy Cold Climate Heat Pump Challenge, are completing field trials for residential systems designed to deliver 100 percent heating capacity down to minus 15 degrees Celsius without backup heaters. Meanwhile, manufacturers are rolling out models using propane as a refrigerant, which offers higher thermodynamic efficiency at low ambient temperatures alongside a lower direct global warming impact.
