Heat pumps pull warmth from outside air even when it feels freezing

A heat pump works in cold weather by extracting heat from the outdoor air and moving it indoors, even when the temperature outside is well below freezing. This sounds counterintuitive — there is heat in cold air, and the heat pump's refrigerant is designed to capture it. The outdoor unit contains a coil that absorbs whatever thermal energy exists in the air, then a compressor pressurizes that refrigerant to raise its temperature further. That warmed refrigerant travels indoors through copper lines, where an indoor coil releases the heat into your home. The cycle repeats continuously.

The colder it gets outside, the less heat is available to extract, so the system works harder and pulls more electricity. At some point — typically around 25 to 35 degrees Fahrenheit, depending on the model — most air-source heat pumps become inefficient enough that a backup heating system kicks in automatically. That backup is usually electric resistance heating (essentially a space heater built into the system) or a gas furnace if your home has one. The heat pump does not stop working in cold weather; it straightforward becomes one part of a two-part heating strategy.

Key Takeaways

  • Heat pumps extract thermal energy from outdoor air and move it indoors using a refrigerant cycle, even in freezing temperatures.
  • Performance drops as outdoor temperature falls, and most systems switch to backup heating around 25 to 35 degrees Fahrenheit.
  • A heat pump in cold climates will use more electricity than in mild climates because the system runs longer and works against a larger temperature difference.
  • Modern cold-climate heat pumps are engineered to operate efficiently down to 0 degrees or lower, though they still use backup heat at the lowest temperatures.

The refrigerant cycle that moves heat from cold air indoors

The heart of cold-weather heat pump operation is the refrigerant — a chemical that boils at very low temperatures. In the outdoor coil, liquid refrigerant evaporates (turns to gas) by absorbing heat from the surrounding air. This happens even when the air temperature is 20 degrees Fahrenheit because the refrigerant's boiling point is far lower than that. The gas then flows to the compressor, which squeezes it under high pressure. Compression raises the temperature of the gas significantly — this is the same principle that makes a bicycle pump warm up when you use it.

That hot, pressurized gas moves through copper lines into your home and enters the indoor coil. There, it condenses back into liquid, releasing its heat into the air or water that circulates through your heating system. A metering device (called an expansion valve) then lowers the pressure of the liquid, cooling it back down so it can return outdoors and repeat the cycle. The whole process is mechanical and requires electricity to run the compressor and the fans that move air across the coils.

In heating mode, the heat pump runs this cycle in the same direction all winter. In cooling mode during summer, the cycle reverses — the indoor coil absorbs heat from your home and the outdoor coil releases it outside. This reversibility is what makes a heat pump different from a furnace or air conditioner alone.

Why cold weather reduces heat pump efficiency

The larger the temperature difference between the outdoor air and the heat you need indoors, the harder the compressor has to work. On a 70-degree fall day, the heat pump only needs to raise outdoor air by 20 or 30 degrees to produce usable heat. On a 0-degree winter day, it needs to raise that air by 70 degrees. The compressor consumes more electricity to achieve that larger lift, and the system produces less heat per unit of energy consumed. This ratio is called the coefficient of performance (COP), and it drops as outdoor temperature falls.

Most air-source heat pumps have a COP of 3 or higher in mild weather, meaning they produce three units of heat for every unit of electricity used. In very cold weather, that same heat pump might have a COP of 1.5 or lower — still better than electric resistance heating (which has a COP of 1), but significantly less efficient than in moderate conditions. This is why heating bills rise sharply during cold snaps even though the heat pump is still running.

Ground-source (geothermal) heat pumps avoid this problem because they extract heat from the ground, which stays around 50 degrees year-round regardless of air temperature. They maintain high efficiency even in extreme cold, but they cost significantly more to install because they require drilling or trenching.

When backup heating turns on and why

Most air-source heat pumps have a built-in thermostat setting called the balance point or switchover temperature. When outdoor temperature drops below this point — often set between 25 and 35 degrees Fahrenheit — the system automatically switches to backup heating. The heat pump may continue running alongside the backup, or the backup may take over entirely depending on how the system is programmed. Homeowners can usually adjust this setting, though manufacturers recommend leaving it where it is because they have calculated the point where backup heating becomes more cost-effective than running the heat pump alone.

If your home has a gas furnace, that furnace becomes the backup. If you have an all-electric heat pump system, the backup is electric resistance heating — essentially heating coils that work like a toaster. Resistance heating is expensive to run because it converts electricity directly to heat with no efficiency multiplier, but it is reliable and requires no additional equipment. Some newer heat pump systems use a hybrid approach, automatically switching between heat pump and gas furnace based on real-time cost and outdoor temperature.

Cold-climate heat pumps rated for extreme temperatures

Manufacturers now produce cold-climate heat pumps specifically engineered to operate efficiently at very low temperatures. These models use larger compressors, improved refrigerants, and variable-speed motors that adjust output based on demand. Many cold-climate heat pumps maintain reasonable efficiency down to 0 degrees Fahrenheit or even lower, reducing the temperature at which backup heating becomes necessary. Some models can heat a home with the heat pump alone down to minus 13 degrees, though efficiency is still lower than in moderate weather.

Cold-climate models cost more upfront than standard heat pumps — typically 15 to 25 percent more — but they reduce heating bills in winter and may eliminate or reduce the need for backup heating. In regions where winter temperatures regularly drop below 20 degrees, a cold-climate heat pump often makes financial sense over the system's 15 to 20 year lifespan. In milder climates, a standard heat pump with backup heating is usually the more economical choice.

How to measure cold-weather performance when shopping

When comparing heat pumps, look for the Heating Seasonal Performance Factor (HSPF), which rates heating efficiency across an entire season rather than at a single temperature. A higher HSPF means lower heating bills. Standard heat pumps typically have an HSPF of 7 to 9; cold-climate models often reach 10 or higher. The HSPF accounts for the fact that your heat pump will run at many different outdoor temperatures throughout winter, so it is more realistic than a single-temperature rating.

You should also ask the manufacturer or installer for the heat pump's performance curve — a graph showing how much heat the unit produces at different outdoor temperatures. This tells you exactly when backup heating will be needed in your climate and how much the system's output drops as temperature falls. Some installers can model your specific heating needs based on your location, home size, and insulation level, then recommend whether a standard or cold-climate heat pump makes sense for you.

Frequently Asked Questions

Do heat pumps stop working when it freezes outside?

No. Heat pumps continue to extract and move heat even at 0 degrees Fahrenheit or colder. However, their efficiency drops significantly, and backup heating usually turns on automatically. The system is designed to keep working; it just becomes less economical to run the heat pump alone at very low temperatures.

Will my heat pump heat my home if it's 10 degrees outside?

Yes, but backup heating will likely be running alongside it or instead of it. The heat pump can still produce heat at 10 degrees, but the compressor has to work very hard, and the amount of heat it produces may be less than what your home needs. Your backup system fills the gap to maintain your set temperature.

Why does my heating bill spike during cold snaps if I have a heat pump?

During very cold weather, your heat pump becomes less efficient and backup heating runs more often. Both use more electricity to maintain your indoor temperature when the outdoor-to-indoor temperature difference is large. This is normal and expected; it does not mean your heat pump is broken.

Is a cold-climate heat pump worth the extra cost?

In regions where winter temperatures regularly drop below 20 degrees Fahrenheit, a cold-climate heat pump usually saves enough on heating bills over 15 to 20 years to justify the higher upfront cost. In milder climates, a standard heat pump with backup heating is usually more economical overall.

Can I use a heat pump in a very cold climate like Minnesota or Canada?

Yes, but you will need a cold-climate heat pump model and a backup heating system. Standard heat pumps are not designed for sustained temperatures below 25 degrees. Cold-climate models work in extreme cold, though they still rely on backup heating during the coldest days and will have higher operating costs than in milder regions.