Heat pumps work in freezing temperatures, but their output drops as the thermometer falls

A heat pump can operate in temperatures well below freezing—most models continue running down to around −13°F (−25°C), and some specialized units work even colder. However, the colder it gets outside, the less heat the pump can pull from the air, so you get fewer BTUs of heating per hour. Below about 32°F (0°C), most heat pumps lose efficiency rapidly, and below 0°F (−18°C), they may struggle to heat your home without help from a backup system.

The reason is straightforward: a heat pump doesn't create heat; it moves it from outside air to inside. When outdoor air is very cold, there is less heat energy available to move, even though cold air still contains some. At −13°F, a typical air-source heat pump delivers only 50 to 70 percent of the heating capacity it provides at 47°F (8°C)—the temperature at which manufacturers usually rate them.

Key Takeaways

  • Most air-source heat pumps continue operating down to −13°F (−25°C), but heating output drops sharply below 32°F (0°C).
  • A heat pump's efficiency is rated at 47°F; below that temperature, you get progressively less heat per hour of operation.
  • In climates colder than 0°F (−18°C) regularly, a backup heating system (electric resistance or gas furnace) is usually necessary to maintain comfort.
  • Ground-source heat pumps work reliably in extreme cold because they pull heat from the earth, which stays warmer than winter air.

Why heat pumps lose power in cold weather

Heat always moves from warmer to cooler places. A heat pump uses refrigerant to absorb heat from outdoor air and pump it indoors. When outdoor air is 47°F, there is plenty of heat energy available. When it drops to 0°F, the same refrigerant cycle still works, but the temperature difference between the cold outdoor air and the refrigerant is smaller, so less heat transfers per cycle.

Think of it like trying to boil water on a stove. On a hot day, the burner transfers heat quickly because the difference between the burner and room temperature is large. On a cold day, the burner is the same temperature, but the room is colder, so the burner has to work harder to warm the pot. A heat pump faces the same problem: the colder the outside air, the harder the compressor must work to move any heat at all.

Below about −13°F (−25°C), the refrigerant itself becomes difficult to compress, and the outdoor coil can ice over even with defrost cycles running. Most manufacturers set the cutoff point here because operation below this temperature risks damaging the compressor or providing almost no useful heat.

How to read your heat pump's cold-weather rating

Manufacturers publish a heating output number called capacity, measured in BTU per hour. This number is always given at a specific outdoor temperature—usually 47°F (8°C). You will also see a second number called coefficient of performance (COP) or heating seasonal performance factor (HSPF), which tells you how many BTUs of heat you get for every BTU of electricity you use.

The problem is that neither number tells you what happens at 0°F or −10°F. To find that, look for a cold-climate specification sheet or performance data at 17°F (some manufacturers include this). If the spec sheet shows capacity at multiple temperatures, you can see exactly how much heating you lose as it gets colder. A unit rated at 24,000 BTU at 47°F might deliver only 12,000 to 15,000 BTU at 0°F.

If you live where winter temperatures regularly drop below 0°F (−18°C), ask the installer whether the unit has a built-in electric resistance heater or whether you will need a separate backup system. Some heat pumps have resistance heating built into the indoor unit; others rely on your existing furnace or a standalone electric heater to take over when the outdoor temperature falls too far.

Defrost cycles and their effect on heating

When outdoor air is cold and humid, frost builds up on the heat pump's outdoor coil, blocking airflow and reducing heat transfer. To clear this, the heat pump switches into defrost mode: it reverses the refrigerant flow temporarily, using outdoor air to melt the frost off the coil. During defrost, no heat is delivered indoors—in fact, the indoor fan may shut off to avoid blowing cold air into your home.

Defrost cycles typically last 5 to 15 minutes and occur every 30 to 90 minutes in cold, humid conditions. If you live in a climate where this happens frequently, you lose a noticeable amount of heating time. Some newer heat pumps use sensors to defrost only when needed, rather than on a fixed schedule, which reduces wasted cycles.

During defrost, your backup heating system (if you have one) usually kicks in automatically to maintain indoor temperature. If you don't have backup heat, your home temperature may drop slightly during each defrost cycle. This is normal and not a sign of failure, but it is worth understanding if you are considering a heat pump in a cold climate.

Ground-source heat pumps in extreme cold

A ground-source heat pump (also called a geothermal heat pump) pulls heat from the earth rather than the air. Because soil temperature stays relatively constant year-round—usually between 45°F and 55°F (7°C to 13°C) depending on depth and location—a ground-source pump maintains high efficiency even when outdoor air is −20°F or colder.

Ground-source systems are more expensive to install because they require digging a loop of pipe either vertically (in a borehole) or horizontally (in a trench). However, they deliver consistent heating in extreme cold without needing backup systems, and they use less electricity than air-source pumps in cold climates. If you live where winter temperatures regularly drop below −10°F (−23°C) and you plan to stay in your home for at least 10 years, a ground-source system may be worth the higher upfront cost.

Backup heating systems and how they work together

In climates colder than 0°F (−18°C), a heat pump is almost always paired with a backup heating system. This is usually either an electric resistance heater (which works like a toaster, converting electricity directly to heat) or a gas furnace. The backup system does not run all the time—it only activates when the heat pump cannot keep up.

Most systems use a thermostat setpoint to decide when to switch. For example, the thermostat might be set to use the heat pump alone down to 20°F (−7°C), then automatically turn on the backup heater below that. Some systems use outdoor temperature sensors to make this decision; others measure how much the indoor temperature is dropping and switch to backup heat if the heat pump falls behind.

The advantage of this setup is that you get the efficiency of a heat pump during milder winter days (when it uses less electricity than a furnace) and the reliability of a furnace on the coldest days. The disadvantage is higher installation cost and the need to maintain two heating systems. Ask your installer to explain exactly when the backup system will set up and whether you can adjust that setpoint yourself.

What to expect in your first cold winter

If you are installing a heat pump in a climate where you have never used one before, the first winter may feel different from what you are used to. The air from heat pump vents is often cooler than furnace air—typically 90°F to 100°F (32°C to 38°C) instead of 120°F to 130°F (49°C to 54°C)—so it takes longer to warm a room, even though the total heat output is the same. This is not a problem; it just feels different.

You may also notice that your heating bill is higher than expected on the coldest days. This happens because the backup heating system is running, and electric resistance heat or gas furnace heat costs more per BTU than heat pump operation. This is normal and expected. Over the entire heating season, a heat pump usually costs less to run than a furnace alone, even with backup heat included, because it operates efficiently on the many milder days.

If your home feels cold or the heating seems weak, do not assume the heat pump is broken. Check that the thermostat is set correctly, that vents are not blocked, and that doors to unused rooms are closed. If the problem persists, call your installer to verify that the system is sized correctly for your home and that the backup heating is activating when it should.

Frequently Asked Questions

At what temperature does a heat pump stop working?

Most air-source heat pumps stop operating around −13°F (−25°C), though some models are rated to −22°F (−30°C). Below this point, the compressor risks damage and heating output is too low to be useful. However, the heat pump becomes inefficient much earlier—below 32°F (0°C), output drops noticeably, and below 0°F (−18°C), a backup heating system is usually necessary.

Do I need a backup heater if I install a heat pump?

It depends on your climate. If winter temperatures rarely drop below 20°F (−7°C), you may not need backup heat. If they regularly drop below 0°F (−18°C), a backup system is strongly recommended. Ask your installer to calculate the heating load for your home and compare it to the heat pump's capacity at your area's typical winter temperature.

Will my heating bills be higher in winter with a heat pump?

On the coldest days when backup heat is running, yes—electric resistance or gas heat costs more per BTU than heat pump operation. However, over the entire season, a heat pump usually costs less because it operates efficiently on the many milder days. Your total heating bill depends on your climate, electricity rates, and how well your home is insulated.

Can I use a heat pump in Alaska or other extreme climates?

Yes, but a ground-source heat pump is usually the better choice because it maintains efficiency in extreme cold. An air-source heat pump can work in Alaska with a properly sized backup heating system, but it will rely on that backup system for much of the winter, which increases operating costs.

What is the difference between a heat pump and a furnace in cold weather?

A furnace burns fuel (gas or oil) to create heat directly, so it works equally well at any outdoor temperature. A heat pump moves heat from outside air, so its output drops as outdoor temperature falls. In very cold climates, a furnace is more reliable, but a heat pump uses less energy on milder days, so a combination of both usually costs less over a full heating season.