Heat pumps lose efficiency as outdoor temperature drops, but most keep working down to around 0°F

A heat pump extracts heat from outside air even when it feels cold to you, but there is a temperature floor where it stops being practical. Most air-source heat pumps—the type on the outside of your house—continue to operate down to 0°F or below, though they work harder and pull less heat as the mercury falls. Below that point, many systems automatically switch to a backup heating source (usually electric resistance heat) because the pump cannot pull enough warmth from the air to heat your home efficiently.

The exact cutoff depends on your specific unit, your climate, and how your system is configured. Some newer cold-climate heat pumps are rated to work down to −13°F or lower. Older or standard models may struggle or shut down around 10°F to 20°F. Your installer should have told you this threshold when the system was set up; if you do not know yours, check your equipment manual or call the installer.

Key Takeaways

  • Most air-source heat pumps continue heating below freezing but become less efficient as outdoor temperature drops.
  • When outdoor temperature reaches your system's cutoff point (often 0°F to 20°F), the backup heater kicks in automatically.
  • Cold-climate heat pumps rated for extreme cold cost more upfront but avoid heavy reliance on backup heat in harsh winters.
  • You can check your heat pump's cold-weather performance in the equipment manual or by contacting your installer.

Why heat pumps slow down in cold weather

A heat pump works by moving heat from one place to another using refrigerant and a compressor. In heating mode, it pulls warmth from outdoor air and moves it inside. The colder the outdoor air, the smaller the temperature difference between outside and inside, and the harder the compressor has to work to move that heat. Think of it like trying to push water uphill—the steeper the hill, the more effort required.

As outdoor temperature drops, the amount of heat available in the air shrinks. At 32°F, there is still usable heat in the air, but at 0°F, there is much less. Your heat pump's capacity—the amount of heating it can deliver—drops by roughly 25 to 50 percent between 47°F (a standard rating point) and 17°F. Below that, the decline accelerates.

At the same time, your home needs more heat when it is colder outside. So the heat pump is delivering less while your house is demanding more. That is when the backup heater takes over.

What happens when your heat pump reaches its limit

Most heat pump systems have a built-in backup heater—usually electric resistance coils or a gas furnace—that turns on automatically when outdoor temperature drops below a set point. This point is called the balance point or cutoff temperature. Your installer programs this into the thermostat during setup.

When the outdoor temperature hits that threshold, the system switches to backup heat, the heat pump shuts down, or both run together. You will not see this happen on your thermostat; it is automatic. Your heating will continue uninterrupted, but you will be using more expensive backup heat instead of the efficient heat pump.

If your system has no backup heater—rare but possible in mild climates—the heat pump will straightforward run continuously and may not keep up with demand on very cold days. This is why backup heat is standard in most climates.

Cold-climate heat pumps rated for extreme cold

If you live in a region with harsh winters, a cold-climate heat pump may be worth the extra cost. These units are designed to extract heat efficiently at much lower temperatures—some are rated to −13°F or even lower—and use variable-speed compressors that adjust their output smoothly rather than cycling on and off.

Cold-climate models reduce your reliance on backup heat, which means lower heating bills and less wear on the backup system. They also tend to be quieter and more durable because the compressor is not constantly ramping up and down. The trade-off is higher upfront cost—typically $1,000 to $3,000 more than a standard unit—and they are not necessary in milder climates where backup heat rarely runs.

If you are considering a heat pump replacement or new installation in a cold climate, ask your installer whether a cold-climate model makes sense for your area and your heating needs.

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

Your equipment manual lists the minimum outdoor temperature at which the heat pump is rated to operate. Look for terms like "minimum operating temperature," "low-temperature cutoff," or "balance point." The manual should also show a performance chart that tells you how much heating capacity the unit delivers at different outdoor temperatures.

If you do not have the manual, you can find it online using your unit's model number (printed on the outdoor condenser) and the manufacturer name. Search "[manufacturer] [model number] manual" or visit the manufacturer's website directly. If you installed the system yourself or bought the house with it already in place, contact the installer or the previous owner for the documentation.

Your thermostat may also display the backup heater status. Some smart thermostats show when auxiliary heat is running, which tells you that outdoor temperature has dropped below your system's efficient range.

What to expect during a cold snap

During extended periods of very cold weather, your backup heater will run frequently or continuously. This is normal and expected. Your heating bills will be higher than they would be in milder weather, because you are using less efficient backup heat. This is not a sign that your heat pump is broken.

If your backup heater is electric resistance, expect a noticeable increase in electricity use. If it is a gas furnace, expect higher gas bills. Neither is a malfunction. The system is doing exactly what it was designed to do.

If your heat pump stops running entirely during cold weather and your backup heater is not coming on, that is a problem. Call your installer or a heat pump technician. The thermostat may be set incorrectly, the backup heater may have failed, or the heat pump itself may have a fault.

Maintenance to keep your heat pump working in cold weather

A well-maintained heat pump works more efficiently in cold weather than a neglected one. Keep the outdoor unit clear of snow, ice, and debris. If snow piles up around the condenser, the system cannot pull air through it. Some units have a defrost cycle that melts ice buildup automatically, but heavy snow still needs to be cleared by hand.

Have your system serviced annually, ideally before winter. A technician will check refrigerant charge, clean coils, inspect electrical connections, and test the backup heater. A system that is low on refrigerant or has dirty coils will lose capacity even faster in cold weather.

If you have a programmable or smart thermostat, make sure the backup heater settings are correct and the thermostat is reading the outdoor temperature accurately. A misreading can cause the backup heater to run when it should not, or not run when it should.

Frequently Asked Questions

Will my heat pump work at all if it is below zero?

Most heat pumps will continue to operate below zero, but your backup heater will be running instead. The heat pump may cycle on periodically to supplement the backup heat, or it may shut down entirely depending on how your system is configured. Either way, your home will stay heated—you will just be using less efficient backup heat.

Can I use a heat pump as my only heat source in a cold climate?

A heat pump alone is not recommended in climates where temperatures regularly drop below 20°F unless you install a cold-climate model with a high efficiency rating. Even then, most installers recommend a backup heater for safety and to avoid extremely high bills on the coldest days. A backup heater ensures you have heat even if the heat pump fails.

Why does my electric bill spike in winter if I have a heat pump?

Your backup heater is electric resistance heat, which is less efficient than the heat pump. When outdoor temperature drops below your system's balance point, the backup heater runs instead of the heat pump, and electric resistance heating uses significantly more electricity to produce the same amount of warmth. This is normal and expected in cold climates.

How do I know if my heat pump's backup heater is running?

If your thermostat is a smart model or a modern programmable unit, it may display "auxiliary heat" or "emergency heat" on the screen when the backup heater is active. You can also check your electric or gas bill—a spike in winter usage suggests the backup heater is running frequently. If you are unsure, ask your installer to show you how to read the status on your specific thermostat.

Should I switch to heat pump-only heating if I live somewhere cold?

A heat pump paired with a backup heater is the standard setup in cold climates and is more cost-effective than heat pump alone. If you are replacing an old furnace, keeping the furnace as backup heat is usually cheaper than installing a heat pump without backup. A may have access to installer can tell you what makes sense for your home and climate.