Heat pumps work in winter by pulling heat from cold air outside and moving it indoors, even when outdoor temperatures drop below freezing

A heat pump doesn't generate heat the way a furnace does—it moves heat from one place to another. In winter, it extracts warmth from the outdoor air, ground, or water and transfers it inside your home. This works because even cold air contains some heat energy. A heat pump uses refrigerant (a special fluid) and a compressor to concentrate that heat and push it indoors, where it warms your living spaces.

The process reverses what happens in summer cooling mode. Instead of pulling heat out of your home and releasing it outside, the cycle flips: the outdoor unit absorbs heat from the cold environment, the compressor pressurizes the refrigerant to make it even hotter, and the indoor unit releases that concentrated heat into your home. This is why heat pumps are called "reversible"—they heat and cool by changing the direction refrigerant flows.

Key Takeaways

  • Heat pumps extract warmth from outdoor air, ground, or water and move it indoors using refrigerant and a compressor, even in freezing temperatures.
  • Air-source heat pumps work down to about 0°F before efficiency drops sharply, while ground-source heat pumps remain efficient in any winter climate.
  • Most modern heat pumps include a backup heating element that automatically switches on when outdoor temperatures fall too low for the pump to keep up.
  • Heat pump efficiency in winter is measured by HSPF (Heating Seasonal Performance Factor), with higher numbers meaning lower heating costs.
  • Defrost cycles run automatically every 30 to 90 minutes in freezing weather to melt frost buildup on the outdoor unit, temporarily reducing indoor warmth.

How the refrigerant cycle works in heating mode

The outdoor unit contains a coil filled with refrigerant. When outdoor air passes over this coil, the refrigerant absorbs heat from that air and evaporates into a gas. The compressor then pressurizes this gas, which raises its temperature significantly—often to 140°F or higher, even though the outdoor air is much colder. This hot, pressurized gas flows to the indoor unit, where another coil releases that heat into your home's air or water system. As the refrigerant cools, it condenses back into a liquid and returns to the outdoor unit to repeat the cycle.

This process works because of a principle in thermodynamics: when you compress a gas, its temperature rises. The compressor is the heart of the system—it's powered by electricity and does the work of concentrating the heat so it can flow from cold to warm. The colder the outdoor air, the harder the compressor has to work, which is why heat pumps use more electricity on very cold days.

Why air-source heat pumps lose efficiency in extreme cold

Air-source heat pumps (the most common type) pull heat from outdoor air. They work well down to about 0°F, but below that temperature, the amount of heat available in the air drops sharply, and the compressor must work much harder to extract it. At temperatures below 0°F, an air-source heat pump may struggle to keep your home warm without help from a backup heating system.

The outdoor unit also faces a physical problem in freezing weather: moisture in the air condenses on the cold coil and freezes into frost. A thick frost layer blocks airflow and reduces heat transfer. To handle this, heat pumps run automatic defrost cycles every 30 to 90 minutes when outdoor temperatures are near or below freezing. During defrost, the system reverses briefly to heat the outdoor coil and melt the frost. This is normal and necessary, but it temporarily reduces the heat available indoors—you may notice a brief warm-air pause or even cool air from vents during defrost.

Ground-source and water-source heat pumps in winter

Ground-source heat pumps (also called geothermal) pull heat from the earth, which stays around 50°F year-round below the frost line. Water-source heat pumps draw from lakes, ponds, or wells. Because the heat source is much warmer than winter air, these systems maintain high efficiency even in the coldest climates. They don't face frost buildup problems and don't need backup heating in most situations.

The tradeoff is installation cost and space. Ground-source systems require drilling or trenching to place pipes underground, which is expensive and disruptive. Water-source systems need access to a suitable water body. Air-source heat pumps are far cheaper to install, which is why they're the standard choice in most homes, even though they're less efficient in extreme cold.

Backup heating and how it activates automatically

Most heat pump systems include a backup heating element—usually electric resistance heating (similar to a space heater) or a gas furnace. When outdoor temperatures drop below a set point (often around 20°F to 35°F, depending on the system), or when the heat pump can't keep up with indoor demand, the backup automatically switches on. You won't see this happen; the system manages it without your input.

Backup heating is less efficient than the heat pump itself, so it costs more to run. However, it ensures your home stays warm even on the coldest nights. Some systems use a two-stage approach: the heat pump runs alone until it can't meet the temperature setpoint, then the backup kicks in to make up the difference. Others use a balance point—a temperature below which the backup takes over entirely. Understanding your system's backup settings can help you manage winter heating costs.

HSPF ratings and what they mean for winter performance

HSPF (Heating Seasonal Performance Factor) measures how much heat a heat pump delivers per unit of electricity used over an entire heating season. A higher HSPF means lower heating costs. Most modern air-source heat pumps have an HSPF of 8 to 10; high-efficiency models may reach 12 or higher. Ground-source systems often exceed 4.0 COP (Coefficient of Performance), which translates to roughly 12+ HSPF.

HSPF is calculated under standard test conditions, so real-world performance varies based on your climate, how cold it gets, and how often backup heating runs. In a mild winter, your heat pump will operate more efficiently and use less backup heat. In a harsh winter with many days below 0°F, backup heating will run more often, raising your overall heating costs. When comparing heat pumps, look for HSPF ratings, but also ask your installer what the system typically costs to run in your specific region.

Common winter problems and how to prevent them

Frost and ice buildup on the outdoor unit is the most common winter issue. The defrost cycle handles most of it automatically, but heavy snow or ice can block airflow around the unit. Clear snow away from the outdoor unit after storms, and make sure the area around it stays open. Don't block vents or cover the unit with tarps—it needs airflow to function.

Reduced heating output during defrost cycles is normal and not a malfunction. If your home feels noticeably cold during these brief pauses, you can adjust your thermostat setpoint slightly higher, or ask your installer about systems with improved defrost controls. Some newer models minimize the comfort impact by using smart defrost scheduling.

If your heat pump isn't keeping your home warm enough, the backup heating may not be activating properly, or the system may need maintenance. Have a technician check refrigerant levels, compressor function, and backup element operation. A clogged air filter or blocked outdoor unit will also reduce heating output.

Frequently Asked Questions

Can a heat pump heat a home when it's 0°F outside?

Yes, but with reduced efficiency. Air-source heat pumps can extract heat down to about 0°F, though the compressor works harder and uses more electricity. Below 0°F, backup heating usually activates to maintain comfort. Ground-source heat pumps remain efficient at any outdoor temperature because they draw from the warmer earth.

Why does my heat pump make noise during winter?

Defrost cycles cause a brief hissing or gurgling sound as refrigerant reverses direction, and the outdoor fan may stop temporarily. You may also hear the backup heating element click on. These are normal. Loud grinding, squealing, or rattling sounds are not normal and warrant a service call.

Does a heat pump use a lot of electricity in winter?

Heat pumps use less electricity than electric resistance heating or most furnaces, but more than they do in mild weather. On very cold days when backup heating runs, electricity use rises. Exact costs depend on your climate, system efficiency, and local electricity rates. Ask your installer for an estimated winter heating cost based on your area's typical weather.

What temperature is too cold for a heat pump to work?

Air-source heat pumps work down to about 0°F, with backup heating handling colder temperatures. Ground-source heat pumps work efficiently at any outdoor temperature. Some cold-climate air-source models are rated to operate down to -13°F, though efficiency is very low. Check your system's specifications or ask your installer about your model's cold-weather performance.

Should I turn off my heat pump in winter and use a furnace instead?

No. Heat pumps are more efficient than electric resistance heating and competitive with gas furnaces in most climates. Switching systems wastes the heat pump's efficiency advantage. Let the system run as designed—it will automatically use backup heating when needed and manage efficiency on its own.