Air source heat pumps move heat from outside air into your home, even when it feels cold

An air source heat pump works by extracting warmth that exists in outside air and moving it indoors through a refrigerant cycle. Even when outdoor temperatures drop below freezing, air still contains heat energy—the pump compresses this refrigerant to concentrate that heat and release it inside your home. The outdoor unit pulls in air across a coil, the refrigerant absorbs the heat, and an indoor unit distributes that warmth through your home's existing ductwork or radiators.

The process reverses in summer: the pump can move heat out of your home to cool it down, making it a year-round system. Most air source heat pumps run on electricity, and they move heat rather than generate it, which is why they use less energy than electric resistance heating or gas furnaces burning fuel to create warmth from scratch.

Key Takeaways

  • Air source heat pumps extract heat from outside air using a refrigerant that circulates between an outdoor unit and an indoor unit.
  • The system compresses refrigerant to concentrate heat and move it indoors, working even in freezing temperatures because air always contains some heat energy.
  • A reversing valve lets the same system cool your home in summer by moving heat outdoors instead.
  • The outdoor unit has a fan and coil that look similar to an air conditioner, but the cycle runs in the opposite direction during heating.

The refrigerant cycle that moves heat indoors

The heart of an air source heat pump is a closed loop of refrigerant—a liquid that boils and condenses at low temperatures. In heating mode, the outdoor unit's coil acts as an evaporator. Cold refrigerant flows through this coil while a fan blows outside air across it. Even air at 20°F contains heat energy; the refrigerant absorbs this heat and boils into a gas, though it stays much colder than room temperature.

The gaseous refrigerant then flows to a compressor, usually located in the outdoor unit. The compressor squeezes the gas, which raises its temperature and pressure significantly—this is the energy-intensive step that requires electricity. The hot, pressurized refrigerant gas then moves to the indoor unit's coil, called the condenser. Here, a fan blows indoor air across the coil, and the hot refrigerant releases its heat into your home while cooling back into a liquid. The liquid refrigerant then flows through an expansion device that lowers its pressure, and the cycle repeats.

This cycle is identical to how a refrigerator works, except the roles are reversed: a refrigerator pulls heat from inside a box and dumps it into your kitchen, while a heat pump pulls heat from outside air and dumps it into your home.

Why air source heat pumps work in cold weather

Many people assume a heat pump cannot work when outdoor air is freezing, but this misunderstands what "cold" means. Cold is the absence of heat, not the absence of energy. Air at 0°F still contains molecular motion and thermal energy; it is straightforward less than air at 70°F. A heat pump's refrigerant is designed to boil at temperatures far below freezing, so it can still extract that energy even in harsh winter conditions.

However, efficiency does drop as outdoor temperature falls. A heat pump pulling heat from 0°F air has to work harder—the compressor runs longer and uses more electricity—than one pulling heat from 40°F air. Most air source heat pumps include an electric resistance heating element (similar to a space heater) that kicks in during extreme cold to supplement the pump. This backup heat is less efficient than the heat pump itself, but it ensures your home stays warm when outdoor temperatures plummet.

The lowest temperature at which a heat pump can heat your home is called its balance point. For most air source systems, this is around 0°F to 10°F, depending on the model and your home's insulation. Below that point, the resistance heater takes over entirely.

The outdoor and indoor units explained

The outdoor unit contains the evaporator coil, compressor, and a fan. It looks similar to an air conditioner's outdoor unit—a metal box with a fan visible through a grille—but the internal plumbing is reversed. Two copper lines connect the outdoor unit to the indoor unit: one carries cold liquid refrigerant out, and the other carries hot gas back. A drain line removes condensation that forms on the coil during heating (and during cooling in summer).

The indoor unit can take several forms depending on your home's heating system. If you have existing ductwork and a furnace, the heat pump's indoor coil replaces or sits alongside the furnace's coil, and the furnace's blower distributes warm air through the ducts. If you have radiators or baseboard heaters, a hydronic heat pump uses the indoor coil to warm water that circulates through those radiators instead. Some homes use ductless or mini-split systems, where small wall-mounted indoor units blow warm air directly into each room without ductwork.

All three setups use the same refrigerant cycle; the difference is how the heat is distributed once it reaches your home.

How the reversing valve switches between heating and cooling

A small component called a reversing valve allows the same heat pump to heat in winter and cool in summer. This valve redirects refrigerant flow so that the outdoor coil becomes the condenser (releasing heat outside) and the indoor coil becomes the evaporator (pulling heat from inside). The compressor and fan continue running, but the cycle reverses.

In cooling mode, warm indoor air passes over the indoor coil, and the refrigerant absorbs that heat and carries it outdoors. The outdoor unit then releases the heat into the outside air. This is why the outdoor unit of a running air conditioner or heat pump in cooling mode feels hot to the touch—it is dumping your home's heat outside.

The reversing valve switches automatically based on a thermostat setting. You set the thermostat to "heat" in winter and "cool" in summer, and the system handles the rest. Some heat pumps can switch between modes automatically if the thermostat detects that heating is no longer needed.

Efficiency and electricity use

Air source heat pumps are measured by a rating called HSPF (Heating Seasonal Performance Factor), which compares the heat delivered to the electricity used over a heating season. A higher HSPF means better efficiency. Modern air source heat pumps typically have an HSPF of 8 to 10, meaning they deliver 8 to 10 units of heat for every unit of electricity consumed. This is possible because the pump is moving heat rather than creating it.

By contrast, an electric resistance heater has an efficiency of 1—it converts electricity directly to heat with no multiplication effect. A gas furnace burns fuel to create heat, and some of that heat escapes up the chimney, so its efficiency is typically 80 to 95 percent. A heat pump's ability to move heat from a colder place to a warmer place (which seems to violate intuition but is allowed by thermodynamics) is what makes it so efficient.

Electricity use depends on outdoor temperature, indoor temperature, and how well your home is insulated. On a 40°F day, a heat pump uses less electricity than on a 0°F day because the temperature difference is smaller. On very cold days, the resistance heater may run frequently, which increases electricity use significantly.

Common misconceptions about air source heat pumps

One widespread belief is that heat pumps cannot heat below freezing. In reality, they work fine at freezing temperatures; they straightforward become less efficient and may use backup resistance heat more often. Another misconception is that the outdoor unit "freezes up" in winter. Modern heat pumps include a defrost cycle that periodically reverses the refrigerant flow to melt ice buildup on the outdoor coil, similar to how a car's defroster works.

Some people think heat pumps are "new technology" that has not been proven. Air source heat pumps have been used in cold climates like Canada, Scandinavia, and northern Europe for decades. They are well-established and reliable, though they do require professional installation and regular maintenance to perform well.

A final misconception is that heat pumps are silent. The outdoor unit produces noise from the fan and compressor—typically 50 to 60 decibels, similar to a window air conditioner. If noise is a concern, the outdoor unit can be placed away from bedrooms or screened with a fence or shrub barrier.

Frequently Asked Questions

Can an air source heat pump heat a whole house in winter?

Yes, in most climates. In regions where winter temperatures rarely drop below 20°F, a heat pump alone can handle all heating. In colder climates, the system includes electric resistance backup heat that activates on the coldest days. Some homes pair a heat pump with an existing gas furnace so the furnace takes over during extreme cold, reducing electricity use.

Why does the outdoor unit sometimes look wet or have frost on it?

During heating, the outdoor coil gets very cold as it absorbs heat from outside air. Moisture in the air condenses on the coil, similar to how dew forms on grass. In freezing weather, this moisture becomes frost or ice. The defrost cycle periodically reverses the refrigerant to warm the outdoor coil and melt the buildup, which drains away.

How long does a heat pump last?

Air source heat pumps typically last 15 to 20 years with regular maintenance. The outdoor unit is exposed to weather and works harder than indoor equipment, so it may need replacement before the indoor unit. Proper installation, annual filter changes, and professional servicing extend the system's lifespan.

Is a heat pump quieter than a furnace?

Heat pumps and air conditioners produce similar noise levels—around 50 to 60 decibels from the outdoor unit. A gas furnace is quieter because it has no outdoor unit, though the indoor blower makes noise when it runs. If noise is a concern, ask the installer about unit placement and sound-dampening options.

What happens if refrigerant leaks out of the system?

A refrigerant leak stops the heat pump from working because the cycle cannot complete without the refrigerant. The system will not heat or cool. A licensed technician must locate the leak, repair it, and refill the refrigerant. This is why regular maintenance is important—technicians check for leaks during annual service visits.