An air source heat pump moves heat from outside air into your home, even in cold weather
An air source heat pump works by extracting warmth from the outdoor air and transferring it inside your home. It does this using a refrigerant—a liquid that changes state easily between liquid and gas—that circulates through an outdoor unit and an indoor unit. Even when outdoor air feels cold to you, it still contains heat energy that the pump can capture and concentrate.
The system runs on electricity and uses the same basic principle as a refrigerator, but in reverse. A refrigerator pulls heat out of its interior and dumps it into your kitchen. A heat pump pulls heat out of the outdoor air and dumps it into your home. In summer, many air source heat pumps can reverse this cycle to cool your home like a traditional air conditioner.
Key Takeaways
- An air source heat pump uses a refrigerant that circulates between an outdoor unit and an indoor unit to move heat from outside air into your home.
- The system can extract usable heat from outdoor air even when temperatures are well below freezing, though efficiency drops as outdoor temperature falls.
- A compressor powered by electricity pressurizes the refrigerant to raise its temperature so it can warm your home's air or water.
- Most air source heat pumps can reverse to provide cooling in summer, combining heating and air conditioning in one system.
- Efficiency is measured in COP (coefficient of performance) for heating and SEER for cooling; higher numbers mean lower operating costs.
The refrigerant cycle: how heat moves through the system
The refrigerant inside an air source heat pump follows a closed loop that repeats continuously. In the outdoor unit, the refrigerant passes through an evaporator coil where it absorbs heat from the outside air. The refrigerant boils and turns into a gas, even though the outdoor air is cold. This is possible because refrigerants are designed to boil at very low temperatures.
That warm gas then travels indoors through copper tubing to a compressor, which is the heart of the system. The compressor squeezes the gas, raising its pressure and temperature significantly. Think of it like pumping air into a bicycle tire—compression creates heat. The now-hot refrigerant gas flows into an indoor condenser coil, where it releases its heat to the air or water circulating through your home's heating system. As the refrigerant cools, it condenses back into a liquid.
The liquid refrigerant then passes through an expansion valve that lowers its pressure, cooling it further before it returns to the outdoor evaporator coil to start the cycle again. This continuous loop—evaporate, compress, condense, expand—is what allows the pump to move heat from cold outdoor air into your warm home.
Why air source heat pumps work even in freezing temperatures
Many homeowners assume a heat pump cannot work when it is freezing outside, but this misunderstands what "cold" means in thermodynamic terms. Cold air still contains heat energy—it straightforward contains less of it than warm air. A heat pump's refrigerant is designed to boil and absorb that energy even when outdoor air is well below 32°F.
However, efficiency does decline as outdoor temperature drops. A heat pump operating at 47°F pulls heat much more easily than one operating at 17°F. The compressor has to work harder and use more electricity to extract and concentrate the heat. Most manufacturers publish a balance point—the outdoor temperature below which the system's efficiency drops enough that a backup heating source (usually electric resistance heat) becomes more cost-effective to run alongside the pump.
In regions with very cold winters, some homeowners install a dual-fuel system that uses the heat pump when outdoor temperatures are moderate and switches to a gas furnace or electric resistance heating when temperatures fall below the balance point. Others use a heat pump alone and accept higher electricity use during the coldest weeks.
Measuring efficiency: COP and SEER ratings
Air source heat pump efficiency is measured differently depending on whether you are heating or cooling. For heating, the rating is called COP, or coefficient of performance. COP is the ratio of heat output to electricity input. A COP of 3 means the system delivers three units of heat for every one unit of electricity consumed. The higher the COP, the lower your heating bills.
For cooling, the rating is SEER (seasonal energy efficiency ratio), which works the same way—higher numbers mean more cooling output per unit of electricity. A modern air source heat pump might have a COP of 2.5 to 4.0 for heating and a SEER of 15 to 22 for cooling, depending on the model and outdoor conditions. These ratings are always measured under specific laboratory conditions, so real-world performance varies based on your climate, installation quality, and how you operate the system.
COP and SEER ratings also assume the system is sized correctly for your home. An oversized heat pump cycles on and off frequently, reducing efficiency. An undersized one runs continuously and cannot keep up on the coldest days. A professional load calculation before installation ensures the pump matches your home's actual heating and cooling needs.
Indoor and outdoor units: what each one does
The outdoor unit contains the evaporator coil, the compressor, and a fan that pulls outside air across the coil to transfer heat to the refrigerant. It looks similar to an air conditioner condenser unit but operates differently. The outdoor unit is where the refrigerant absorbs heat from the air, so it must be placed where air can flow freely around it—typically on the ground or mounted on a wall, away from obstructions.
The indoor unit contains the condenser coil and a fan or blower that circulates your home's air across it. In an air-to-air system, the indoor unit looks like a wall-mounted or ceiling-mounted air handler and distributes warm air through ducts or directly into rooms. In an air-to-water system, the indoor unit is a heat exchanger that warms water circulating through radiators or radiant floor heating. Some systems use a combination of both.
The two units are connected by insulated refrigerant lines and an electrical control line. A thermostat tells the compressor when to run and how hard to work. Modern heat pumps can modulate—meaning the compressor runs at different speeds rather than straightforward on or off—which improves efficiency and comfort by reducing temperature swings.
Defrosting: how the system handles frost buildup
When outdoor air is cold and humid, frost can accumulate on the outdoor coil, blocking airflow and reducing heat transfer. To prevent this, air source heat pumps include a defrost cycle. The system periodically reverses the refrigerant flow, using hot refrigerant to warm the outdoor coil and melt the frost. During defrost, the indoor unit may blow cool air briefly, or a backup heater may set up to maintain comfort inside.
Defrost cycles typically last 5 to 15 minutes and occur several times per day during cold, damp weather. They use extra electricity and temporarily reduce heating output, which is why heat pump efficiency drops in very cold, humid climates. Modern systems use sensors to detect frost and run defrost only when needed, rather than on a fixed schedule, which saves energy.
Air-to-air versus air-to-water heat pumps
An air-to-air heat pump transfers heat from outdoor air directly to the air inside your home. The indoor unit blows warm air into your rooms through ducts or directly into the space. This is the most common type for residential heating and cooling. It works well in homes with existing ductwork or in newer homes designed for ducted systems.
An air-to-water heat pump transfers heat from outdoor air to water, which then circulates through radiators, baseboard heaters, or radiant floor systems. This type is common in Europe and is becoming more popular in North America for homes with hydronic heating systems. Air-to-water pumps can also provide domestic hot water, combining space heating and water heating in one unit.
Both types use the same refrigerant cycle and operate on the same principles. The choice between them depends on your home's existing heating system, available space, and whether you want to replace your water heater at the same time.
Frequently Asked Questions
Can an air source heat pump heat a home in winter without backup heat?
Yes, in mild climates where winter temperatures rarely drop below 30°F. In colder regions, most systems need backup heat for the coldest weeks. Some homeowners accept higher electricity bills during those periods; others install a dual-fuel system that switches to gas or electric resistance heat below a certain temperature.
How much electricity does an air source heat pump use?
Usage depends on your home's size, insulation, climate, and how cold it gets. A typical home might use 5,000 to 15,000 kWh per year for heating with a heat pump, compared to 10,000 to 20,000 kWh with electric resistance heating. Gas heating is often cheaper in very cold climates, but a heat pump is usually more efficient than electric-only heating.
What happens to an air source heat pump in very cold weather?
The system continues to work but becomes less efficient. The compressor must work harder to extract and concentrate heat, using more electricity. Most systems have a balance point—typically around 20°F to 35°F depending on the model—below which backup heating becomes more cost-effective. Defrost cycles also become more frequent.
Do air source heat pumps require special maintenance?
Maintenance is minimal compared to gas furnaces. The outdoor coil should be kept clear of debris, and the system should be inspected annually by a technician. Refrigerant levels rarely need adjustment if the system is properly sealed. Most heat pumps last 15 to 20 years with routine care.
Can I use an air source heat pump if I have a gas furnace?
Yes. A dual-fuel system runs the heat pump when outdoor temperatures are moderate and switches to the furnace during very cold weather. This setup maximizes efficiency by using the most cost-effective heating method for each condition. A technician can design a system that uses both without replacing your existing furnace.