Heat pumps move warmth from outside air or ground into your home, rather than burning fuel to create heat
A heat pump works by capturing thermal energy that already exists in the air or soil outside, then moving that energy indoors. Even on a cold day, there is heat energy in the outdoor air—a heat pump extracts it, concentrates it, and releases it inside your home. In summer, the process reverses: the pump pulls heat out of your indoor air and moves it outside, working like an air conditioner. This is why heat pumps can both heat and cool, and why they use less energy than furnaces or space heaters that generate heat from scratch.
The core of a heat pump is a closed loop of refrigerant—a liquid that easily changes between liquid and gas states. As the refrigerant cycles through the system, it absorbs heat at one location and releases it at another. The outdoor unit contains a coil that pulls heat from the air or ground. That heat warms the refrigerant, turning it into a gas. The gas travels indoors to a second coil, where it releases that heat into your home and turns back into a liquid. A compressor powered by electricity drives this cycle continuously.
Key Takeaways
- Heat pumps extract existing heat from outdoor air or ground and move it indoors, rather than generating heat by burning fuel or using electric resistance.
- The refrigerant cycle—evaporation outdoors and condensation indoors—is what transfers heat, and a compressor powered by electricity keeps the cycle running.
- Air-source heat pumps pull heat from the outdoor air and work down to roughly 0°F, while ground-source heat pumps use soil temperature and work in colder climates.
- Heat pumps reverse their cycle in summer to cool your home by pulling heat indoors and moving it outside.
- Efficiency depends on the temperature difference between indoors and outdoors—the smaller the gap, the less work the compressor has to do.
The refrigerant cycle: how heat actually moves
The refrigerant is the working fluid that makes heat transfer possible. It is chosen because it boils and condenses at temperatures that occur naturally in heating and cooling systems. When the outdoor coil is colder than the outside air, heat flows into the refrigerant and it evaporates into a gas. This gas is then sucked into the compressor.
The compressor is an electric pump that squeezes the refrigerant gas, raising its pressure and temperature. A higher-pressure, hotter gas flows to the indoor coil. There, the refrigerant is hotter than the air inside your home, so it releases heat to that air and condenses back into a liquid. The liquid then flows through an expansion valve that lowers its pressure, cooling it further, and the cycle begins again. This loop runs continuously as long as you need heating or cooling.
The compressor is the only moving part that uses significant electricity. Everything else—the flow of refrigerant, the heat transfer at the coils—happens because of pressure and temperature differences. This is why heat pumps are more efficient than electric resistance heaters: they move heat rather than creating it, so the compressor does not have to work as hard.
Air-source heat pumps and outdoor temperature limits
Air-source heat pumps pull heat from the outdoor air through a coil exposed to the weather. They are the most common type because they are cheaper to install than ground-source systems and work in most climates. On a 35°F day, there is still enough heat in the air for the pump to extract and move indoors. Even at 0°F, an air-source pump can still operate, though it becomes less efficient because the temperature difference between the outdoor air and the refrigerant is smaller.
Below roughly 0°F, most air-source heat pumps lose effectiveness quickly. At that point, a backup heating system—usually electric resistance heat or a gas furnace—kicks in automatically. Some newer air-source models with variable-speed compressors and improved refrigerants can operate down to −13°F or lower, but this is not standard. If you live in a climate where outdoor temperatures regularly drop below 0°F for extended periods, a ground-source pump or a hybrid system combining a heat pump with a gas furnace may be more practical.
Air-source pumps also work in reverse during summer. The indoor coil becomes the evaporator (pulling heat out of your home), and the outdoor coil becomes the condenser (releasing that heat outside). The system cools your home while also heating water for showers and laundry if it is a heat pump water heater model.
Ground-source heat pumps and stable soil temperature
Ground-source heat pumps (also called geothermal) bury a loop of refrigerant-filled pipe underground, where soil temperature stays relatively constant year-round—usually between 45°F and 60°F depending on your location and depth. Because the outdoor temperature is stable and moderate, the compressor does not have to work as hard to extract heat in winter or reject heat in summer. This makes ground-source pumps more efficient than air-source models, especially in cold climates.
The tradeoff is cost and installation complexity. Burying the loop requires excavation, which can mean drilling a deep vertical well or trenching a horizontal loop across your property. Installation costs are typically two to three times higher than air-source systems. Ground-source pumps are most practical if you are replacing an old heating system, have space for the loop, and plan to stay in the home long enough to recoup the higher upfront cost through energy savings.
Ground-source systems are not affected by outdoor air temperature swings, so they maintain consistent heating and cooling performance even during extreme cold or heat waves. They also tend to have longer compressor lifespans because the system operates under less stress.
Efficiency ratings and what they mean
Heat pump efficiency is measured by HSPF (Heating Seasonal Performance Factor) for heating and SEER2 (Seasonal Energy Efficiency Ratio) for cooling. HSPF compares the total heat output over a heating season to the total electricity input. A higher HSPF means more heat delivered per kilowatt-hour of electricity used. Current air-source models typically range from HSPF 8 to 13, with higher numbers indicating better performance in cold climates.
SEER2 measures cooling efficiency the same way. A higher SEER2 means more cooling output per kilowatt-hour. Current air-source models typically range from SEER2 8 to 22. Ground-source systems often have higher ratings because they operate under more favorable temperature conditions.
Efficiency also depends on how you use the system. A heat pump is most efficient when the temperature difference between indoors and outdoors is small—for example, heating on a 40°F day uses less compressor energy than heating on a 0°F day. Running the thermostat at a lower temperature in winter and higher in summer improves efficiency. Proper maintenance, including cleaning outdoor coils and replacing air filters, also keeps the system running at rated efficiency.
Backup heating and how it activates
Most heat pump systems include a backup heating source that turns on automatically when outdoor temperature drops below a set point or when the heat pump cannot keep up with demand. In air-source systems, this is usually electric resistance heat (similar to a space heater) or a gas furnace. The backup activates without any action from you—the thermostat senses that the heat pump alone is not maintaining your set temperature and switches on the backup.
Electric resistance backup is simpler and cheaper to install but uses more electricity than the heat pump itself, so it should run as little as possible. Gas furnace backup is more efficient but requires a gas line and annual maintenance. Some newer systems use a hybrid approach, automatically switching between heat pump and gas furnace based on outdoor temperature and cost, to minimize energy use.
The backup system does not mean the heat pump is broken or inefficient—it is a normal part of operation in cold climates. Even with backup running occasionally, a heat pump typically uses less total energy over a heating season than a furnace alone, especially in moderate climates where outdoor temperatures stay above 20°F most of the time.
Common misconceptions about how heat pumps work
One widespread misunderstanding is that heat pumps "create" heat. They do not. They move heat that already exists. On a cold day, there is still thermal energy in the air; the heat pump concentrates and relocates it. This is why they cannot heat your home to a comfortable temperature if the outdoor air is extremely cold and the temperature difference is too large—the compressor would have to work so hard that it becomes impractical, which is why backup heating exists.
Another misconception is that heat pumps are silent. They are quieter than many furnaces, but the outdoor unit produces noise from the compressor and fan, typically 50 to 60 decibels—similar to a conversation at normal volume. Indoor units are usually very quiet. If noise is a concern, ask about the decibel rating before purchasing and consider placement away from bedrooms or outdoor living areas.
A third misunderstanding is that heat pumps work only in warm climates. Modern air-source heat pumps work effectively down to 0°F and sometimes lower. Ground-source systems work in any climate. The question is not whether they work, but whether they are the most cost-effective choice for your specific location and heating needs.
Frequently Asked Questions
Why does my heat pump run constantly in winter?
Heat pumps run longer than furnaces because they move heat gradually rather than generating it all at once. On very cold days, the compressor may run almost continuously to extract enough heat from the cold outdoor air. This is normal. If the compressor runs constantly and your home is still cold, the backup heating system should be activating—if it is not, contact a technician.
Can a heat pump heat a home below freezing?
Yes, but with reduced efficiency. Even at 0°F, there is heat energy in the air that a heat pump can extract. Below 0°F, most air-source models become impractical and the backup heating system takes over. Ground-source systems continue to work efficiently because soil temperature remains stable regardless of outdoor air temperature.
What is the difference between a heat pump and an air conditioner?
An air conditioner only cools by moving heat outdoors. A heat pump does the same thing in summer but reverses the cycle in winter to move heat indoors. Both use the same refrigerant cycle; a heat pump straightforward has a reversing valve that changes the direction of flow.
Do heat pumps work if there is no outdoor unit?
No. Every heat pump has an outdoor unit where the refrigerant absorbs or releases heat. Ductless mini-split systems have a small outdoor unit mounted on an exterior wall, but it is still there. The outdoor unit is essential to the heat pump cycle.
Why is my electric bill higher with a heat pump than my old furnace?
If you are heating to a higher temperature than before, or if backup heating is running frequently, electricity use will increase. Heat pumps are efficient, but they still use electricity to run the compressor. On very cold days when backup heating activates, total energy use may exceed what a gas furnace would use. Over a full heating season in moderate climates, heat pumps typically use less energy overall.