A heat pump moves warmth from outside air or ground into your home, rather than burning fuel to create heat
A heat pump is an appliance that pulls heat energy from the air or ground outside your home and moves it indoors. It does not generate heat the way a furnace burns gas or oil. Instead, it uses electricity to run a refrigeration cycle — the same basic process that makes a refrigerator cold — but in reverse. In winter, it extracts warmth from outdoor air (even when it feels cold) or from the ground, concentrates that heat, and pushes it into your home. In summer, it reverses and pulls heat out of your home to cool it.
The reason this matters is efficiency. Moving heat takes far less energy than creating it from scratch. A heat pump can deliver three to four units of heating energy for every unit of electricity it uses. A traditional electric heater delivers only one unit of heat per unit of electricity. This is why heat pumps cost less to run, even though they use electricity instead of gas.
Key Takeaways
- A heat pump moves existing heat from outside into your home using a refrigeration cycle, rather than generating heat by burning fuel or running electric resistance.
- The system contains a compressor, two heat exchangers (called the evaporator and condenser), and a refrigerant fluid that circulates between them and changes state between liquid and gas.
- In winter, the outdoor unit absorbs heat from cold air or ground, the refrigerant carries that heat indoors, and the indoor unit releases it into your home.
- Heat pumps work in cold climates, but their efficiency drops as outdoor temperature falls, and some models include a backup electric heater for the coldest days.
- The most common types are air-source (pulls from outdoor air) and ground-source (pulls from underground), with air-source being cheaper to install but less efficient than ground-source.
The refrigeration cycle that moves heat
The core of a heat pump is a closed loop of tubing filled with refrigerant, a fluid that boils and condenses at low temperatures. The loop contains four main parts: a compressor, a condenser, an expansion valve, and an evaporator. In winter heating mode, the cycle works like this:
The evaporator (the outdoor unit) contains cold refrigerant. Outdoor air passes over it, and even though the air is cold, it is warmer than the refrigerant inside the coil. Heat flows from the warmer air into the colder refrigerant, causing the refrigerant to boil and turn into a gas. The compressor then sucks in this refrigerant gas and squeezes it. Compression heats the gas further — this is the key step. The hot, pressurized gas flows to the condenser (the indoor unit), where indoor air passes over it. The refrigerant cools and condenses back into a liquid, releasing its heat into your home. Finally, the expansion valve reduces the pressure on the liquid refrigerant, cooling it back down, and the cycle repeats.
The refrigerant is the workhorse. It is chosen because it boils and condenses at temperatures that make this cycle practical. Common refrigerants include R-410A and R-32. The refrigerant itself does not leave the system — it circulates in a closed loop for years, though it can leak if a connection corrodes or breaks.
How the outdoor unit pulls heat from cold air
The outdoor unit of an air-source heat pump looks like an air conditioner condenser — a metal box with a fan and coils. In winter, this unit is where the magic happens. The refrigerant inside the coils is kept cold (usually between 20°F and 40°F) by the expansion valve. Outdoor air, even at 30°F or below, is still warmer than this cold refrigerant. Heat always flows from warmer to colder, so warmth from the outdoor air transfers into the refrigerant coil.
This is counterintuitive: a heat pump can extract usable heat from air that feels freezing to you. The reason is that cold air still contains thermal energy. At absolute zero (−459°F), all molecular motion stops and there is no heat. At 0°F, molecules are still moving and colliding, and that motion is heat. A heat pump exploits the temperature difference between the outdoor air and the even colder refrigerant to pull that energy out.
As outdoor temperature drops, the temperature difference between the air and the refrigerant shrinks, and the heat pump becomes less efficient. At around 32°F to 35°F, many air-source heat pumps lose enough efficiency that a backup heating system (usually electric resistance heat) kicks in automatically. This is why heat pumps work best in mild climates and why they are paired with a furnace or electric heater in cold regions.
How the indoor unit releases heat into your home
The indoor unit is typically mounted on a wall or in a closet and contains the condenser coil. Hot, pressurized refrigerant gas arrives from the compressor and flows through this coil. A fan blows indoor air across the coil, and the refrigerant releases its heat into that air. The warmed air is then distributed through your home via ductwork (if you have a ducted system) or directly into the room (if you have a ductless mini-split system).
The amount of heat released depends on how much refrigerant flows through the coil and how hot that refrigerant is. The compressor speed controls the refrigerant flow. Modern heat pumps use variable-speed compressors that ramp up and down based on how much heating or cooling your home needs. On a mild day, the compressor runs slowly, moving less refrigerant and releasing less heat. On a very cold day, it runs faster. This modulation makes the system more efficient than older fixed-speed models that ran at full power or not at all.
Air-source versus ground-source heat pumps
An air-source heat pump pulls heat from outdoor air. It is the most common type because it is cheaper to install — you do not need to dig or drill. The outdoor unit sits on the ground or mounted to a wall. Air-source heat pumps work well in climates where winter temperatures stay above 20°F most of the time. In colder climates, they lose efficiency and may need backup heat on the coldest days.
A ground-source heat pump (also called a geothermal heat pump) pulls heat from the ground or groundwater. Underground temperature stays relatively constant year-round — usually between 45°F and 55°F depending on your location. Because the ground is warmer than winter air, the temperature difference is larger, and the heat pump stays efficient even when outdoor air is very cold. Ground-source systems work well in cold climates and are more efficient overall, but they cost significantly more to install because they require drilling or trenching to bury the heat exchanger loop.
The choice between the two depends on your climate, your budget, and your long-term plans. Air-source is the entry point for most homeowners. Ground-source makes sense if you are replacing a failed heating system and plan to stay in the home for many years, since the higher upfront cost is offset by lower operating costs.
Why heat pumps are more efficient than traditional heating
A traditional furnace burns fuel (gas, oil, or propane) and converts that chemical energy into heat. Even a high-efficiency furnace wastes some energy up the chimney. An electric resistance heater (like a space heater or the backup heat in a heat pump) converts electricity directly to heat with nearly 100% efficiency, but electricity is expensive, so the operating cost is high.
A heat pump moves heat instead of making it, so it uses far less energy. If a heat pump delivers 3 units of heat for every 1 unit of electricity it consumes, its coefficient of performance (COP) is 3. A traditional electric heater has a COP of 1. Over a heating season, this difference translates to lower utility bills, even though the heat pump costs more upfront.
The efficiency advantage shrinks in very cold climates. As outdoor temperature drops, the heat pump must work harder to extract heat, and the compressor consumes more electricity. Below about 20°F, many air-source heat pumps become less efficient than a gas furnace. This is why cold-climate heat pumps are often paired with a gas furnace or electric backup heat. The system automatically switches to the furnace when the heat pump can no longer keep up.
Maintenance and common issues
A heat pump has fewer moving parts than a furnace and no combustion, so it is generally reliable. The main maintenance tasks are cleaning the outdoor unit and replacing the air filter on the indoor unit. Leaves, dirt, and snow can block the outdoor coil and reduce efficiency, so clear the area around the outdoor unit in fall and after heavy snow. The indoor filter should be checked monthly and replaced every one to three months, depending on dust levels in your home.
Common issues include refrigerant leaks, which show up as reduced heating or cooling output. If you notice your heat pump is not keeping up with demand, have it serviced by a technician who can check the refrigerant charge and look for leaks. Compressor failure is rare but expensive — it usually means replacing the entire outdoor unit. Icing on the outdoor coil in winter is normal; most heat pumps have a defrost cycle that reverses the refrigerant flow periodically to melt ice. If ice builds up continuously, the unit may need service.
Frequently Asked Questions
Can a heat pump work when it is below freezing outside?
Yes. A heat pump can extract heat from air well below freezing because the refrigerant inside is even colder. However, efficiency drops as outdoor temperature falls. Most air-source heat pumps lose significant efficiency below 20°F and may need backup electric heat or a furnace to maintain comfort on very cold days. Ground-source heat pumps remain efficient in cold climates because ground temperature stays relatively constant.
What is the difference between a heat pump and an air conditioner?
An air conditioner cools by moving heat out of your home. A heat pump does the same thing in summer but can reverse the cycle in winter to move heat into your home. Both use the same refrigeration cycle; a heat pump straightforward has a reversing valve that changes the direction of refrigerant flow. This is why a heat pump can both heat and cool, while an air conditioner only cools.
Do heat pumps work in humid climates?
Yes. Heat pumps work in any climate, but they are most cost-effective in mild to moderate climates. In very humid climates, the indoor unit may produce condensation as it cools air, which is normal. The condensate drains away through a small pipe. In heating mode, humidity is usually not an issue because the heat pump warms and dries the air.
How long does a heat pump last?
A well-maintained heat pump typically lasts 15 to 20 years. The compressor is the most expensive component, and if it fails after the warranty period, replacement may cost more than repairing other parts. Regular maintenance — cleaning coils, replacing filters, and having the system serviced annually — extends the lifespan.
What happens if the refrigerant leaks?
If refrigerant leaks, the heat pump will not heat or cool effectively because there is less fluid to carry heat. You will notice reduced output or the system running constantly without reaching the set temperature. A technician must locate and repair the leak, then refill the refrigerant. Refrigerant is not something you can top up yourself — the system must be properly charged by a licensed technician.