The Basic Cycle: How Heat Pumps Move Warmth
A heat pump works by moving heat from one place to another using a refrigerant—a liquid that changes between liquid and gas states as it flows through the system. The pump doesn't create heat; it captures warmth that already exists (even in cold air) and transfers it indoors during winter, or moves indoor heat outside during summer. This cycle repeats continuously, driven by an electric compressor that pressurizes the refrigerant and forces it through a closed loop of pipes.
The key to understanding a heat pump is recognizing that heat exists everywhere, even at temperatures below freezing. A heat pump extracts this low-grade heat, concentrates it using pressure and a compressor, and releases it where you need it. This is why heat pumps work in cold climates—they're not waiting for warm air to appear; they're pulling usable thermal energy from the outdoor air, ground, or water and upgrading it to room temperature.
Key Takeaways
- A heat pump uses a refrigerant that cycles between liquid and gas to move heat from outdoors to indoors (or vice versa) rather than generating heat from fuel.
- The compressor pressurizes the refrigerant, which is the engine that makes the whole cycle work and requires electricity to run.
- Two heat exchangers—one outside and one inside—allow the refrigerant to absorb heat in one location and release it in another.
- A reversing valve lets the system switch between heating mode (winter) and cooling mode (summer) by changing the direction refrigerant flows.
- Heat pumps move existing heat rather than burning fuel, which is why they use less energy than furnaces or electric resistance heaters.
The Four Main Parts and What They Do
The compressor is the heart of the system. It draws in low-pressure refrigerant gas from the indoor unit, squeezes it under high pressure, and heats it up in the process. This pressurized, hot gas then travels to the outdoor heat exchanger. The compressor runs on electricity and is the reason heat pumps have an electric bill rather than a gas bill.
The outdoor heat exchanger (called the condenser in cooling mode, or the evaporator in heating mode) is where the pressurized hot refrigerant releases its heat to the outside air, ground, or water source. As the refrigerant cools and condenses back into a liquid, it sheds the thermal energy it picked up from the compressor. This liquid then travels indoors through a thin copper line.
The indoor heat exchanger (called the evaporator in heating mode, or the condenser in cooling mode) is where the liquid refrigerant evaporates back into a gas, absorbing heat from indoor air in the process. A blower fan pushes room air across this heat exchanger, warming it up and circulating it through your home. The now-gaseous refrigerant returns to the compressor to repeat the cycle.
The reversing valve is a four-way switch that changes the direction refrigerant flows through the system. In heating mode (winter), it directs hot refrigerant to the indoor unit so you get warm air. In cooling mode (summer), it reverses the flow so the indoor unit absorbs heat and releases it outside. This single component is what allows one unit to both heat and cool.
The Heating Cycle Step by Step
During winter, the reversing valve positions the outdoor unit as the evaporator and the indoor unit as the condenser. The cycle begins when the compressor draws in cold, low-pressure refrigerant gas from the indoor unit. It compresses this gas, raising its temperature and pressure significantly—often to 140°F or higher.
This hot, pressurized gas travels outdoors through a copper line to the outdoor heat exchanger. Even though outdoor air is cold (say, 35°F), the refrigerant is much hotter, so heat flows from the refrigerant into the outdoor air. As the refrigerant cools, it condenses into a liquid. This liquid then passes through an expansion device (a metering valve or capillary tube) that reduces its pressure, cooling it further before it enters the indoor unit.
Inside your home, the cold liquid refrigerant enters the indoor heat exchanger. Indoor air (around 70°F) is warmer than the refrigerant, so heat flows from the air into the refrigerant. The refrigerant evaporates back into a gas, and a blower fan pushes room air across the heat exchanger, warming it and sending it through your ducts. The now-gaseous refrigerant returns to the compressor, and the cycle repeats.
The Cooling Cycle Step by Step
During summer, the reversing valve flips, positioning the outdoor unit as the condenser and the indoor unit as the evaporator. The compressor still draws in refrigerant gas and pressurizes it, but now this hot gas is sent to the indoor unit instead of outdoors. The indoor heat exchanger becomes the place where heat is released, not absorbed.
The pressurized, hot refrigerant enters the indoor heat exchanger. Room air (around 75°F) passes across it, and heat flows from the refrigerant into the air—but wait: the refrigerant is much hotter than the room, so the blower fan actually pulls heat out of your home and into the refrigerant. This cools your indoor air. The now-liquid refrigerant travels outdoors and enters the outdoor heat exchanger, where it releases all that absorbed indoor heat to the outside air. The cycle repeats continuously until your home reaches the set temperature.
Why Pressure and Temperature Matter
The reason a heat pump can move heat from cold outdoor air into your warm home is pressure. When the compressor pressurizes a gas, its temperature rises automatically—this is a law of physics. A refrigerant compressed to 300 pounds per square inch can reach 140°F even if outdoor air is only 35°F. This temperature difference is what allows heat to flow from the refrigerant to the outdoor air during heating mode.
The expansion device (metering valve) does the opposite: it releases pressure, and the refrigerant temperature drops. This creates a large temperature difference between the cold refrigerant and your room-temperature indoor air, allowing heat to flow into the refrigerant during heating mode. Without pressure changes, the refrigerant would stay at the same temperature throughout the cycle and no heat transfer would occur.
Common Variations: Air-Source, Ground-Source, and Water-Source
An air-source heat pump exchanges heat with outdoor air. It's the most common type because it requires no digging or water access. The outdoor unit has a fan that blows air across the heat exchanger, improving heat transfer. Air-source pumps work in cold climates but lose efficiency as outdoor temperature drops below 32°F, which is why many cold-climate homes pair them with a backup electric heater.
A ground-source (geothermal) heat pump exchanges heat with soil or groundwater instead of air. Ground temperature stays around 50°F year-round, even when air is freezing, so these systems maintain high efficiency in very cold climates. They require trenches or a deep borehole, making installation more complex and expensive than air-source systems.
A water-source heat pump exchanges heat with a pond, lake, or well. Like ground-source systems, water temperature is stable, but water-source pumps are less common because they require a suitable water body nearby. All three types operate on the same refrigerant cycle; the only difference is what medium the outdoor heat exchanger exchanges heat with.
Why Heat Pumps Use Less Energy Than Furnaces
A furnace burns fuel (gas or oil) to create heat, converting chemical energy into thermal energy. A heat pump moves existing heat using electricity to run the compressor. Because moving heat requires far less energy than creating it, a heat pump can deliver three to four units of heat for every unit of electricity it consumes. A gas furnace, by contrast, converts only about 95 percent of fuel energy into usable heat—the rest escapes up the chimney.
This efficiency advantage is why heat pumps lower heating bills in most climates, even though electricity costs more per unit than natural gas. The compressor is the only moving part that uses significant power; the refrigerant cycle itself requires no fuel. However, heat pump efficiency does decline in very cold weather (below 0°F) because the outdoor air contains less extractable heat, and the compressor must work harder to concentrate it.
Frequently Asked Questions
How does a heat pump work when it's freezing outside?
Even at 0°F, outdoor air contains thermal energy that a heat pump can extract. The compressor pressurizes the refrigerant to a much higher temperature than the outdoor air, creating a temperature difference large enough for heat to flow into the refrigerant. As outdoor temperature drops, the compressor must work harder and longer, reducing efficiency. Most cold-climate heat pumps include a backup electric heater that activates below 32°F to maintain comfort and reduce compressor strain.
What's the difference between the indoor and outdoor units?
The outdoor unit contains the compressor, one heat exchanger, a fan, and the reversing valve. The indoor unit contains the other heat exchanger and a blower fan that circulates air through your home. Refrigerant lines connect them, allowing the cycle to continue. In heating mode, the outdoor unit absorbs heat; in cooling mode, the indoor unit absorbs heat. They're identical in function but positioned differently.
Can a heat pump heat and cool at the same time?
No. The reversing valve switches the entire system between heating and cooling modes, but not simultaneously. However, some multi-zone systems can heat one room and cool another at the same time by using separate indoor units connected to the same outdoor compressor. Each indoor unit has its own thermostat and can operate independently.
Why does my heat pump sometimes make a hissing or gurgling sound?
These sounds are normal and come from refrigerant flowing through the copper lines and expansion device. Hissing occurs when high-pressure refrigerant passes through the metering valve. Gurgling happens when liquid refrigerant moves through the lines. If sounds are loud or accompanied by reduced heating or cooling, the system may have a refrigerant leak and needs service.
Do heat pumps work in humid climates?
Yes. Heat pumps work in any climate, but in humid regions the indoor unit removes moisture from air as it cools, which is actually beneficial. During heating season in humid climates, the indoor unit may add some moisture back to the air. Humidity control is a side effect of the cooling cycle, not a primary function, but it generally improves comfort.