A heat pump AC pulls heat out of your indoor air and moves it outside
A heat pump air conditioner works by circulating refrigerant through a closed loop of pipes and coils. The indoor coil absorbs heat from the air in your home, cooling the rooms. That heat is then pumped outside through the outdoor coil, where it is released into the air. A compressor powered by electricity drives the whole cycle by pressurizing the refrigerant, which makes it hot enough to shed heat outdoors and cold enough to absorb heat indoors.
The key difference from a traditional air conditioner is that a heat pump can reverse this cycle. In heating mode, it pulls heat from outside air (even in cold weather) and moves it inside. In cooling mode, it works exactly like a standard AC unit. Most heat pumps sold today are air-source heat pumps, meaning they exchange heat with the outdoor air rather than the ground.
The process is continuous. As long as the system runs, refrigerant circulates, heat moves from inside to outside, and your indoor temperature drops. When your thermostat reaches the set temperature, the compressor shuts off and the cycle pauses until the house warms up again.
Key Takeaways
- Heat pump AC uses refrigerant in a closed loop to absorb heat from indoor air and release it outside, powered by an electric compressor.
- The indoor coil gets cold and pulls heat from your home; the outdoor coil gets hot and releases that heat to the outside air.
- A reversing valve lets the same system heat your home in winter by flipping the direction refrigerant flows.
- Heat pumps move existing heat rather than generating it, which is why they use less electricity than traditional AC or electric resistance heating.
The refrigerant cycle: how heat actually moves
Refrigerant is a chemical that boils and condenses at temperatures useful for heating and cooling. When it boils, it absorbs heat; when it condenses, it releases heat. A heat pump exploits this property by forcing the refrigerant to change state over and over.
The cycle starts at the indoor coil (called the evaporator in cooling mode). Refrigerant enters as a cold, low-pressure liquid. Indoor air blows across the coil, and the refrigerant absorbs that heat and boils into a gas. Your home gets cooler; the refrigerant gets warmer and becomes a vapor.
That warm vapor travels to the compressor, which squeezes it under high pressure. Compression heats the refrigerant further—this is where electricity is used. The hot, pressurized gas then flows to the outdoor coil (the condenser). Outside air blows across this coil, and the hot refrigerant releases its heat to the outdoor air and condenses back into a liquid.
The liquid refrigerant then passes through an expansion device (usually a metering valve or capillary tube) that reduces its pressure. This pressure drop cools the refrigerant dramatically, and it re-enters the indoor coil to start the cycle again. The whole loop repeats as long as cooling is needed.
Why the outdoor unit gets hot and the indoor unit gets cold
The temperature difference between the two coils is not accidental—it is the result of pressure. High-pressure refrigerant is hot; low-pressure refrigerant is cold. The compressor creates this pressure difference, and that difference drives the heat transfer.
When you stand in front of the outdoor unit on a hot day, you feel hot air blowing out. That is the heat your home rejected, plus the heat generated by the compressor itself. The indoor unit blows cool air because the refrigerant inside it is cold and under low pressure. The larger the pressure difference the compressor creates, the colder the indoor coil and the hotter the outdoor coil.
This is also why heat pumps struggle in very cold weather. When outdoor air is already cold, the outdoor coil cannot absorb enough heat to warm your home efficiently. Most heat pump systems include electric resistance heating (like a space heater) as a backup for temperatures below freezing.
How the reversing valve switches between heating and cooling
A heat pump's ability to heat and cool comes from a reversing valve, a four-way valve that changes the direction refrigerant flows through the system. In cooling mode, refrigerant flows one direction. In heating mode, the valve switches, and refrigerant flows the opposite way.
When the valve reverses, the outdoor coil becomes the evaporator (the cold one that absorbs heat from outside air), and the indoor coil becomes the condenser (the hot one that releases heat into your home). The compressor still runs and still pressurizes the refrigerant—only the direction changes. This is why the same unit can heat and cool without any additional equipment.
The reversing valve is controlled by your thermostat. When you set the system to heating mode and indoor temperature drops below your set point, the thermostat signals the valve to reverse. When you switch to cooling mode, it signals the valve to reverse again.
The compressor: the engine that makes it all work
The compressor is the most energy-intensive part of a heat pump. It is an electric motor that compresses refrigerant gas, raising its pressure and temperature. Without the compressor, refrigerant would not circulate, and no heat transfer would occur.
Most modern heat pumps use a variable-speed compressor that adjusts its output based on how much cooling or heating is needed. On a mild day, the compressor runs slowly and uses less electricity. On a very hot or cold day, it runs faster and uses more power. This modulation makes heat pumps more efficient than older fixed-speed systems, which ran at full power or not at all.
The compressor also generates heat as a byproduct of compression. In cooling mode, this waste heat is released outdoors along with the heat pulled from your home. In heating mode, this waste heat is actually useful—it contributes to warming your home.
Why heat pumps are more efficient than traditional AC
A traditional air conditioner and a heat pump cooling system work almost identically. The real efficiency difference shows up in heating. A heat pump moves heat from outside to inside; a traditional electric heater generates heat by running electricity through a resistance wire, like a toaster. Moving heat uses far less electricity than generating it.
Heat pumps are rated by their Seasonal Energy Efficiency Ratio (SEER) for cooling and Heating Seasonal Performance Factor (HSPF) for heating. A SEER of 16 or higher and an HSPF of 8 or higher are considered efficient for most climates. The higher the number, the more cooling or heating you get per dollar of electricity.
However, efficiency drops in very cold climates. Below about 35 degrees Fahrenheit, the outdoor coil cannot absorb enough heat, and the system switches to electric resistance backup heating, which is less efficient. This is why heat pumps work best in mild to moderate climates and why they are paired with backup heating in cold regions.
Common issues that affect how well your heat pump cools
A heat pump that does not cool properly usually has one of a few problems. Low refrigerant charge is the most common—a leak in the line set or coils means less refrigerant circulates, and cooling capacity drops. A technician must locate the leak, repair it, and recharge the system with the correct amount of refrigerant.
A dirty outdoor coil also reduces cooling. Leaves, dust, and pollen block airflow across the coil, so the refrigerant cannot release heat efficiently. Cleaning the coil with a garden hose or calling a technician usually restores performance. A clogged indoor filter has the same effect on the indoor side—air cannot flow across the indoor coil, and cooling suffers.
A failing compressor is more serious and more expensive. If the compressor does not pressurize refrigerant properly, the temperature difference between the coils shrinks, and cooling weakens. Compressor failure usually means replacing the entire outdoor unit, which costs several thousand dollars.
Frequently Asked Questions
Does a heat pump AC work the same way as a window air conditioner?
Yes, the cooling cycle is identical. Both use a compressor to pressurize refrigerant, an indoor coil to absorb heat, and an outdoor coil to release it. The difference is that a heat pump can reverse the cycle to provide heating, while a window AC only cools. A window unit also exhausts all its heat outdoors through a single vent; a heat pump uses separate indoor and outdoor units connected by refrigerant lines.
Can a heat pump cool a house as fast as a traditional AC unit?
A properly sized heat pump cools just as fast as a traditional AC. Cooling speed depends on the compressor power and the temperature difference between indoor and outdoor air, not on whether the system can also heat. An undersized heat pump will cool slowly, but that is a sizing problem, not a technology problem.
Why does my heat pump outdoor unit stay on even when the house is cool enough?
The compressor may be running at low speed to maintain temperature, or the system may be in defrost mode. In cold weather, frost builds up on the outdoor coil, blocking airflow. The system reverses to heating mode briefly to melt the frost, which uses energy but keeps the coil clear. This is normal and necessary in winter.
What happens to the heat that comes out of the outdoor unit?
That heat is released into the outdoor air. In cooling mode, it is the heat your home rejected plus the heat generated by compressing the refrigerant. In heating mode, it is the heat absorbed from outside air plus compressor waste heat, all being moved into your home through the indoor unit.
Can a heat pump cool if it is below freezing outside?
Yes, but not efficiently. The outdoor coil can still absorb heat from air below freezing, but the temperature difference is small, so the compressor must work harder and use more electricity. Most systems switch to electric resistance backup heating below 35 degrees Fahrenheit because it becomes more cost-effective than running the compressor at maximum power.