A heat pump AC unit moves heat from inside your home to outside, rather than generating cold air
A heat pump air conditioner works by circulating refrigerant through a closed loop of pipes and coils. The refrigerant absorbs heat from the air inside your home, carries it outside, and releases it there. A compressor pumps the refrigerant to keep it moving, and fans blow air across the coils to speed up the heat transfer. The result feels like cooling because the indoor air loses its heat, even though the unit is not making cold—it is moving warmth out.
This is different from a traditional air conditioner, which uses energy to create a temperature difference. A heat pump uses less energy because it is moving heat that already exists rather than fighting against the laws of physics to manufacture it. In summer, it pulls heat out of your home. In winter, many heat pumps reverse direction and pull heat from outside air (even cold air contains some heat) to warm your home.
Key Takeaways
- A heat pump AC moves heat from inside to outside using refrigerant that cycles through indoor and outdoor coils.
- The compressor is the engine of the system—it pressurizes the refrigerant so heat can flow in the direction you need.
- Indoor and outdoor fans blow air across the coils to transfer heat faster than would happen on their own.
- Heat pumps use less electricity than traditional air conditioners because they move existing heat instead of creating temperature from scratch.
- The expansion valve controls refrigerant flow and pressure, allowing the system to switch between cooling and heating modes.
The refrigerant loop: how heat actually moves
The refrigerant is the working fluid that does all the heat moving. It is a chemical compound (often HFC-410A or a newer low-GWP refrigerant) that boils and condenses at temperatures useful for air conditioning. As it circulates, it changes state—from liquid to gas and back—and each state change involves absorbing or releasing heat.
Inside your home, the refrigerant flows through the evaporator coil, a set of metal tubes with fins. The refrigerant is cold and at low pressure here. Indoor air is blown across the coil by a fan. Heat from that air transfers into the refrigerant, warming it until it boils into a gas. The air loses heat and feels cooler. The gaseous refrigerant then travels outside through copper tubing.
Outside, the refrigerant enters the condenser coil, another set of finned tubes. Here the refrigerant is hot and at high pressure (the compressor made it that way). An outdoor fan blows outside air across the condenser coil. The refrigerant cools down and condenses back into a liquid, releasing the heat it picked up from inside. You may notice warm air blowing out of the outdoor unit—that is the heat your home rejected.
The compressor: the pump that makes it all work
The compressor is an electric pump that pressurizes the refrigerant gas coming from the evaporator coil. Pressurizing a gas heats it, so the compressor raises the refrigerant temperature high enough that it can release heat to the outside air even on a warm day. Without the compressor, the refrigerant would not circulate, and no heat would move.
The compressor runs on electricity and is the single largest energy consumer in the system. It cycles on and off based on a thermostat signal. When your home reaches the temperature you set, the compressor shuts down. When the temperature drifts above the setpoint, the compressor starts again. Modern heat pumps often use variable-speed compressors that run at different speeds rather than straightforward on or off, which saves energy by matching the cooling demand more precisely.
The expansion valve: controlling pressure and flow
Between the outdoor condenser coil and the indoor evaporator coil sits the expansion valve (also called a metering device or thermostatic expansion valve). This valve is a narrow opening that restricts refrigerant flow. As the liquid refrigerant is forced through this small opening, its pressure drops suddenly. When pressure drops, temperature drops too, so the refrigerant becomes very cold—cold enough to absorb heat from indoor air.
The expansion valve also acts as a control point. It senses the temperature of the refrigerant leaving the evaporator coil and adjusts how much refrigerant flows through to keep the system running efficiently. If the evaporator coil gets too cold, the valve closes slightly to reduce flow. If it is not cold enough, the valve opens to let more refrigerant through. This automatic adjustment keeps the system balanced.
Indoor and outdoor fans: speeding up heat transfer
Heat transfer happens naturally—warm air always moves toward cool air—but it is slow. The fans in a heat pump speed up this process dramatically. The indoor fan (called the blower) pulls air from your home through a filter and across the evaporator coil, then pushes that cooled air back into your home through ducts or vents. Without this fan, heat would transfer to the refrigerant very slowly.
The outdoor fan pulls outside air across the condenser coil and pushes it away from the unit. On a hot day, the outdoor fan works hard because the temperature difference between the hot refrigerant and the outside air is small. On a cool day, the fan does not need to work as hard. Some heat pump systems modulate fan speed—running the fan faster when needed and slower when the system is not under stress—to save energy.
How the system switches between cooling and heating
In cooling mode (summer), the system works as described above: the evaporator coil is inside, the condenser coil is outside, and heat flows out of your home. To switch to heating mode (winter), a reversing valve redirects the refrigerant flow. The outdoor coil becomes the evaporator (pulling heat from outside air), and the indoor coil becomes the condenser (releasing that heat into your home).
The compressor still pressurizes the refrigerant the same way, but now the high-pressure hot refrigerant goes to the indoor coil instead of the outdoor one. The expansion valve still controls flow. The fans still blow air across the coils. Only the direction of refrigerant flow changes. This reversibility is what makes a heat pump useful year-round in mild climates, though in very cold regions many heat pumps need a backup electric heater for the coldest days.
Why heat pumps are more efficient than traditional air conditioners
A traditional air conditioner uses the compressor to create a temperature difference—it forces refrigerant to expand and contract in a way that produces cold. A heat pump does the same thing, but because it can reverse direction, it also heats. More importantly, moving heat that already exists requires less energy than creating temperature from scratch.
The efficiency of a heat pump is measured by its SEER2 rating (Seasonal Energy Efficiency Ratio) for cooling and HSPF2 rating (Heating Seasonal Performance Factor) for heating. A higher number means more cooling or heating output per unit of electricity consumed. Modern heat pumps typically have SEER2 ratings between 15 and 22, meaning they move 15 to 22 units of heat for every unit of electricity the compressor uses. This is why heat pumps lower electricity bills compared to older window units or traditional central air systems.
Common issues and what causes them
If your heat pump is not cooling well, the most common cause is a clogged air filter. The indoor filter restricts airflow, which reduces heat transfer at the evaporator coil. Check and replace the filter every one to three months during cooling season. Low refrigerant charge is another possibility—if the system has a leak, there is less refrigerant to carry heat, and cooling suffers. This requires a technician to find the leak, repair it, and recharge the system.
If the outdoor unit is very noisy, the fan may be hitting debris or the compressor may be failing. If the system cycles on and off very frequently (short cycling), the thermostat may be set too low, or the system may be oversized for your home. If you see ice on the outdoor coil in winter, the defrost cycle (which temporarily reverses the system to melt ice) may not be working. These issues all require professional service.
Frequently Asked Questions
Can a heat pump cool as well as a traditional air conditioner?
Yes. A heat pump uses the same refrigeration cycle as a traditional AC unit for cooling. The difference is that a heat pump can also reverse to provide heating. In cooling mode, performance is equivalent—the SEER2 rating is the same measurement used for both types of systems.
Why does my heat pump outdoor unit blow warm air?
That warm air is the heat your home rejected. The refrigerant absorbed heat from inside and released it at the outdoor condenser coil. The outdoor fan then blows that heat away. This is normal and expected—it is how the system works.
Do heat pumps work in cold climates?
Heat pumps work in cold climates, but efficiency drops as outdoor temperature falls because there is less heat to extract from cold air. Many cold-climate systems include a backup electric heater that turns on when outdoor temperature drops below about 20°F. Newer cold-climate heat pumps can operate efficiently down to 0°F or lower.
How often does the compressor run?
The compressor runs whenever your home temperature is above the thermostat setpoint. On a hot day, it may run most of the time. On a mild day, it cycles on and off every 15 to 30 minutes. The exact pattern depends on your setpoint, outdoor temperature, and how well your home is insulated.
What is the refrigerant, and is it dangerous?
Modern refrigerants are non-toxic and non-flammable, though older systems may use R-22, which is being phased out. Refrigerant should never be released into the air—it must be recovered by a certified technician. If you smell a sweet odor near your unit, you may have a leak and should call for service.