A heat pump reverses its cycle to pull heat out of your home
A heat pump cools by doing the opposite of what it does in winter. Instead of pulling heat from outside air and moving it indoors, the system reverses direction and pulls heat from inside your house and pushes it outdoors. The refrigerant that circulates through the pipes changes direction, the indoor coil becomes cold instead of warm, and a fan blows your home's air across that cold coil. The heat transfers to the refrigerant, which carries it outside where another coil releases it into the outdoor air.
This reversal happens automatically when you switch your thermostat from heating to cooling mode. A component called a reversing valve flips the flow of refrigerant through the system. You do not need to do anything manually—just set the mode and the temperature you want, and the heat pump handles the rest.
Key Takeaways
- A heat pump cools by reversing its refrigerant flow so the indoor coil becomes cold and pulls heat out of your house instead of adding it.
- The reversing valve is the part that switches the system between heating and cooling modes automatically.
- Heat pumps cool more efficiently than traditional air conditioners because they move heat rather than generating cold air from scratch.
- The outdoor unit becomes hot during cooling because it is releasing all the heat pulled from inside your home.
- A heat pump cools the same way a refrigerator works—by circulating refrigerant through coils to transfer heat from one place to another.
The refrigerant cycle that makes cooling happen
Refrigerant is a chemical that boils and condenses at low temperatures. As it circulates through your heat pump, it changes between liquid and gas states. In cooling mode, the refrigerant enters the indoor coil as a cold liquid. Your home's warm air passes over this coil, and heat transfers from the air into the refrigerant, turning it into a gas. A compressor then pressurizes this gas and sends it to the outdoor coil.
In the outdoor coil, the pressurized refrigerant gas releases the heat it absorbed from inside. As it cools down, it condenses back into a liquid. A fan blows outdoor air across the outdoor coil to help carry away that heat. The liquid refrigerant then flows back indoors through an expansion device that lowers its pressure, making it cold again so it can absorb more heat from your home. This cycle repeats continuously until your thermostat senses the indoor temperature has dropped to your set point.
The same refrigerant does all the work—it just changes direction and state. This is why heat pumps are more efficient than air conditioners that generate cold air by compressing refrigerant and venting the heat outdoors without recovering any of it.
Why the outdoor unit gets hot during cooling
The outdoor unit of a heat pump becomes noticeably hot when the system is in cooling mode. This is normal and expected. The outdoor coil is releasing all the heat that was pulled from inside your house, so it radiates warmth into the outdoor air. The fan on the outdoor unit runs to help move that heat away faster.
If you stand near the outdoor unit on a hot day while cooling is running, you will feel warm air blowing out. That air contains the heat from your home. The hotter it is outside, the harder the system has to work to push that heat into the outdoor air, which is why cooling costs more on very hot days than on mild ones.
The reversing valve that switches between heating and cooling
The reversing valve is a small but critical part that allows one heat pump to both heat and cool. It is a solenoid-operated valve that changes the direction refrigerant flows through the system. When you set your thermostat to cooling mode, an electrical signal tells the reversing valve to flip. Refrigerant that was flowing one direction now flows the opposite way, which swaps which coil is hot and which is cold.
This happens automatically and takes only a few seconds. You do not control it directly—the thermostat and the heat pump's control board manage it. Some heat pump systems have a manual mode selector on the thermostat so you can choose heating, cooling, or auto mode, where the system switches automatically based on temperature. In auto mode, the reversing valve flips whenever the indoor temperature drifts too far from your set point.
How a heat pump compares to a traditional air conditioner
A traditional air conditioner only cools. It compresses refrigerant, runs it through an indoor coil to cool your home, and vents the heat outdoors through the outdoor unit. The system generates cold air but wastes the energy used to create that cold.
A heat pump does the same cooling job but can also reverse to provide heat in winter. Because it moves heat instead of generating it, it uses less energy to cool than a traditional air conditioner of the same capacity. In mild climates, a heat pump can heat your home in winter using less electricity than a resistance heater or furnace would use. In very cold climates, a heat pump's cooling efficiency advantage remains, but it may need a backup heater for the coldest days.
Both systems use the same refrigerant and similar components. The main difference is that a heat pump has a reversing valve and can operate in both directions, while an air conditioner only moves heat one way.
What happens inside the indoor coil during cooling
The indoor coil is where the actual heat transfer happens. It is a network of thin metal tubes filled with cold refrigerant, surrounded by aluminum fins that increase the surface area. Your home's air is pulled through the coil by a blower fan. As warm air passes over the cold fins, heat transfers from the air into the refrigerant. The air cools down and returns to your home through the ductwork or vents.
Moisture in the air also condenses on the cold coil, just like water beads on a cold glass. This condensation drips into a pan beneath the coil and drains outside or into a condensate line. This is why you may see water dripping from the indoor unit or near your outdoor unit during cooling—that is normal operation. If the drain clogs, water can back up and damage the system or create mold, so keeping the drain clear is part of regular maintenance.
Common cooling problems and what causes them
If your heat pump is not cooling well, the most common causes are a clogged air filter, a blocked outdoor coil, or low refrigerant. A dirty air filter restricts airflow over the indoor coil, so less heat gets absorbed. Check and replace the filter every one to three months during cooling season. If the outdoor coil is covered in dirt, leaves, or grass clippings, the system cannot release heat efficiently. Hose it off gently or have a technician clean it.
Low refrigerant means the system cannot absorb and move as much heat. This is usually caused by a leak somewhere in the refrigerant lines. You cannot refill refrigerant yourself—a licensed technician must find and repair the leak, then recharge the system. If your heat pump cools but not as well as it used to, or if it runs constantly without reaching your set temperature, have it inspected for leaks.
Another issue is short cycling, where the system turns on and off frequently without running long enough to cool the house. This can be caused by a dirty filter, a blocked coil, low refrigerant, or a faulty thermostat. If your heat pump cycles on and off every few minutes, turn it off and call a technician rather than running it repeatedly.
Frequently Asked Questions
Does a heat pump cool as well as a regular air conditioner?
Yes. A heat pump cools using the same refrigerant cycle as an air conditioner, so cooling performance is equivalent. The difference is that a heat pump can also heat, making it a two-in-one system. In cooling mode, you will not notice any difference in comfort or speed compared to a traditional air conditioner of the same size.
Why does my heat pump make noise when cooling?
Heat pumps make several normal sounds during cooling: a hum from the compressor, a whoosh from the outdoor fan, and a gurgling sound from refrigerant flowing through the lines. A loud bang or hissing usually means something is wrong—a loose part, a refrigerant leak, or a compressor issue. If the noise is new or unusually loud, have a technician inspect it.
Can I run my heat pump in cooling mode all summer without switching to heating?
Yes. Once you set the thermostat to cooling mode, you can leave it there for the entire cooling season. The system will run whenever the indoor temperature rises above your set point and stop when it reaches that temperature. You do not need to switch modes manually unless the weather changes and you need heating again.
What temperature should I set my heat pump to for cooling?
Most people set cooling to 72 to 78 degrees Fahrenheit depending on comfort preference and energy costs. Each degree lower uses more electricity, so setting it to 78 instead of 72 can reduce cooling costs noticeably. Using a programmable or smart thermostat to raise the temperature when you are away or asleep also saves energy without sacrificing comfort when you are home.
Is it normal for the outdoor unit to stay on after I turn off the cooling?
Yes. After you switch the thermostat off or raise the set temperature, the outdoor unit may run for a minute or two longer to clear refrigerant from the lines and equalize pressure. This is called the pump-down cycle and protects the system. If the outdoor unit runs for more than five minutes after cooling stops, or if it cycles on and off repeatedly, contact a technician.