A heat pump reverses its cycle to pull heat out of your house
In summer, your heat pump works backward from how it heats in winter. Instead of pulling warmth from outside air and moving it indoors, it pulls warmth from inside your home and pushes it outdoors. The same refrigerant that circulates through the system in winter still circulates in summer — the difference is the direction it flows and which coil does the pulling.
The indoor coil (called the evaporator in cooling mode) gets cold and absorbs heat from the air inside your house. A fan blows your home's air across this cold coil, and the heat transfers into the refrigerant. That warm refrigerant then travels outside to the outdoor unit, where a compressor pressurizes it and makes it even hotter. The outdoor coil (now called the condenser) releases all that heat to the outside air, and a fan helps move that warm air away from your home.
The cooled refrigerant then flows back indoors to start the cycle again. This continuous loop keeps running as long as your thermostat senses your home is warmer than your set temperature.
Key Takeaways
- A heat pump cools by reversing its winter operation — the indoor coil absorbs heat from your home instead of releasing it.
- The same refrigerant loop that heats in winter cools in summer by flowing in the opposite direction through the system.
- The outdoor unit releases the heat your home pulls out, so it will feel warm to the touch during summer operation.
- Heat pumps cool more efficiently than traditional air conditioners because they move heat rather than generating cold air from scratch.
- Summer cooling performance depends on outdoor temperature, indoor humidity, and how well your home is insulated and sealed.
The reversing valve switches the refrigerant direction
The component that makes summer cooling possible is called a reversing valve. This four-way valve sits in the refrigerant line and changes which coil receives cold refrigerant and which one releases heat. In winter, it directs refrigerant one way; in summer, your thermostat signals it to flip, and refrigerant flows the opposite direction.
When you switch your thermostat from heating to cooling mode, you are not just changing a setting — you are telling the reversing valve to redirect the entire refrigerant loop. This happens automatically in most modern heat pumps. Some older or simpler models require you to manually switch the mode, but the valve itself still does the physical work of reversing flow.
If the reversing valve gets stuck or fails, your heat pump will not cool properly in summer, even though the compressor and fans are running. This is one of the most common reasons a heat pump stops cooling while still heating fine in winter.
The compressor pressurizes refrigerant to move heat outdoors
The compressor is the engine of your heat pump. In summer cooling mode, it takes the warm refrigerant coming from your indoor coil and squeezes it under high pressure. This compression heats the refrigerant even more — similar to how a bicycle pump gets warm when you push air into a tire.
This highly pressurized, very hot refrigerant then flows to the outdoor coil, where it is much hotter than the outside air. Heat naturally flows from hot to cold, so the refrigerant sheds its heat to the outdoor air through the condenser coil. A fan blows across the outdoor coil to carry that heat away from your home.
The compressor runs continuously as long as your thermostat calls for cooling. If outdoor temperatures are very high (above 95°F), the compressor works harder to move heat against a larger temperature difference, which uses more electricity and can reduce cooling efficiency.
Why the outdoor unit feels hot in summer
During summer cooling, the outdoor unit of your heat pump will be noticeably warm or even hot to the touch. This is normal and expected — it is literally releasing all the heat your home pulled out of the air. The hotter your home's interior and the hotter the outdoor air, the more heat the outdoor unit has to reject, and the warmer it will feel.
You should never block the outdoor unit with plants, furniture, or debris, because it needs to release heat efficiently. Restricted airflow around the outdoor coil forces the compressor to work harder and raises your cooling costs. Keep at least two feet of clear space around all sides of the outdoor unit during the cooling season.
If the outdoor unit is not warm at all during summer operation, the system may not be cooling. Check that your thermostat is set to cooling mode and that the temperature setting is lower than your home's current temperature. If the unit still feels cold, contact a technician — the reversing valve or compressor may not be functioning.
Summer cooling efficiency depends on temperature difference
Heat pumps cool most efficiently when the outdoor temperature is moderate — roughly 60°F to 80°F. In this range, the temperature difference between indoors and outdoors is small enough that the compressor does not have to work extremely hard to move heat outside.
On very hot days (above 90°F), the outdoor air is already warm, so the refrigerant has a harder time shedding heat to the outside. The compressor must run longer and work harder to achieve the same cooling, which increases electricity use. On extremely hot days, some heat pumps may struggle to cool your home below your set temperature, especially if your home is poorly insulated or if you have large windows facing the sun.
Humidity also affects cooling performance. When indoor air is humid, the evaporator coil must remove moisture as well as heat, which requires more refrigerant circulation. This is why heat pumps often run longer on humid days than on dry days at the same temperature.
How heat pump cooling compares to traditional air conditioning
A traditional air conditioner cools by expanding refrigerant through an expansion device, which causes it to become very cold. The cold refrigerant absorbs heat from indoor air, and a compressor pumps that heat outdoors. A heat pump does the same thing in summer — the cooling cycle is physically identical to a standard air conditioner.
The difference is that a heat pump can reverse this cycle in winter to provide heating as well. This reversibility makes heat pumps more versatile, but it also means they have one additional component (the reversing valve) that can fail. In pure cooling performance on a hot day, a heat pump and a traditional air conditioner of the same size and age will cool your home at roughly the same rate.
Heat pumps are generally more efficient than air conditioners over a full year because they provide both heating and cooling from one system, reducing the need for a separate furnace. However, in summer alone, the efficiency advantage is smaller — it comes mainly from the fact that you are not running a separate heating system.
Maintenance that keeps summer cooling working
To maintain cooling performance through the summer, clean or replace your indoor air filter every one to three months. A clogged filter restricts airflow across the evaporator coil, which reduces cooling and forces the compressor to work longer. Check the filter monthly during heavy cooling season.
Keep the outdoor unit clear of leaves, grass clippings, and debris. Hose off the outdoor coil gently with a garden hose if it looks dirty — do not use a pressure washer, which can bend the fins. A clean outdoor coil releases heat more efficiently, which lowers your cooling costs and reduces strain on the compressor.
Have a technician inspect your heat pump before summer cooling season begins. They will check refrigerant charge, test the reversing valve, clean coils, and verify that the compressor is running at the correct pressure. This annual maintenance typically costs $100 to $200 and can prevent expensive repairs later in the season.
Frequently Asked Questions
Why does my heat pump cool slower on very hot days?
On days above 90°F, the outdoor air is already warm, so the refrigerant cannot shed heat to the outside as easily. The compressor must work longer to move the same amount of heat, which slows cooling. This is normal — all cooling systems slow down in extreme heat. If your home does not reach your set temperature even after running for hours, the system may be low on refrigerant or the compressor may be failing.
Can I run my heat pump in both heating and cooling mode at the same time?
No. Your thermostat switches the system to either heating or cooling mode, not both. The reversing valve cannot flow refrigerant in two directions simultaneously. If you need heating in one room and cooling in another, you would need a zoned system with separate thermostats, which is a major installation and not standard in most homes.
What does the outdoor unit do during summer cooling?
The outdoor unit releases the heat your home pulled from the indoor air. The warm refrigerant from inside flows to the outdoor coil, where it sheds heat to the outside air. A fan helps move that warm air away. This is why the outdoor unit feels hot — it is working correctly.
Does a heat pump cool as well as a window air conditioner?
A properly sized heat pump cools a whole home more evenly than a single window unit, but a window unit may cool a single room faster because it is smaller and more focused. For whole-home cooling, a heat pump is more efficient and convenient than multiple window units, though it costs more to install.
What temperature should I set my thermostat to in summer?
Most people set their thermostat between 72°F and 78°F in summer. Each degree lower increases cooling costs by roughly 3 percent. Setting it to 78°F when you are away and 72°F when you are home can reduce summer electricity use without sacrificing comfort.