A heat pump reverses itself 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 heat from inside your home and pushes it outdoors. The same refrigerant loop that warms you in January cools you in July—the system just reverses direction.
This reversal happens through a component called the reversing valve, a four-way switch that changes which coil acts as the evaporator (the cold part) and which acts as the condenser (the hot part). When you set your thermostat to cooling mode, the reversing valve flips, and the indoor coil becomes cold instead of warm. Warm air from your rooms blows across this cold coil, loses its heat, and returns to you as cooler air.
The heat that was pulled from your home gets compressed and sent to the outdoor unit, where it is released into the outside air. This is why the outdoor unit of a heat pump feels hot to the touch on a summer day—it is literally dumping your home's heat outside.
Key Takeaways
- A heat pump cools by reversing its refrigerant flow, making the indoor coil cold and the outdoor coil hot.
- The reversing valve is the component that switches the system between heating and cooling modes.
- Summer cooling uses the same refrigerant loop as winter heating, just running in the opposite direction.
- Heat pumps are more efficient at cooling than traditional air conditioners because they move heat rather than generate cold.
- The outdoor unit will feel hot in summer because it is actively releasing heat pulled from your home.
The refrigerant cycle that makes summer cooling work
The cooling cycle starts when warm indoor air passes over the cold indoor coil (now the evaporator). Refrigerant inside that coil is at a low pressure and low temperature, so it absorbs heat from the air. As the refrigerant absorbs heat, it evaporates into a gas.
This warm gas refrigerant then travels to the outdoor unit, where a compressor squeezes it. Compression raises both the pressure and temperature of the gas—this is the energy-intensive step that requires electricity. The now-hot, high-pressure gas flows to the outdoor coil (now the condenser), where it releases the heat it picked up from your home into the outside air. As it releases heat, the refrigerant condenses back into a liquid.
The liquid refrigerant then passes through an expansion device that lowers its pressure again, and the cycle repeats. This continuous loop—evaporate indoors, compress, condense outdoors, expand, repeat—is what keeps your home cool all summer.
Why the outdoor unit gets so hot
The outdoor unit of a heat pump running in cooling mode will be noticeably warm or hot, even on a mild day. This is normal and expected. The unit is working hard to reject the heat your home is generating, plus the extra heat created by the compressor itself.
On very hot days, the outdoor unit has to work harder because the temperature difference between the refrigerant and the outside air is smaller. When it is 95 degrees outside, the refrigerant cannot dump heat as efficiently as it can on a 75-degree day. This is why heat pumps lose cooling capacity in extreme heat, and why some homes in very hot climates may need a backup cooling system.
Keep the area around your outdoor unit clear of leaves, grass clippings, and debris. Restricted airflow makes the unit work harder and reduces cooling efficiency. A straightforward annual cleaning of the outdoor coil (done by a professional) can restore lost cooling capacity.
How the thermostat tells the heat pump to switch modes
Your thermostat controls whether the heat pump heats or cools by sending a signal to the reversing valve. When you set the thermostat to cooling mode and the indoor temperature rises above your set point, the thermostat sends a signal that energizes the reversing valve. The valve flips, and the system begins pulling heat from indoors.
Most modern thermostats allow you to set a heating setpoint and a separate cooling setpoint. For example, you might set the system to heat if the temperature drops below 68 degrees and cool if it rises above 74 degrees. During spring and fall, when temperatures swing, the system can switch between modes automatically without your input.
Some older or manual thermostats require you to switch the mode yourself—you have to physically change it from "heat" to "cool" when the season changes. If you forget and leave it in heating mode during summer, the system will not cool your home, even if the outdoor unit is running.
Summer efficiency and why heat pumps cool differently than air conditioners
A heat pump cools more efficiently than a traditional air conditioner because it moves heat rather than generating cold. An air conditioner uses refrigerant to create cold at the indoor coil, then discards the heat outdoors. A heat pump does the same thing, but because it uses the same equipment for both heating and cooling, it avoids the cost of installing and maintaining two separate systems.
The efficiency of a heat pump in cooling mode is measured by its SEER2 rating (Seasonal Energy Efficiency Ratio). A higher SEER2 means the system uses less electricity to move the same amount of heat. Modern heat pumps typically have SEER2 ratings between 15 and 22, depending on the model and size. Older units may be as low as 10 or 12.
Your actual cooling costs depend on how hot your summers are, how much you run the system, and how well your home is insulated. A well-sealed, well-insulated home needs less cooling than a drafty one, even with the same outdoor temperature.
When to call a professional for summer cooling problems
If your heat pump is running but not cooling, or cooling much less than it used to, several things could be wrong. The most common are a refrigerant leak, a clogged filter, a dirty outdoor coil, or a failed reversing valve. All of these require a licensed HVAC technician to diagnose and repair.
Do not attempt to add refrigerant yourself. Refrigerant is pressurized, toxic, and regulated by law. Only a certified technician can handle it. If you suspect a leak—you hear hissing from the outdoor unit or see oily residue—turn off the system and call a professional.
A straightforward maintenance step you can do yourself is checking and replacing your indoor air filter every one to three months during cooling season. A clogged filter reduces airflow, makes the system work harder, and can damage the compressor over time. If the filter is visibly dirty, replace it.
How to get the most cooling from your heat pump
Set your thermostat to a temperature you can live with and leave it there. Constantly adjusting the temperature does not cool your home faster—it just makes the system cycle on and off more often, which wastes energy. A setting of 78 degrees uses significantly less electricity than 72 degrees.
Close blinds and curtains during the day to block direct sunlight, especially on windows that face south or west. This reduces the heat load your heat pump has to remove. Use ceiling fans to circulate cool air—fans use very little electricity and help distribute cooled air more evenly through your home.
Avoid running heat-generating appliances like the oven, dishwasher, or clothes dryer during the hottest part of the day. If you must use them, run them in the early morning or evening. Seal air leaks around windows, doors, and ductwork so the cool air you are paying for does not escape.
Frequently Asked Questions
Why is my heat pump outdoor unit so loud in summer?
The outdoor fan runs faster in summer to move more air across the condenser coil and reject heat more quickly. A louder fan means the system is working hard, which is normal on hot days. If the noise is a grinding or squealing sound rather than a steady hum, call a technician—that can indicate a bearing problem or refrigerant issue.
Can I use my heat pump to cool if it is over 100 degrees outside?
Yes, but the system will lose cooling capacity as outdoor temperature rises. Most heat pumps are rated for cooling down to about 95 to 100 degrees. Above that, the refrigerant cannot reject heat efficiently enough to keep up with demand. If you live in a very hot climate, ask your installer whether your system needs a backup air conditioner or a larger capacity unit.
Should I turn off my heat pump at night in summer to save money?
Turning it off at night saves electricity, but your home will warm up, and the system will have to work harder the next morning to cool it back down. You will likely use more total energy than if you had kept it running at a slightly higher temperature all night. A better approach is to raise the setpoint by a few degrees at night rather than turning it off completely.
What does it mean if my heat pump is running but the indoor coil is not cold?
The most common causes are a clogged air filter, a dirty outdoor coil, or low refrigerant from a leak. Less often, the reversing valve has failed and the system is stuck in heating mode even though you set it to cool. All of these require a technician to diagnose and fix.
Is it normal for my heat pump to frost up in summer?
No. Frost on the indoor coil in summer usually means the coil temperature has dropped below freezing, which happens when refrigerant flow is restricted or the system is malfunctioning. Turn off the system and call a technician. Running a frosted heat pump can damage the compressor.