A heat pump is an air conditioner running in reverse
A heat pump works by moving heat from one place to another instead of creating heat from scratch. In winter, it pulls warmth from the outside air (even when it feels cold) and pushes that heat indoors. In summer, it reverses direction and pulls heat out of your house and releases it outside—exactly like an air conditioner. The same refrigerant fluid cycles through the system year-round; a valve straightforward switches which direction the heat flows.
This is why heat pumps are more efficient than furnaces or space heaters. A furnace burns fuel to create heat. A heat pump just moves heat that already exists, using far less energy. On a 35-degree day, there is still warmth in the outside air; the heat pump's job is to concentrate it and bring it indoors.
Key Takeaways
- A heat pump moves heat between indoors and outdoors using refrigerant fluid, rather than generating heat by burning fuel or running electric resistance coils.
- The outdoor unit absorbs heat from the air (or ground) and the indoor unit releases that heat into your home, with a compressor doing the work to move the heat uphill.
- In summer, the reversing valve flips the cycle so the heat pump pulls warmth out of your house and dumps it outside, working like an air conditioner.
- Heat pumps work down to about 25 to 35 degrees Fahrenheit; below that, most systems switch to electric resistance heating or a backup furnace to maintain comfort.
The outdoor unit absorbs heat from the air
The outdoor unit contains a coil filled with cold refrigerant. Outside air passes over this coil, and the refrigerant absorbs the warmth from that air—even air that feels chilly to you. The refrigerant boils and turns into a gas as it warms up. This is the key difference from an air conditioner: in an air conditioner, the outdoor coil is hot and rejects heat; in a heat pump running in heating mode, the outdoor coil is cold and absorbs heat.
A fan pulls air across the outdoor coil to speed up this heat transfer. The colder the outside air, the slower the heat transfer happens, which is why heat pumps become less efficient as outdoor temperature drops. At around 25 to 35 degrees Fahrenheit (depending on the model), the system usually switches to a backup heat source—either electric resistance coils inside the house or a furnace—because pulling enough heat from very cold air would require the compressor to work so hard that it becomes wasteful.
The compressor moves the heat indoors
Once the refrigerant has absorbed heat from outside and turned into a warm gas, it flows to the compressor. The compressor is an electric pump that squeezes the refrigerant, raising its temperature and pressure even higher. Think of it like pushing down on a bicycle pump: the air inside gets hotter as you compress it. The refrigerant leaving the compressor is now hot enough to warm your house.
This compressed, hot refrigerant flows to the indoor coil. A fan blows your home's air across this coil, and the heat transfers from the refrigerant into your living space. As the refrigerant cools down and releases its heat, it condenses back into a liquid. Then it flows through an expansion valve that lowers its pressure, cooling it further, and the cycle starts over.
The reversing valve switches the system for summer cooling
A four-way reversing valve is a small component that changes which coil is hot and which is cold. In winter heating mode, the outdoor coil is cold (absorbing heat) and the indoor coil is hot (releasing heat). When you switch the thermostat to cooling mode in summer, this valve flips the refrigerant flow. Now the indoor coil becomes cold and absorbs heat from your house, while the outdoor coil becomes hot and rejects that heat outside.
From the indoor unit's perspective, the system works exactly like a standard air conditioner in summer. The reversing valve is why a heat pump can heat and cool with the same equipment—no separate furnace needed, no separate air conditioner needed. One set of coils and one compressor handle both jobs.
How the thermostat controls when heating and cooling happen
Your thermostat tells the heat pump when to run and in which direction. In winter, you set it to heating mode and choose a target temperature—say, 70 degrees. When the indoor temperature drops below 70, the thermostat signals the heat pump to start. The compressor turns on, the outdoor fan starts, and warm refrigerant flows to the indoor coil. Once the house reaches 70 degrees, the compressor shuts off and waits.
If the outdoor temperature is very cold and the heat pump cannot keep up, the thermostat automatically switches on the backup heat (electric resistance or furnace) to reach your target temperature faster. This is called auxiliary heat or emergency heat. You may see "aux heat" light up on your thermostat display when this happens. In summer, you flip the thermostat to cooling mode, set your target temperature, and the reversing valve switches the system to pull heat out of the house instead.
Why the outdoor unit sometimes frosts over in winter
On cold, humid days, frost or ice can build up on the outdoor coil. This happens because the coil is so cold that moisture in the outside air freezes on it. A thick frost layer blocks airflow and reduces the heat pump's ability to absorb warmth, so efficiency drops. Most heat pumps have a defrost cycle that runs automatically: the system briefly reverses to heating mode, warming the outdoor coil and melting the frost. During defrost, you may notice warm air stopping at your indoor vents for a few minutes, or you may hear the outdoor fan shut off.
Defrost cycles are normal and happen several times per day in winter if conditions are right. They use a small amount of energy, but they keep the system working properly. If frost builds up and never melts, or if your heat pump stops heating altogether, that is a sign to call a technician.
Ground-source heat pumps pull heat from the earth instead of air
An air-source heat pump (the most common type) pulls heat from outside air. A ground-source heat pump (also called a geothermal heat pump) pulls heat from the ground or groundwater instead. The ground stays at a relatively constant temperature year-round—usually 45 to 55 degrees Fahrenheit depending on your location—so ground-source systems can extract heat even when the air is much colder.
Ground-source systems are more efficient than air-source systems, especially in very cold climates, because they do not have to work as hard to pull heat from a warmer source. However, they require digging trenches or drilling a well to bury the refrigerant loop underground, which is expensive and not practical for every property. Air-source heat pumps are far more common in homes because they cost less to install and work well in most climates.
Frequently Asked Questions
Can a heat pump heat a house when it is below freezing outside?
Yes, but with limits. Heat pumps work down to about 25 to 35 degrees Fahrenheit, depending on the model. Below that temperature, the outdoor coil becomes so cold that it cannot absorb enough heat efficiently, so the system switches to backup heat. Even at freezing temperatures, there is still warmth in the air; the heat pump just has to work harder to pull it out.
Why does my heat pump make noise when it switches to heating mode?
You may hear a hissing sound or a brief whoosh when the reversing valve switches direction. Some models also make a clicking noise as the valve moves. These sounds are normal. If the noise is loud, grinding, or continuous, that is a sign to have a technician inspect the unit.
Is a heat pump the same as an air conditioner?
An air conditioner only cools. A heat pump cools and heats using the same equipment and refrigerant loop. The reversing valve is the main difference—it lets the heat pump run in both directions. If you have a heat pump, you do not need a separate air conditioner.
What happens to my heating if the heat pump breaks down in winter?
If you have a backup furnace or electric resistance heating, it will turn on automatically when the heat pump stops working. If you have only a heat pump with no backup, you will lose heat until it is repaired. This is why many homes in cold climates pair a heat pump with a furnace as a backup.
Do heat pumps work in humid climates?
Yes. Heat pumps work in any climate, but they are most efficient in moderate temperatures. In very humid climates, the indoor unit may remove moisture from the air as it cools, which is actually helpful for comfort. In winter, humidity is usually lower, so moisture is less of an issue.