In this blog post, we’ll explore the principles behind heat pump HVAC systems—which perform both cooling and heating with a single unit—the refrigerant circulation process, and the advantages and limitations of heat pumps.
Heat Pumps: One Unit, Two Functions
Throughout our lives, people seek out and strive to create objects or tools that can perform multiple different functions at once. Just a few years ago, middle and high school students were busy searching for places with computers or using the internet in computer labs. But now, cell phones have taken on the functions of computers, allowing us to access the internet anytime, anywhere. To give another example, when working with others, don’t you tend to look for someone who excels at two or three things rather than just one?
A heat pump heating and cooling system is similarly a device that performs two functions with a single unit. This may raise a question: As the name suggests, we know it enables both cooling and heating, but what exactly does the term “heat pump” mean? A “pump” originally refers to a mechanical device that uses pressure to transport liquids or gases from one place to another; in this context, it means transporting heat from one place to another to provide cooling and heating. Just as water flows through a water pump, heat moves from one place to another through a heat pump. The reason I’m emphasizing this point is that a heat pump heating and cooling system is not a device that creates or destroys heat from scratch; rather, it is a device that transfers heat from one place to another to provide cooling and heating. Of course, since components like the compressor use electrical energy during the heat transfer process, it’s not entirely energy-free. How can heat—which is neither visible nor tangible—be moved to the desired location? Let’s take a look at that process now.
What are the components and refrigerant of a heat pump?
A heat pump consists of major components such as a compressor, expansion valve, indoor heat exchanger, outdoor heat exchanger, and four-way valve; the refrigerant circulates between these components to enable heating and cooling indoors. Just as we need a means of transportation to move heat in the direction we want, we need a medium to transfer heat—this is called a refrigerant. A refrigerant is a substance that transfers heat; as its pressure, temperature, and state change during the circulation process, it absorbs or releases heat. In other words, the refrigerant itself does not generate heat; rather, it plays the role of transferring heat between the indoor and outdoor environments as it undergoes processes such as compression, condensation, expansion, and evaporation. Furthermore, while the four-way valve is not a device that causes a change in the refrigerant’s state, it is a very important component that adjusts the internal valves to change the direction of refrigerant flow, thereby enabling the heat pump to perform both cooling and heating functions. Therefore, in the cooling and heating processes described below, we will assume that the four-way valve is adjusted to the appropriate direction for cooling or heating.
How does the refrigerant produce cooling?
First, let’s examine the cooling process. Since the refrigerant circulates continuously, there are no distinct starting and ending points in the process; however, for the sake of explanation, we will consider the compressor as the starting point of the cooling process. ① When the refrigerant—in a low-temperature, low-pressure gaseous state—enters the compressor, the compressor compresses this gas, turning it into a high-temperature, high-pressure gas. ② Since the refrigerant in this high-temperature, high-pressure gaseous state is much hotter than the outside air in summer, it releases heat to the outside air via the outdoor heat exchanger. As a result, the refrigerant loses heat and approaches a high-pressure liquid state.
③ As the refrigerant in this liquid state passes through the expansion valve, its pressure drops sharply, and part of it vaporizes, turning it into a low-temperature, low-pressure refrigerant. As the pressure drops and part of the refrigerant vaporizes, the refrigerant draws the necessary energy from its own thermal energy, causing its temperature to drop as well. This principle can be understood as similar to the phenomenon that occurs when you press the nozzle of a spray can, such as an insecticide: the liquid inside the high-pressure container expands as it exits into the lower-pressure exterior, and part of it vaporizes, causing it to cool. ④ The refrigerant, now in a low-temperature, low-pressure state, enters the indoor heat exchanger and absorbs heat from the warm indoor air. As a result, the temperature of the indoor air decreases, providing cooling. During this process, the refrigerant absorbs heat and becomes a gas with a slightly higher temperature; it then re-enters the compressor to repeat the process. Thus, in the cooling cycle, the process of the refrigerant absorbing heat from the indoor air and releasing it outdoors is continuously repeated.
Would reversing the flow of the refrigerant result in heating?
Simply put, the heating process involves making the refrigerant flow in the opposite direction of the cooling process. For the sake of explanation, let’s consider the compressor as the starting point for the heating process as well: ① The refrigerant becomes a high-temperature, high-pressure gas in the compressor and flows into the indoor heat exchanger—the opposite of the cooling process. ② Since the high-temperature, high-pressure refrigerant entering the indoor heat exchanger is hotter than the indoor air, it releases heat to the indoor air. As the refrigerant loses heat, its temperature drops, while the indoor air absorbs heat and becomes warmer, thereby providing heating. The heat released by the refrigerant during this process is used to warm the indoor space. After releasing heat, the refrigerant becomes a high-pressure liquid, and ③ as it passes through the expansion valve, its pressure decreases and part of it vaporizes, resulting in a low-temperature, low-pressure state. This operates on the same principle as the expansion process described in the cooling cycle. ④ The refrigerant, now cooled, enters the outdoor heat exchanger. Since the refrigerant’s temperature is lower than that of the outdoor air, it can absorb heat from the outdoor air; as a result, it becomes a low-temperature, low-pressure gas with a slightly higher temperature and flows back into the compressor. However, when the outdoor temperature drops significantly, it becomes difficult to maintain a sufficient temperature difference between the refrigerant and the outside air. Since the process of extracting the heat needed for heating from the outside air becomes challenging, heating capacity and efficiency may decrease. For this reason, depending on the type of heat pump, systems that utilize heat sources with relatively stable temperatures—such as the ground, water, or wastewater—in addition to air-source heat are also employed. Heat pumps are also classified based on the location of the heat source, such as air-source, ground-source, and water-source heat pumps.
How will heat pump heating and cooling systems evolve in the future?
As described above, by regulating the flow of refrigerant using the four-way valve, a single unit can provide both cooling and heating depending on the season and weather. This is the basic principle of heat pump heating and cooling systems. In the past, the widespread adoption of heat pumps was limited due to relatively high upfront costs, reduced performance in cold weather, and installation constraints. However, with recent advancements in products designed to maintain performance even in low-temperature environments and the development of high-efficiency technologies, the range of applications for heat pumps continues to expand. According to the International Energy Agency (IEA), global heat pump sales in 2025 decreased by approximately 2% compared to the previous year; however, sales in Europe increased by 11%, and in the United States, heat pumps have outsold gas boilers for the fourth consecutive year. Furthermore, the 2026 IEA report assesses that heat pumps are showing increasing potential not only for building heating but also in the industrial and district heating sectors.
Just as when working with people, while someone who excels at everything would be ideal, if that’s not the case, we tend to prefer someone who does at least one thing well over someone who does many things poorly—the same principle applies here. In other words, it’s not enough to simply offer the advantage of being able to provide both cooling and heating; it’s also crucial how efficiently and reliably each function is performed. Heat pumps must also overcome drawbacks—such as reduced performance in cold environments, high initial installation costs, and efficiency variations depending on the installation environment—while maintaining the advantage of performing both cooling and heating functions in a single unit. Therefore, for heat pumps to evolve into systems that properly perform a variety of functions in the future, it will be necessary to continuously research and develop technologies that address these shortcomings and enhance efficiency and reliability.
