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How Do Heat Pumps Work?

If you position a fan near a window or a door, you can either draw hot air out of the room or bring cool air into it, depending upon the direction you point the fan. When bringing cool air into a room, the fan acts as a refrigerator; when forcing it out, it works as a heat pump.

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Air conditioning systems work on the same principle by forcing air over a heat exchange device, which can either be hot or cold, and forcing it either inward or outward, as needed. Thus, a heat pump may be used either to bring heat from the outside into a cold room or to draw heat from a warm room and push it outward. Heat pumps tend to be relatively efficient because the heating energy comes from the environment, and you need only run the compressor and the air blower, which require less electricity than it would take to cool the air directly. The transferred heat can be as much as three or four times greater than the electrical power consumed. And, of course, insulating the space will reduce the need even for this energy expenditure.

Heat naturally flows from warmer to cooler spaces. A heat pump, then, consists of a circulatory system—usually some configuration of copper tubes, of which one side acts as a condenser, and the other side as an evaporator. Between the two sides, at one end, is a compressor, which compresses a liquid refrigerant gas (usually an ozone-safe compound like HFC R-410A, or water/antifreeze mixtures) into a liquid. At the other end, an expansion valve or other metering device releases the hot, compressed liquid from the condenser into the evaporator at a controlled rate where it becomes a gas again. When a gas is condensed, it stores heat; when a liquid evaporates, it releases heat. Air flowing over the condenser side can be used to heat a room; when it flows over the evaporator side, it can be used to cool it. The same air conditioning system can be used both in summer and in winter simply by reversing the compressor and the metering valve so that the hot side becomes cold and the cold side becomes hot—or, of course, by blowing the air over one side of the circulatory system or the other.

The compressor pressurizes and circulates the refrigerant into the condenser tubes. Since it’s under pressure, it’s hot, and the condenser tubes can be used to heat the air blown over it by a fan. At the other end, the condensed refrigerant is released through the metering device into the evaporator side at a controlled rate. The metering device is typically an expansion valve or a restrictive capillary tube. Encountering a relative absence of pressure there, the liquid turns into a gas. As it ‘boils’ in this way, it loses energy and becomes cooler. Air blown over the evaporator becomes cooler, as well. The refrigerant then returns to the compressor and the cycle is repeated. High-efficiency aluminum fins increase evaporator efficiency.

The greater the difference between the temperatures of the heat exchange system and the environment to be cooled, the more pressure needed to compress the fluid. Because of the physics of pressurizing the refrigerant, heat pumps are most efficient around 90°F or below.

About the Author:

Zelick Gimelstein, Marketing Director of The AC Outlet, is a frequent contributor on home energy-related topics. Having learned the many facets and nuances of the industry from the company’s principals—who possess years of general construction and HVAC distribution experience—he enjoys sharing knowledge about both residential and commercial heating & cooling solutions so that end-users may have a better understanding of their needs in terms of comfort, energy usage, and sustainability.

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