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Building Science

How Does a Heat Pump Get Heat From Cold Air?

The physics of heat pumps isn’t really that difficult to understand

A heat pump in heating mode pulls heat out of cold outdoor air and sends it into a warm building. Does this violate the second law of thermodynamics?
Image Credit: Energy Vanguard

Cold weather is coming back to Atlanta this week, so let’s talk about heat. An increasingly popular way to heat buildings these days is with heat pumps, even in cold climates. But how do they work?

How the heck can this device take heat out of cold outdoor air and put it inside a building? Ask people how a refrigerator or air conditioner works, and they think they understand it. Those are devices that make coldness. Right? But when you think about the bigger picture, heat pumps do the same thing as refrigerators and air conditioners: They move heat from a cooler area to a warmer area. Does that mean heat pumps also “make coldness”?

Follow the heat

It really is possible to get heat out of cold air, so to understand heat pump operation, let’s start with the basics. Heat flows when you have a temperature difference (ΔT). If you’re trying to get heat out of 40°F air, what do you have to do? Put it in contact with something that’s at a temperature lower than 40°F! That’s the job of the refrigerant in a heat pump.

Heat pumps work on exactly the same principle as air conditioners and refrigerators. All three of these devices are, in physics terminology, refrigerators (or better yet, refrigerator engines). They move heat in what seems like the opposite direction specified by the second law of thermodynamics: from cooler to warmer. But recall the full statement of the second law, as given by Rudolf Clausius: “Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.”

That bit about “some other change” is what keeps heat pumps from violating the second…

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