Everyone’s talking about heat pumps. These machines for heating buildings can exchange heat with the outdoors in multiple ways, of course. Ground-coupled heat pumps pull heat out of the ground or water and send it to heat the building. But most heat pumps are of the air-coupled variety. In winter, they pull heat from the outdoor air and send it inside. And this is where a great many people ask, how the heck do heat pumps get heat out of cold air?
That last link goes to my first article on this topic, which I wrote in 2010. It’s still valid, but today I want to approach this question a bit differently. I’m not going to talk about the refrigeration cycle or the second law of thermodynamics. Instead, let me start with what the real question usually is.
The question behind the question
When someone asks how a heat pump can pull heat out of cold air, there’s often another question behind it that’s really what they want to know. Drew Tozer mentioned this in a LinkedIn post this week. He said he usually doesn’t answer the above question because:
What they’re usually trying to say is “Someone told me that heat pumps don’t work when it’s cold. Is that true? Am I going to regret buying one?”
Drew lives in Ontario, and this is becoming a big deal there because Canada is going nuts with heat pumps, just as we are here in the US. When I drove through Nova Scotia this past summer, I saw heat pumps all over the place. Yes, even out in the rural areas. The photo above is one of those homes.
So, let’s begin there.
The answer is in your kitchen
You have a device in your kitchen that’s already providing the answer to that question. A refrigerator works on the same principle and technology as an air-coupled heat pump. Unlike a heat pump for space heating, though, a refrigerator works in only one direction. It always removes heat from the fridge whereas a heat pump can move heat from outdoors to indoors in winter and then reverse the flow to move heat from indoors to outdoors in summer.
How does this show that a heat pump can pull heat out of cold air? Because your fridge is constantly pulling heat out of both the refrigerator and freezer portions of your fridge. In fact, it pulls enough heat out of the freezer to keep the temperature there at about 0 °F (-18 °C). That’s more than enough heat removal to freeze water into ice. (Wondering about that photo? I wrote about the reason the ice spike forms a while back.)
Believe it now?
Arbitrary temperature scales
OK, maybe you remain skeptical. Let me give you another reason not to worry about the capability of heat pumps to remove heat from cold air. When we talk about temperature in our daily lives, we use either the Fahrenheit or Celsius temperature scales. They’re great for giving us numbers we can assign to different levels of thermal activity. But they’re arbitrary.
Daniel Fahrenheit chose the zero on his scale following the lead of Ole Roemer. Roemer chose his zero because he didn’t want to record any negative temperatures, and the temperature that now equates to 0 °F was about as cold as it ever got in Denmark in his day. (Excellent article on that here.)
Anders Celsius chose the zero of his scale to be the freezing point of water. That’s useful, but still arbitrary.
The absolute answer
In contrast, Lord Kelvin developed the absolute temperature scale with zero being the lowest possible temperature, also called absolute zero. And that, dear reader, is how we can clear up this confusion once and for all. How, you ask? Because the thermal energy in air is proportional to the absolute temperature.
Knowing that, we can do a bit of math here to show how much less heat is in really cold air versus room temperature air. Let’s say those temperatures are 0 °F (-18 °C) and 70 °F (21 °C). The Kelvin equivalents are 255 K and 294 K. The difference in temperature here is 39 K. And that means there’s only a 13 percent drop in thermal energy as the air goes from 70 °F down to 0 °F.*
In other words, when it’s really cold outdoors, the outdoor air still has 85 to 90 percent as much heat available as the nice cozy indoor air.
I know not everyone finds these details as interesting as I do. But on the issue of the physics and the engineering behind heat pumps and their ability to heat your home in winter, the short answer is that you don’t need to worry. They not only can do it, but they ARE doing it. I showed that in my article about Gary Nelson’s house in Minnesota. And I showed the data for my own house in Atlanta when we had really cold weather a year ago.
So, yes, heat pumps can and do work in cold weather. Do some heat pumps struggle? Yes, and the reasons vary. It could be poor design, faulty equipment, bad distribution, or any number of other problems. But it’s not because heat pumps don’t work when it’s cold. Because that they certainly do!
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can subscribe to Energy Vanguard’s weekly newsletter and follow him on LinkedIn.
* The calculation here is (294 K – 255 K) / 294 K = 0.13 = 13%.
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