How Do Heat Pumps Work?

Now stretch it really hard and quickly hold it to your upper lip, which is sensitive to temperature. You will feel that it’s warmer than it was before. That’s because you’re adding energy to the rubber band, which increases its temperature.

Are you ready for the awesome part? Keep it stretched for a little while until it returns to room temperature. Now let the rubber band relax and quickly touch it to your lip again. It’s now colder than room temperature! Seriously, try this for yourself.

So if you had a big enough rubber band, could you use this to cool your house? Wait a minute, you’re gonna say: In the first stage, when we stretched the rubber band, it got hot, and then it cooled back to its original temperature—and in doing that, it heated the air. You’re right. But what if we could vent that warmer air outside? Then you could keep just the cooling phase inside.

Boom. You just reinvented the air conditioner! Instead of a rubber band, an AC has a fluid called a refrigerant that circulates in a closed loop from inside to outside. This fluid has a low specific heat—so it changes temperature quickly—and a very low boiling point, turning into a gas at something like –15 degrees Fahrenheit.

How’s it work? The gas is first compressed, causing it to heat up to around 150 degrees. The hot gas circulates in a set of copper coils outside, with a fan blowing over them, so the gas loses thermal energy to the atmosphere. (Copper also has a low specific heat.)

Then it’s pumped back inside, where the pressure is quickly reduced, causing it to expand and instantly cool down to around 40 degrees. As the now cold fluid circulates through indoor coils, a fan blows warm inside air over it, heating the fluid again and cooling the indoor air in the process. As the system circulates, it basically picks up thermal energy indoors and carries it outdoors.

By the way, this is exactly the same process that your fridge uses to keep your cheese and soda cold. In both cases, the process makes something inside cooler and something outside warmer. Put your hand behind the fridge and you’ll see what I mean. Just for kicks, here’s a guy who actually built a refrigerator that runs on rubber bands.

So Heat Pumps Aren’t New!

You thought this was going to be an article about heat pumps, right? Well, guess what—we’ve been talking about heat pumps this whole time, because they run on the same principles. A heat pump cools your home just like an air conditioner: by circulating a refrigerant and varying the pressure to change its temperature, so it takes thermal energy from one place and puts it in a different place.

So back to the big mystery: How can a heat pump increase the temperature of indoor air on a cold day without actually generating any heat? Simple: Just run it in reverse! This time we let the hot compressed refrigerant cool off inside the house to raise the indoor air temperature. The low-pressure, cold gas then goes outside to warm up.

Warm up outside? Yep. Even on a freezing day, the air still has thermal energy. So long as it’s above absolute zero (which, believe me, it is, since that’s around –460 degrees Fahrenheit), the air molecules are in motion. And since we’re cooling the refrigerant to, say, –15 degrees, which is lower than winter temperatures in most places, it will wring thermal energy out of even frigid air.

Of course, you can’t get energy for free. Heat pumps rely on electricity to drive the compressor and fans. But if you have solar panels at home, or if the electricity in your area is even partly from non-carbon sources, replacing a gas furnace with a heat pump can make a big difference in reducing greenhouse gas emissions. And it’ll probably lower your utility bills in the process.