There is no such thing as cold

How do heat pumps work? I couldn’t find a clear explanation, so I wrote one.

You walk outside. The frigid air could turn water into ice. You wear a heavy down coat to keep from shivering and thick wool socks to keep your feet from going numb. Inside, you burn gas or oil and maybe even light a wood fire to stay warm. 

You are freezing. 

Galactically speaking, you’re in the tropics.

In outer space, the average temperature runs around minus 455 degrees Fahrenheit. Even in terms of our own planet, you’re technically warm; the mean temperature on Earth, the third-balmiest world in our solar system, is 59 degrees. In straight-up scientific terms, you’re always hot, because even if it’s zero outside, the mercury is 459.67 degrees above absolute zero

Where the hell am I going with this? Bear with me.

Saturns’s moon, Enceladus, surface temperature minus 330 degrees Fahrenheit. Courtesy NASA.

You may think of heat as something you feel, but “heat” is a specific term in thermodynamics: It’s when molecules in a system have enough energy to transfer some to another system. Say one system is a metal chair that’s been sitting in the sun; another system is your butt. You sit down on the chair, you burn your ass. That’s energy transfer, that is heat. 

Meanwhile, if that chair was chilled to absolute zero, not only could you lose a butt-cheek to frostbite, but the only energy transfer would be from you to the chair; you have body heat, so you’d warm it up. At absolute zero, the chair’s molecules have nothing to spare. 

That, however, doesn’t make the chair cold. I mean, sure, you perceive it as cold, but really it just doesn’t have any heat, and those two are not the same thing. While heat is that 👆state of energy, cold is a human observation. You might feel cold or note that a thermometer is reading cold, but technically you’re just observing lower levels of heat. Cold is our construct, while heat is a law of the universe.

This may be pedantic (it is VERY pedantic) but it’s actually key to understanding something that’s been in the news a lot lately: heat pumps. They’re practically every other paragraph in the Inflation Reduction Act; your local utility has probably sent you a bunch of flyers begging you to buy one; and the president fired up the Defense Production Act to get more of them made. 

Let’s imbibe from the cup of honesty and be real for a minute: Nobody understands how heat pumps work, and they are an important part of how we’re going to lower our greenhouse gas emissions. You’re not going to find a better explanation of how this tech works; I know because I tried. The best I could do was call a bunch of people and write my own.

You want to know what’s going on in that box the HVAC contractor wants to sell you? Keep reading. Oh, and invite your friends to the bar of knowledge so we can all get drunk off hydrofluorocarbons.

You’ve probably heard the term “heat pump,” and wondered how these miracle devices are going to live up to the hype and end Global Warming. On their own, they won’t, of course. But they can help us reduce our greenhouse gas emissions while efficiently making our spaces comfortable—even livable in some areas.

Heat pump is a pretty confusing term for something that’s commonly used as an air conditioner, so maybe you googled it and ran into even more confusing terms: geothermal, ground-source heat pump, air-source heat pump, mini split, inverter, ductless unit. Gah, marketers need to chill (😉) with the proprietary monikers.

Good news: Everything in the list above is some version of the same thing. There are slight differences in the way some of those products function, but they all rely on the same base technology that, as the special guest in a lot of recent legislation, will become increasingly important in our efforts to not cook our planet while we keep ourselves cool—and warm. 

Fire in the basement

Do you heat your home? If so, you probably burn gas or oil. Even as recently as 2020, 55 percent of the houses built in the U.S. employ gas heat. That tracks pretty closely with the national average, which means somewhere around 70 million residences have small fossil fuel fires in their utility rooms, heating air or water that is then circulated throughout the building. 

I’ll spare you the guilt trip about residential emissions and just suggest that you look into heat pumps if you need to make any major changes to your HVAC system. If you can afford it, this kind of upgrade can measurably decrease your personal emissions. In addition to directly creating greenhouse gasses, and financially tying you to a shitty industry, petroleum-based climate control wastes a ton of energy to heat loss. You know how you tell your kids not to touch the furnace because it’s hot? That’s wasted energy; the fire in that box should be conditioning your living spaces, not waiting to burn someone. 

We also made a fire-obsessed child.

Most furnaces and boilers employ somewhere around  70 percent of their combustion energy for climate control. If you have the absolute most efficient gas furnace, you might get up to 99 percent in ideal conditions. “There’s something called the coefficient of performance, which has to do with the energy transferred relative to the energy provided,” says Sean O’Brien, a mechanical engineer in the building space. “If you could take 100 percent of the energy contained within your fuel and transfer it to heating, you would have a coefficient of performance of 1.” So an average furnace would have a coefficient of performance of around 0.7, and that really fancy one would be up to 0.99.

“With a heat pump, though, you can have a coefficient of performance of 3—maybe even 4,” says O’Brien. “That’s like getting three or four hundred percent efficiency.” How does that not violate the laws of physics? Unlike a combustion-based setup, “you’re not generating heat,” he says. “You’re using a little bit of energy to move a larger amount of energy.” You’re pulling heat into the system from its surroundings.

Remember how heat describes the movement of energy? Well, the laws of thermodynamics dictate that heat always flows from a higher-energy state to a lower one. And because any substance that’s warmer than absolute zero has energy in it, there’s energy everywhere on this planet. There is no naturally occurring air, water, or ground on Earth that is at—or even close to—absolute zero. There is heat energy all around us, and heat pumps grab it.

Air conditioner ancestor 

Let’s start with the close technological relative of the heat pump, something you have probably experienced: an air conditioner. Inside, its fan blows air over a cold coil to create cool air. That coil is called the evaporator. Outside, a fan blows on a hot coil, creating that nasty plume of waste heat you sometimes get blasted by when you walk past the ass-end of a window unit. 

That heat came from the room the A/C was cooling, brought outside by refrigerant circulating through a bunch of metal (probably copper) coils. Refrigerant is an engineered substance designed to transfer heat really well and to boil at a low temperature; a compressor moves it through the system. This motor pushes the refrigerant into that hot coil, which is known as the condenser, and then through an expansion valve into the evaporator. When it’s in the condenser, it’s a hot liquid; when it flows through the expansion valve into the evaporator, though, it changes state—this is where that low boiling point comes in. 

“Picture a can of hairspray,” says O’Brien. “Inside the can you’ve got a liquid under pressure, but when you release it, it sprays through a nozzle and becomes a mist.” You could almost say it evaporates, which is why that cold coil is called an evaporator. And when a substance changes from liquid to gas, it uses heat energy to complete that transformation. That causes its temperature to drop. We perceive that as cold.

The hot coil, meanwhile, is called the condenser, because when the refrigerant is compressed, it becomes liquid, and compressing things makes them hotter. “You need to expend energy to compress something,” says O’Brien, “and whatever you’re compressing stores a portion of that energy.” You squash something down, and its energy level goes up. A refrigerant can get quite hot in this kind of situation (since that’s what it’s designed to do). R410a, a commonly used refrigerant, gets well above 100 degrees at the pressures used in a modern heat pump system.

Condenser: HOT, full of compressed liquid
Evaporator: COLD, full of hairspray gas

Throwing it in reverse

So while our window unit grabs heat from inside the house and blasts it outside, a heat pump, thanks to a bit of hardware called a reversing valve, can flip the whole process the other direction and go from chilly to cozy: Your evaporator becomes your condenser, and your condenser becomes your evaporator. (They’re both metal coils with fans blowing over them.) And because there is always heat energy in any substance that’s above absolute zero—whether that’s 10 degree winter air or the consistently 50 to 60 degree dirt a hundred feet below the ground—your system can usually find some heat to grab and then concentrate it by compressing it. 

Take an air source heat pump, the most common variety. “Let’s say it’s 50 degrees outside and you’re trying to heat your space,” says O’Brien. “All you have to do is run that evaporator coil down to 25 degrees or so by running that refrigerant loop.” The more refrigerant you vaporize, the lower the temperature in the coil will drop, and the more readily ambient heat will be attracted to it. “The 50 degree air is going to naturally flow heat into the coil, because it’s at a lower temperature,” he says. 

If it’s colder outside, the system just needs to run the evaporation cycle over and over, to bring the temperature of the coil lower. Modern air-source heat pumps can make this work down to about minus 14 degrees Fahrenheit.

In a ground-source heat pump, commonly referred to as geothermal, your coils are operating in a much narrower window; they can always pull heat from a 50 to 60 degree source, because, below a dozen or so feet, the ground is always in that temperature range. (This depends on where you live, of course.) When you’re cooling, the system can exhaust heat into the limitless heat sink that is the Earth’s mantle. There are also water-source heat pumps, which use a reservoir of H2O. Some people even use ponds or wells. 

The beauty of the heat pump is that the source doesn’t matter too much; as long as you can get your evaporator coil to a lower temperature than what’s around it, you can extract heat energy. This holds true whether you’re talking about a sweltering room or a chilly winter day.

I can’t recommend which type of heat pump people should use. (Or even if you should use one.) That depends on where you live, how large your house is, and many other factors. Fortunately, tons of helpful businesses will likely spin up in order to grab some of the money made available by the Inflation Reduction Act. But these products are here, and they are an important part of our efforts to halt Global Warming.

If we’re taking a hard look at the tech, though, we have to acknowledge the harm that the hydrofluorocarbon refrigerants can do if they get out of their closed loops. And “things always leak,” says O’Brien. R410a, a commonly used HFC, isn’t an ozone depleting substance, but “you shouldn’t inhale the stuff,” he says. You can imagine the impact a leak could have in dense urban areas, where efficient cooling can be a life-saver in the increasingly hot summers. It’s something to keep an eye on. 

That all said, two weeks ago a buddy and I carried my old oil tank out my basement door. And as I type this, HVAC dudes are snaking refrigerant lines and ductwork through my house, which used to rely on baseboard heating and window units. There’s a compressor that weighs more than my whole family bolted to wall under my deck. We went with an air-source heat pump that’s rated to work down to sub-zero temperatures.

It’s a huge weight off my mind. This past winter, I spent a lot of cold nights and mornings writing about global warming while a 30-year-old boiler kept me cozy. I was all-too aware that domestic heating and cooling currently accounts for 441 million tons of CO2 emissions annually, and I was contributing pretty heavily. We had wanted to ditch that thing for a while, but a whole-house HVAC overhaul is not cheap. It took us months to pick the right system and longer to navigate the state and local incentives. It’s yet another bit of environmentalism that’s out of some people’s reach due to the amount of money and time you need to invest.

This does seem to be changing, though, and you should celebrate that. Hopefully, when it’s time for you to make a decision about how you control the temperature in your home, this tech will be even better and even cheaper, and you’ll have a good idea of how it works.

Take care of yourself—and the rest of us too,


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