The Presto Chango Hot 'n' Cold Machine
We're back, baby!
My partner and I spent the last three weeks cat-sitting in Barcelona for two adorable if possibly feral kitties. We had a great time! I ate roughly 400 olives (stuffed with anchovies, naturally), enjoyed a classic 'vermut' or two at lunch, wandered around, mucked about, and generally marveled at what a country can be like when it's not actively hostile towards its own citizenry.
Now I'm back in my favorite chair in our kitty-less apartment, sipping a Celsius, fired up to tell you about some Cool Shit that most of us Americanos know next to nothing about. It's like I never left!
Let's get into it.
[ESPN guy voice]
What if I told you there was a way to heat and cool your home with a single appliance that's up to 550% more efficient than your gas boiler or furnace... and it could save you up to $500 a year on your utility bill?
What if I told you this mystery appliance is way, way better for the environment than your current fossil fuel-fulminating HVAC setup—so much so that clean energy icon David Roberts has called it 'the main way we are going to decarbonize home heating and cooling', which account for a whopping 20% of America's overall CO2 emissions?
And what if I told you that this appliance is on sale for 30% off—every brand, every style—until the end of the year?
[ESPN guy voice off]
I can sense you're on the edge of your gaming chair, so let's get to the big reveal. The name of this mysterious, magical appliance is...
The heat pump!
The name kind of stinks, frankly. That's because most heat pumps can satisfy both your home's heating and cooling needs. It's a two-fer! Which is huge, because by combining your two main HVAC systems into a single A/C-sized appliance, you can get rid of that giant gas boiler or basement furnace and install a Golden Tee arcade machine in the reclaimed space.
If I were tasked with renaming the heat pump, I'd call it...
- A reversible air conditioner
- A two-way heat sucker
- An all-seasons A/C unit
- A Presto Chango Hot 'n' Cold Machine
Something like that, anyway.
But what are heat pumps, exactly? What makes them so darn efficient? Should everybody get one? How come the rest of the world has Heat Pump Madness but Americans don't? And can they really save you $500 a month on your utility bill?
These questions and more, answered below.
1. What's a heat pump?
You've probably seen heat pumps in the wild without knowing it. That's because the most common variety—'air source', also known as 'air-to-air' heat pumps—look a lot like air conditioning units. There's an inside component that uses either ducts, like central air, or mini-splits, and an outside component that sits on or behind the building. Here's a classic example:
As heat pumps have started to take off, we're starting to see more fun, design-forward iterations coming out of the startup world.
This is the Octopus Energy air source heat pump. Octopus is massive in the UK, and they're gearing up to cross the pond in short order.
This is the sleek mini-split designed by Quilt, an American ductless heat pump purveyor.
Increasingly, we're seeing more window and portable heat pumps coming onto the market, like this spiffy specimen from Midea.
Though this post is focusing on air source heat pumps, there are many different varieties of heat pump available today. For instance, there are ground source (AKA 'geothermal') heat pumps, water source heat pumps, absorption heat pumps, heat pump water heaters that double as thermal batteries... the list goes on. We'll talk more about the alternative versions next week.
But no matter what kind of heat pump it is, all heat pumps have one thing in common: they don't make heat. They move it.
2. How heat pumps work: no fire, no problem
Humans have been setting shit on fire to keep warm ever since Prometheus came downstairs for a smoke and ended up giving his whole pack away to some teenage homunculi.
Overall, fire's been a mixed bag for humanity. While it allowed early adopters to move into caves (imagine being able to afford your own cave today...), it's also burned down countless homes, cities, forests, granaries, fireworks emporiums, the Cuyahoga River, etc.
Besides, keeping a fire going is hard work. You ever stay in one of those cute little cabins upstate with only a wood-burning stove to keep you comfy through the night? It's not as romantic as it sounds! Fire eats fuel. It doesn't matter if you're burning wood, oil, or explosive, deadly natural gas: setting stuff on fire is a wildly inefficient way to heat your home.
How do we know that? Well, some smart people came up with a way to calculate the efficiency (or lack thereof) of modern heating and cooling appliances. It's called the Coefficient of Performance (COP), and it tells us the ratio of 'useful' heating or cooling output compared to energy input.
Said another way, COP tells us how much energy we gotta put in to get hot or cold air out. The higher the COP, the more efficiently the appliance makes use of energy.
Examples of appliance COPs
- A space heater converts energy directly into heat by sending electricity into metal coils, so it has a COP of exactly 1. There's no chemical or phase change occurring that would lead to energy losses.
- A gas boiler has a COP of 0.8 to 0.9, meaning it turns 80-90% of the gas it burns into heat. The other 10-20% is lost as exhaust—carbon dioxide and water vapor with trace amounts of nitrogen oxide and carbon monoxide, sometimes. Fun fact: exhaust is what creates the pollution that is actively incinerating our planet.
- A furnace burning oil or propane has an average COP of 0.84 to 0.96, respectively, with the rest lost as exhaust or flue gases.
- Air conditioners have an average COP of 2 to 4. They're already super-efficient.
- Heat pumps can have a COP of anywhere from 2 to over 5. This means you get two to five times more heating and cooling out of a heat pump than the amount of electricity you put in.
Five times!
That level of efficiency is a big deal. Heating systems are the biggest energy guzzlers in the home, accounting for 40-50% of our overall energy bills on average. If you suddenly use five times less energy on heating, your utility bill should contract like it's injecting Ozempic into its buttox. But more on this later.
First, a question: how's it even possible to get more heat out of less energy? Doesn't that defy the first law of thermodynamics, which states that energy cannot be created or destroyed but only transformed?!
It certainly would... if heat pumps generated heat. But they don't. They move existing heat around.
Heat pumps are just reversible air conditioners
Air source heat pumps are literally just air conditioning units with a few extra parts added that allow them to work in reverse. Easy peasy!
(I'm picturing my reader squinting, nodding slowly... and realizing they don't actually know how their A/C unit works.)
Honestly, same.
That's why I asked Dr. Aarati, Green Juice illustrator extraordinaire and actual scientist, to explain how my air conditioner keeps me chilly on a warm summer's day. This is not strictly necessary information, but it sure is interesting.
3. Dr. Aarati explains how air conditioners work
While it feels like air conditioners create cold air out of the aether, that's an illusion. In fact, what they really do is suck hot air out of the room, expel the heat outside, and then blow cooled air back into the room.
Air conditioners accomplish this feat by combining physics with a special kind of fluid called a 'refrigerant'.
Physics comes into play in two specific ways, here.
1. When you raise the pressure of a gas, its temperature goes up; when you lower the pressure, its temperature drops.
2. The second law of thermodynamics tells us that heat always flows spontaneously from hotter to colder regions.
Refrigerants, meanwhile, are special because they have incredibly low boiling points. As in, sub-zero boiling points. For example, one refrigerant called R-134a has a boiling point of -15°F. Another, R-410A, boils at -60°F!
Refrigerants work kind of like our own body’s cooling system: sweat. On a hot day, liquid sweat on our skin turns into gas (i.e. evaporates) by absorbing the heat from our skin. When sweat evaporates, it releases its heat into the air, which makes us feel cooler.
Refrigerants in an A/C unit work similarly. They start off as a very cold, low-pressure liquid that travels through the coils in the unit's 'evaporator'. As the refrigerant moves through the coils, warm air in the room naturally flows towards it. This heats up the refrigerant and, thanks to that incredibly low boiling point, causes it to evaporate into a gas.
That gas—still inside the coils—gets moved to the outside of the building, where a 'compressor' squeezes the gas molecules, increasing their pressure (and therefore their temperature). Next, the heated gas then enters a 'condenser', which uses the cooler outside air to do the opposite: lower the gas pressure, cooling the gas and turning it back into a liquid. The heat released by this state change naturally flows outside, drawn by the cooler air. A fan helps get the heat out, too.
Finally, the re-liquified refrigerant passes through an 'expansion valve', which sharply reduces its pressure , making it very cold and ready to absorb heat once again, so the process can start all over—evaporation, compression, condensation, and expansion.
Got all that?
I think I get it: cold air is like hipsters, and hot air is like finance bros. Finance bros will always follow hipsters wherever they move to. (After waiting a couple years until it's 'safe enough' to take over the neighborhood, of course.)
But I wondered about one thing. If heat pumps are just air conditioners that work in reverse by sucking up hot air from outside and moving it inside... how do they provide heat in the wintertime, when it's, y'know, cold out??
4. How heat pumps work even when it's really cold out
The thing is, our concept of 'warmth' is relative. Even when it's freezing out, there's still some heat energy in the air. If there was no heat at all, Earth would look and feel like Mars.
So as long as your heat pump's refrigerant boils at a lower temperature than the temp of the outside air, you're (pretty much) golden. A standard-issue heat pump should have no trouble humming along at temperatures of around 0°F... though its efficiency will fall somewhat if its compressor has to work harder.
Should you happen to live in one of the few remaining places on Earth where winter temps regularly dip into the negatives, fear not. There's a whole specialty series of cold-climate heat pumps outfitted with 'variable-speed' compressors that provide effective heating at temps as low as -30°F.
(However, folks who live in truly frigid climates often keep a backup heating source on hand, such as electric baseboard heaters or even a gas furnace.)
But for the vast, vast majority of us, a standard heat pump is far more efficient and safe and better for the environment than triggering constant small explosions in the home where your precious family sleeps.
...but will it really save you $500 a month?
I'd love to answer that question, but I've once again run out of space. We'll get into it next time.
However, if you're already convinced that you must have a heat pump, then you should absolutely take advantage of the Energy Efficient Home Improvement Credit (IRC §25C), which allows folks to claim 30% of the cost of installing qualifying heat pumps (up to $2,000).
This wonderful tax credit will permanently expire on January 1, 2025. So if you've got an older boiler or creaky furnace, now's the time to start thinking about what you'll do when it inevitably breaks down. My vote? Achieve fossil fuel freedom.
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