Everything you need to know about the wild world of heat pumps – MIT Technology Review
Heat pumps could help address climate change and save you money. Here’s how they work.
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We’re entering the era of the heat pump.
The concept behind heat pumps is simple: powered by electricity, they move heat around to either cool or heat buildings. It’s not a new idea—they were invented in the 1850s and have been used in homes since the 1960s. But all of a sudden, they’ve become the hottest home appliance, shoved into the spotlight by the potential for cost savings and climate benefits, as well as by recent policy incentives.
Simple though the basic idea may be, the details of how heat pumps work are fascinating. In the name of controlling your home’s temperature, this device can almost seem to break the laws of physics. Heat pumps are also getting better: new models are more efficient and better able to handle cold weather.
So let’s dive in and uncover what makes a heat pump tick.
At a high level, a heat pump gathers heat from one place and puts it in another place. We’ll mostly talk about heat pumps in the context of heating, but they can also be used for cooling, gathering heat from inside and sending it outside like an air conditioner. Many heat pumps can actually be run in reverse, either heating or cooling depending on what’s needed.
The hero in a heat pump is the refrigerant: a fluid that moves in a circuit, soaking up and releasing heat as it goes. Electricity powers the system, pushing the refrigerant around the cycle.
As the refrigerant moves through the heat pump, it’s compressed and expanded, switching between liquid and gas forms to allow it to gather and release heat at different points in the cycle. (If this is enough detail for you, feel free to skip to the next question. Otherwise, join me on a journey inside a heat pump to understand how this all works.)
Picture this: it’s a chilly winter day, say 25 °F (-5 °C). You’re sitting on the couch in your living room with a good book, and your cat is curled up nearby. You look over at the thermostat, which is set to 68 °F. Sensible, but a little chilly. You walk over and bump it up a bit, to 70 °F.
Your heat pump has been quietly humming along in the background. Now it kicks things up a notch to raise the temperature: the fan and compressor inside speed up, and the refrigerant starts moving faster to transfer more heat from outside to inside.
It may seem counterintuitive to collect heat from outside when it’s so cold out, so let’s follow the refrigerant for one cycle to see how it works. For most heat pumps, the trip takes just a few minutes.
Heat pump refrigerants have very low boiling points, typically below -15 °F (-25 °C). So at the beginning of our journey, the refrigerant is around that temperature, and in liquid form. Even in the coldest places, a refrigerant in this state is usually significantly colder than the outside air (in our case, more than 40 degrees colder).
In the first stage of its trek, the refrigerant flows through a heat exchanger, past that outside air and warms up enough to start boiling, changing from a liquid to a gas.
The second phase of its journey is a trip through the compressor. The compressor squeezes the refrigerant into a smaller volume, increasing its pressure and boiling point (this will become important in a minute). This also warms it further, so by the time the refrigerant is past the compressor, it’s warmer than the room indoors.
The third leg of the refrigerant’s journey takes it through another heat exchanger. But by now, the refrigerant is a warm gas, above 100 °F, and it’s flowing past a relatively colder room. As it transfers some of that heat into the room with the help of a fan, it starts turning back into a liquid.
Finally, in the fourth stage, the liquid refrigerant will go through an expansion valve, releasing the pressure. Just as squeezing a material heats it up, expanding it allows it to cool down again, so now the liquid is back to a low temperature and ready to absorb more heat to bring inside.
The claim that heat pumps don’t work well in really cold weather is often repeated by fossil-fuel companies, which have a competing product to sell.
There’s a kernel of truth here—heat pumps can be less efficient in extreme cold. As the temperature difference between inside and outside increases, a heat pump will have to work harder to gather heat from that outside air and disperse it into the room, so efficiencies drop.
Hurricane Ida offers the latest reminder that we need to rapidly rebuild our systems to withstand increasingly extreme events.
But even if heat pumps aren’t running at peak efficiency in colder climates, “they work everywhere,” says Sam Calisch, head of special projects at Rewiring America, a nonprofit group focused on electrification.
There are heat pumps running everywhere from Alaska to Maine in the US. And about 60% of buildings in Norway are heated with heat pumps, along with 40% in Sweden and Finland.
Heat pumps can work efficiently even in the coldest places. Still, choosing the right heat pump is key to making sure it works well when temperatures drop, says Andy Meyer, senior program manager at Efficiency Maine, an agency that runs energy efficiency programs in the state.
Some heat pumps won’t be equipped to warm a room when it’s below zero, but there are models that will work efficiently in colder temperatures, Meyer says. Small space heaters can help provide backup for cold snaps, but if you choose a well-sized system, you shouldn’t need them, he adds.
Improvements in several of their main components have helped boost the efficiency and performance of heat pumps, especially in the cold, Meyer says.
One major improvement is in the refrigerants. Freon, also called R-22, used to dominate the market, but it has been phased out in the US and other major markets for its ozone-depleting effects.
Today, a mixture of chemicals referred to as R-410A is one of the most widely used refrigerants in heat pumps. In addition to being slightly less harmful for the ozone layer, R-410A has a lower boiling point than R-22, meaning it can absorb more heat at lower temperatures, boosting efficiency in the cold.
Other components have improved as well. New compressors used in heat pumps today can get refrigerants to higher pressures using less power. There are also new so-called variable-speed compressors that allow heat pumps to ramp their power up and down. Finally, the heat exchangers that transfer heat between the air and the refrigerant are getting bigger and better, so they can move heat around more effectively.
There’s already a wide range of heat pumps available today. About 85% of those installed are air-source heat pumps like the one I’ve described. These come in a wide range of shapes and sizes. But other models—so-called ground-source or geothermal heat pumps—gather heat from underground instead of collecting it from the air.
Heating buildings frequently relies on natural gas or heating oil, which is why the sector accounts for about 10% of global emissions today. Heat pumps will be the central technology used to cut heating’s climate impact, predicts Yannick Monschauer, an energy analyst at the International Energy Agency.
Heat pumps run using electricity from the grid. While fossil-fuel plants still help power grids around the world, renewables and low-carbon power sources also contribute. So with the current energy mix in all major markets, heat pumps are better for the climate than directly fossil-fuel-powered heating, Monschauer says.
Heat pumps’ real climate superpower is their efficiency. Heat pumps today can reach 300% to 400% efficiency or even higher, meaning they’re putting out three to four times as much energy in the form of heat as they’re using in electricity. For a space heater, the theoretical maximum would be 100% efficiency, and the best models today reach around 95% efficiency.
The gulf in efficiency between heat pumps and heaters comes down to how they work. Space heaters work by transforming energy from the form of electricity into another form, heat.
Heat pumps, on the other hand, aren’t turning electricity into heat—they’re using electricity to gather heat and move it around. It’s a subtle difference, but it basically means that a heat pump can return significantly more heat using the same amount of electricity.
A heat pump’s maximum efficiency will depend on the refrigerant and the system that’s installed, as well as the temperature difference between the room it’s heating and the outside.
Up-front costs for heat pumps are a major barrier to adoption: purchasing and installing a single unit today can cost between $3,000 and $6,000, and larger homes often require multiple units.
But over their lifetime of about 15 years, heat pumps are already cheaper to buy and operate than other systems for some consumers, especially if they’re used to both heat and cool a home during different parts of the year, Monschauer says.
And over 30 countries around the world have incentive programs for heat pumps, often with bonuses for low-income households or those purchasing high-efficiency equipment. Italy has especially generous subsidies for heat pumps that are installed when retrofitting buildings for energy efficiency, with customers getting up to 110% of the purchase price back as a tax credit.
In the US, the Inflation Reduction Act offers a 30% tax credit on the purchase price of a heat pump, with additional rebates for low- and moderate-income households. For some households, the funding could cover 100% of the cost. Rewiring America has a calculator to help people determine what IRA subsidies they qualify for.
While heat pumps are significantly better than they were a decade ago, there’s still plenty of potential growth ahead for the technology.
New designs, like self-contained window units from startup Gradient, could cut down on installation costs. Other companies, like Midea and LG, have also started offering small, portable units. These new options could allow heat pumps to break into new spaces, like older apartment buildings where installation might otherwise be expensive or impossible.
One ripe area for further progress is in refrigerants. While today’s refrigerants are an improvement over older options, even the newer ones are powerful greenhouse gases. Careful handling and precise manufacturing are required to avoid leaks. The climate benefits from heat pumps outweigh the warming potential of leaking refrigerants, but alternatives could help cut this risk further.
Gradient, for example, uses a refrigerant called R-32, which has a lower global warming potential than R-410A. Other classes of refrigerants, like the hydrocarbons propane and butane, pose even less climate risk. However, some of these more climate-friendly refrigerants tend to be extremely flammable, so safety systems are required.
New technological advances will help expand the already massive array of heat pumps on the market. And costs should come down over time as the technology becomes more common.
Global heat pump sales grew by 15% in 2021. Europe has seen some of the quickest growth, with 35% sales growth in 2021, a trend that’s likely to continue because of the energy crisis. North America still has the largest number of homes with heat pumps installed today, but China takes the prize for the most new sales.
Wherever you look, the era of the heat pump has officially begun.
Update: This story was updated to add additional information about the cost of a whole-home heat pump installation.
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