Stephen Cass: Hello and welcome to Fixing the Future, an IEEE Spectrum podcast where we look at concrete solutions to tough problems. I’m your host, Stephen Cass, a senior editor at IEEE Spectrum. And before we start, I just want to tell you that you can get the latest coverage from some of Spectrum‘s most important beats, including AI, climate change, and robotics, by signing up for one of our newsletters. These are free, and you just have to go to to subscribe.

Heat pumps don’t have a reputation as being a particularly glamorous technology. They sort of act like a combination of an air conditioner and a refrigerator, pumping heat out of a home in the summer and pumping it back inside during the winter. But governments around the world increasingly see heat pumps as a chance to make some big improvements in energy efficiency, and some recent technological developments could expand dramatically the number of homes that could employ heat pumps. Here to talk about those developments today, we have Spectrum’s new power and energy editor, Emily Waltz, who recently joined the staff after many years contributing to Spectrum as a freelance writer. So Emily, welcome to Spectrum and welcome to Fixing the Future.

Emily Waltz: Thanks. I’m glad to be here.

Cass: So first off, when we talk about heat pumps, I think one of two pictures form in people’s minds. One is a geothermal system in which pipes are buried in the ground outside a home and the ground acts as a heat reservoir where you can dump heat during the summer and then you extract it during the winter. But today we’re going to be focusing on the other type of heat pump, the air source heat pump. Can you sketch out how that works?

Waltz: Yeah. So what’s great about heat pumps is that they transfer heat rather than generate it. And that’s part of what makes them more energy-efficient than other sources of heating. They can both heat and cool a home. And I’ll describe how they work in heating mode. So in heating mode, what they do is they pull ambient heat from outside air and compress it and then release it into the home. And there’s an outdoor unit, which from the exterior looks like a big box with a fan. And then there’s some connection lines and then an indoor unit. And so what happens is the air gets drawn into the system in the outdoor unit. It passes over a heat exchanger, which contains a refrigerant that has a very low boiling point. So the most common refrigerant is called R410A, and it has a boiling point at about negative 48 degrees Celsius. So it may be 0 degrees outside, but when that air passes over the refrigerant, the refrigerant boils. So the refrigerant boils, and then it evaporates into a vapor. And then the compressor increases the temperature and pressure so that it becomes this superheated vapor. And the superheated vapor moves to an indoor unit and goes over through a set of coils. And there a fan blows across it, and it moves the heat into the home. So the heat is distributed through the home, usually through ductwork, but there are ways to do it without ductwork too. And then in the summer, the system works in reverse. It pulls warm air out of the home and moves in cooler air.

Cass: And so what kind of homes are suitable for hit heat pumps? I mean, obviously, you need some land for geothermal heat pump because we talk about burying things, but this seems to be able to work on a much smaller footprint in homes in denser areas.

Waltz: Yes, that’s right. So as you mentioned, the ground source or geothermal heat pumps, they do require quite a bit of land. But the air source heat pumps, just a small outdoor space is needed. These can be installed, obviously, standalone homes, but also townhomes, apartment buildings, and even high-rises. There are ways to make it work. I know that the outdoor units are frequently installed on roofs and on balconies.

Cass: So what kind of energy savings kind of typical homeowner gain from installing a heat pump?

Waltz: Yeah. There was a good study published on this last month in the journal Joule. They looked at 550,000 homes that are representative of the entire housing stock in the US. And they looked at both energy use and then energy bills. And the study found that if every home in the United States switched to a heat pump, home energy use, that is the residential sector, would drop by 31 to 47 percent on average. And that national carbon dioxide emissions would fall by 5 to 9 percent overall. So that’s pretty good. But the reductions depend on what kind of heating system is being replaced, how well the home is sealed up and insulated, and whether the home’s electricity comes from renewable sources. So they found that emissions reductions are highest when replacing a fuel oil heating system. But whether that will translate into lowering a home’s heating bill is widely variable. And it depends a lot on what kind of heat pump is installed, so whether it’s a high-efficiency heat pump or a low-efficiency, so a newer one or an older one, and then what kind of heat’s being replaced and whether the home had previously had air conditioning. But bottom line, what they found is if replacing fuel oil or propane for those homes, 87 to 100 percent of those homes would see a reduction in their energy bill. That percentage is smaller for natural gas and electric resistance heating.

Cass: Wow. That’s still considerable. And this idea, how many homes can this be used in? And this is where I want to turn out to recent developments. So you recently published this terrific story for us, which will be linked to in the show notes, titled “Heat Pumps Take On Cold Climates”. Can you tell us why heat pumps up to now haven’t fared well in cold climates? And what’s the key new advance that’s changing that?

Waltz: Yeah. Yeah. So most air source heat pumps on the market currently work pretty well until the outdoor temperature gets to about 4 degrees Celsius, which is 40 degrees Fahrenheit. Colder than that, they still work, but they’re often operating at less than full capacity. So when the temperature gets down to about negative 15 degrees Celsius, which is 5 degrees Fahrenheit, they stop doing their job. And they switch over to emergency heating mode, which is an all-electric resistance heating. But that’s what’s currently available, and that’s changing. And one of the key advances has been in optimizing how the compressor works in concert with the rest of the system. So that includes controlling the compressor motor speed, improving the timing when the vapor is injected into the compressor. So heat pump manufacturers have been playing with these cycles to optimize them. And it sounds like they finally got it sorted. One manufacturer I spoke with, Trane Technologies, they found that if they inject refrigerant at just the right time, right when the system begins to lose its capacity to heat, it gives it the boost it needs. So that’s been the main advancement. And there’s also technology that improves the way that the indoor and outdoor units communicate with each other and with a thermostat that optimizes the system.

Cass: And this was kind of demonstrated in a big test recently, wasn’t it?

Waltz: It was. The Department of Energy has set up this challenge. The goal is to get cold climate heat pumps working efficiently at full capacity at negative 15 degrees Celsius and even down as low as negative 26 degrees Celsius. So the agency law launched a challenge to inspire companies to achieve that. There are eight companies competing in it, and they’re in the middle of field testing that right now.

Cass: And where are those field test tests taking place? Do you know?

Waltz: Yes. They are in several US states, mostly northern states, and in a couple of Canadian provinces.

Cass: So how long before we might see these cold weather pumps hit the market?

Waltz: Yeah. It depends partly on how you define cold-climate heat pumps. The ones we’re talking about that are in this DOE challenge, I think we’ll see them next year. Both the Department of Energy and training representatives I spoke to at those places both said, “We should see this in the market by next year.” But it’s important to remember that there is a big upfront cost to installing these. So widespread adoption will probably require government incentives and some good marketing.

Cass: You know, with all these great results coming out from these DOE trials and so on, what kind of incentives is the US putting toward heat pumps??

Waltz: Right so the US is putting some pretty good incentives toward it. The Federal government offers tax credits and states will be rolling out rebates to offset the cost of installations which is very very high. In the systems I’ve seen its 10 to 20 thousand to install these things. We’ve also seen 9 US states, last month they pledged to accelerate heat pump sales and then 25 governors have vowed to quadruple heart pump sales, so there is an all-out effort in the US to make this happen and it seems to be working so far, cause heat pumps outsold gasoline furnaces for the second year in a row last year.

Cass: So you mentioned some pretty impressive figures there for things like reducing climate emissions and so on. And yes, it depends on what you’re switching from. But why are they so much better than conventional HVAC systems? Is this related to the electrification of everything?

Waltz: Yeah. So it’s partly because they run on electricity rather than fossil fuels. But it’s also because they transfer heat rather than generate it. So I mean, there is all electric heating, but heat pumps are different. So with electric resistance heating an electric current passes through conductive materials and releases heat. But with heat pumps, they’re powered by electricity. They’re plugged in. But the electricity powers equipment that enables it to transfer and concentrate heat. So they’re more efficient than all-electric. So it’s a combination of those things and the fact that it’s not relying on fossil fuels.

Cass: But is there a danger that all the advantages we could gain from heat pumps will be wiped out depending on how the electricity is generated? Does this really have to go hand-in-glove with renewables to see these advantages? Or is this something that even if you aren’t changing your generation profile, you’re still going to see some advantages?

Waltz: Right. I think you’ll still see advantages. I mean, if electricity comes from renewable energy, then that’s a bonus. But these are so much more energy efficient that even if they don’t come– even if you’re not powered by renewables, it’s still an advantage.

Cass: And Europe seems to be very interested in heat pumps as well. Why is that?

Waltz: Yeah. So Russia’s gas exports to Europe have fallen sharply because of the tensions over Ukraine over the last couple of years. And so Europe is pushing pretty hard for people to replace their gas heating systems with heat pumps. The European Commission has called for expedited deployment of heat pumps, and they also recommended that member states phase out the use of fossil fuel heating systems in all buildings by 2035. And so we’re seeing many European countries subsidizing residential heat pump installation and offering grants to homeowners. Yeah. So we’re seeing a pretty hard push in Europe.

Cass: I just want to talk then, just to come back to geothermal heat pumps, it’s still the case though that if you have the chance, the geothermal— if you have the ground, I guess, basically, the geothermal system is more efficient than these air source heat pumps in an ideal kind of world.

Waltz: Yes. Especially if you live in a very cold climate because underground is going to maintain a more consistent temperature. And so the source of the heat that’s coming in is already warmer. So yes, they can be more efficient. They just require a lot of land. I was looking at one commercial developer and they were sketching out what that might look like in a home. And it looked like it was almost probably a quarter of an acre that it took up. And they have to dig up trenches. And I mean, your yard, your garden is all dug up. But I love the idea of it. I do have some land and I was thinking about doing it myself.

Cass: Well, you’ll have to let us know how that goes and maybe give us a peek into how your bills have been going. Well, that is all fascinating, but I’m afraid we’ll have to leave it there. But thanks very much, Emily, for coming on and making your first appearance on Fixing the Future.

Waltz: Well, thank you. I enjoyed it.

Cass: So today we were talking with Emily Waltz about cold climate heat pumps. For IEEE Spectrum, I’m Stephen Cass, and I hope you’ll join us next time.

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