A Colorado Home Wins the Solar Decathlon, But Still Helps Cook the Planet

Air-source heat pumps perform well in cold climates. Why do they rely on a potent greenhouse gas?

Kristen Taddonio confers with the CU Boulder students working on the home they were constructing for her and her husband in Fraser, Colorado, which was the students' 2021 Solar Decathlon entry. Credit: Casey A. Cass/University of Colorado
Kristen Taddonio confers with the CU Boulder students working on the home they were constructing for her and her husband in Fraser, Colorado, which was the students' 2021 Solar Decathlon entry. Credit: Casey A. Cass/University of Colorado

Share this article

When Kristen Taddonio, a former EPA official and climate policy analyst, decided to build a clean energy home in the mountains of Colorado, she had mixed feelings about how to best heat it.

She and her husband wanted to use heat pumps that draw warmth out of the air outside the home because of the technology’s reputation as one of the cleanest, most energy efficient heat sources available. Using this rapidly emerging form of heating in Fraser, Colorado, with annual, average temperatures among the coldest in the contiguous U.S., would also serve as a proof of concept.

Until recently, air-source heat pumps were limited to more moderate climates. The devices performed poorly in extreme cold. However, recent advances meant the pumps, which use a fan to pull heat from outside air and a compressor to concentrate it before pumping it through the home, could now be used in even the coldest climates.

If air-source heat pumps could work in Fraser, near Colorado’s Winter Park ski resort, the technology could work anywhere, the couple reasoned.

Their home, which was designed and built by students at the University of Colorado Boulder, took first place in this year’s Solar Decathlon, a collegiate home-building competition run by the U.S. Department of Energy.


We deliver climate news to your inbox like nobody else. Every day or once a week, our original stories and digest of the web’s top headlines deliver the full story, for free.

U.S. Energy Secretary Jennifer Granholm praised the home during a virtual award ceremony in April, saying it “showcased energy efficient performance in its high-altitude cold climate.”

What Secretary Granholm did not mention is that the chemical refrigerants used in heat pumps are also cooking the planet. 

Devices sold in the United States rely on hydrofluorocarbon (HFC) 410a, a synthetic chemical refrigerant that is 4,260 times more potent as a greenhouse gas than carbon dioxide over the near term. As the refrigerant circulates between an outdoor compressor and heat exchangers within the home, a small amount of HFC-410a slowly leaks into the atmosphere. While the amount of gas they release is tiny compared to the exhaust from a gas furnace, over the years, those emissions add up.

The typical heat pump system will leak 12 pounds of HFC-410a over the course of its lifetime, according to a recent assessment by the California Public Utilities Commission. Those emissions, the majority of which come when the device is destroyed at the end of its useful life, equal the near-term climate impact of 23 metric tons of carbon dioxide. That’s the greenhouse gas emissions equivalent of burning 54 barrels of oil, or driving a car for five years, according to the EPA’s greenhouse gas equivalency calculator

“It’s critically important that we get rid of these chemicals because when they do get accidentally emitted they have a very potent warming impact,” said Taddonio, a senior climate and energy advisor with the Washington-based Institute for Governance and Sustainable Development. “If we continue using 410a in air conditioners and heat pumps it will produce an outsized climate harm.”

Interest in heat pumps has soared in recent years as cities and states from California to New England have proposed or enacted bans on natural gas hookups in new construction or incentivized heat pump installations. In areas where electricity primarily comes from fossil fuels, the majority of greenhouse gas emissions associated with heat pumps—roughly 80 percent—stems from electricity production. However, in places with clean electricity, the refrigerant’s contribution to the pumps’ total greenhouse gas emissions is higher. 

In most cases the total emissions associated with heat pumps are less than those that come from burning natural gas or other fossil fuels for heating. However, refrigerant emissions are still significant. Eliminating the use of HFC-410a and other HFCs with high global warming potential in all applications, including heat pumps, air conditioners and refrigeration, could prevent as much as 0.5 degrees Celsius of warming by the end of the century. 

The Climate Pros and Cons of Heat Pumps

Regulations approved In May by the Environmental Protection Agency could go a long way to help reduce those emissions by allowing the substitution of HFC-32, another hydrofluorocarbon refrigerant but one with significantly lower emissions, for HFC-410a.

“Finally we have the opportunity to catch up with the rest of the world,” said Taddonio, who has spent much of her career focusing on HFCs and other climate super-pollutants and knows better than most the outsized impact they have on global warming. “Most of the rest of the world uses heat pumps for heating and air conditioning needs and most leading countries have already allowed HFC-32 to be utilized in these systems.”

HFC-32 is still a potent greenhouse gas, but only half as bad as HFC-410a. It also does a better job of transferring heat than HFC-410a so less refrigerant is needed to provide the same amount of heating or cooling. The end result is that a heat pump running on HFC-32 packs only about one third the amount of warming from refrigerant emissions than one using HFC-410a.

“It’s a big decision,” said David Calabrese, a senior vice president for government affairs with Daikin U.S. Corporation. “We’ve known it all the time because we have been using it and selling it around the world but now to have it available here in the United States is a big move.”

Kristen Taddonio sits in her new home, which took first place in the U.S. Department of Energy Solar Decathlon Build Challenge as the entry from the University of Colorado Boulder. One of the "mini-splits' of the heat pump system that warms and cools the house is on the wall above her head. Photo Courtesy of Kristen Taddonio
Kristen Taddonio sits in her new home, which took first place in the U.S. Department of Energy Solar Decathlon Build Challenge as the entry from the University of Colorado Boulder. One of the “mini-splits’ of the heat pump system that warms and cools the house is on the wall above her head. Photo Courtesy of Kristen Taddonio

Daikin first petitioned the EPA to be able to use HFC-32, a synthetic chemical developed by the company, in heat pumps in 2011 and waited a decade for its approval. It belongs to a class of “mildly flammable” refrigerants and, as a result, had to go through a lengthy vetting process by UL, a private safety standards company that certifies the safety of thousands of different consumer goods. UL approved mildly flammable, or “A2L”, refrigerants, including HFC-32, for use in heat pumps in 2019.  

Just three days before the EPA’s approval of HFC-32 in 2021, the agency proposed a much broader rule that seeks to phase down HFCs starting with the worst climate super pollutants—chemicals, including HFC-410a, with a high “Global Warming Potential” (GWP).

“With the new federal law we are really on an inexorable path toward these lower GWP refrigerants,” Calabrese said. “I feel in many respects the tide has turned.”

Unexpected Resistance

But not every chemical manufacturer has been happy to see HFC-32 enter the market. Before it can be put to use in heat pumps the refrigerant still has to pass one additional hurdle, one that a competing business interest has thus far largely succeeded in blocking.

In addition to passing the UL safety standards and getting approval from the EPA, HFC-32 must also be approved for use in state and local building codes. The latest standards approved by UL and other safety standard organizations are typically written into “model codes”—universal building codes developed by professional organizations such as the International Code Council—before being adopted by government agencies.  

Model codes effectively determine what types of equipment and materials can and can’t be used in buildings nationwide and therefore have tremendous sway over the building, consumer appliance and chemical industries. The codes are hashed out about every three years in hearings that, while public, are highly technical and arcane.   

In 2019, chemical manufacturer Honeywell International appears to have been the driving force behind a successful effort to block the approval of refrigerants in the mildly flammable class in an update to the ICC’s International Mechanical code, the most widely used model code in the country focusing on heating, ventilation and air conditioning systems. 

During an unusually contentious hearing in Las Vegas, other manufacturers and UL representatives took issue with Honeywell’s efforts. Those who spoke accused Honeywell of stoking unwarranted fears of mildly flammable refrigerants in an attempt to corner the market for their own non-flammable alternative refrigerant, which the company is still in the process of developing.

“Usually, manufacturers are friendly and work together toward a common good and codes and standards,” Andrew Klein, a representative of chemical manufacturer Chemours, said. “But that’s not what’s happening right now.”

Federal regulations could soon require heating and air conditioning manufacturers to stop using refrigerants with high global warming potential, including the 410a that is currently used in heat pumps, though a phase-out date has not yet been set. If mildly flammable refrigerants are not approved in building codes, Honeywell would likely have the market to themselves.

During the hearing Honeywell representatives said more time was needed to ensure the safety of HFC-32 and other mildly flammable refrigerants. Other industry experts suggested this was a ploy by Honeywell to block HFC-32 and other mildly flammable refrigerants indefinitely. 

“How much time do we need?” Klein asked. “Enough time just for all (appliance) manufacturers to use a single refrigerant?”

The 2021 Solar Decathlon house entry for the University of Colorado Boulder being built in Fraser, Colorado. Credit: Casey A. Cass/University of Colorado
The 2021 Solar Decathlon house entry for the University of Colorado Boulder being built in Fraser, Colorado. Credit: Casey A. Cass/University of Colorado

Bill Koffel, a consultant speaking on behalf of the Air Conditioning, Heating and Refrigeration Institute, an industry group representing appliance and chemical manufacturers, including Honeywell, also questioned the company’s motives, noting that Honeywell sells mildly flammable, or “A2L,” refrigerants in Europe.

“You can go to the Honeywell website and they will tell you A2L is safe for use,” Koffel said in public testimony given during the hearing. “They market it in Europe. I have a slide presentation, where they specifically say that A2L refrigerants are difficult to ignite…. And yet they’re going to stand up here today and potentially tell you it’s not safe to use A2L in America. What’s the difference?”

Koffel did not respond to an interview request, however a slide presentation on a Honeywell Europe website does state that A2L refrigerants are “difficult to ignite.”

Honeywell supports the use of A2L refrigerants in residential heating, ventilation and air conditioning systems, Tehani Manochio, a spokesperson for the company, said in a written statement. 

“We support the adoption of A2L solutions in these applications and we believe that A2L solutions can be designed for use in these applications so long as the industry safety standards … and all of the building codes are responsibly updated for the protection of homeowners, first responders and end customers,” Manochio said.   

Representatives from UL, the safety standards company that spent close to a decade vetting the safety of A2L refrigerants, said they found the chemicals to be safe and urged ICC to allow their use.

“We’d like to see the most current updated standard in the codes,” said Guy Tomberlin, who at the time was the senior regulatory engineer for codes and regulatory services at UL, and is now a vice president at the ICC.

The ICC ultimately sided with Honeywell, effectively blocking HFC-32 and other mildly flammable refrigerants from inclusion in the model code approved in 2020. The ICC only revises the codes every three years, although they are sometimes approved faster, so A2Ls could have a long wait before they are included.

Keep Environmental Journalism Alive

ICN provides award-winning climate coverage free of charge and advertising. We rely on donations from readers like you to keep going.

Donate Now

While most states follow the ICC model codes, Washington State and Florida write their own building codes and have already approved the use of A2L refrigerants in heat pumps. Washington State approved their use in 2020, despite fire safety concerns expressed by Honeywell to the state’s building code council. California and Colorado are now considering taking a similar approach.

Honeywell declined to comment on whether it will oppose the new codes at a state-by-state level as it previously did in Washington and instead reiterated its support for lower Global Warming Potential (GWP) refrigerants as required under an international agreement known as the Kigali Amendment to the Montreal Protocol.

“The safe deployment of lower GWP alternatives is crucial to the industry’s collective commitment to meet the HFC phasedown targets that were agreed to in the Kigali Amendment to the Montreal Protocol, and Honeywell will continue to support all industry efforts to help the world meet ambitious goals for reducing greenhouse gas emissions,” Manochio said. 

A Climate-Friendly House Could Do Even Better

After weighing several options, including geothermal heat pumps that draw heat from the earth rather than the air, and radiant solar heating, the CU Boulder team decided to use air-source heat pumps for Taddonio and her husband’s home.  

The system kept their home comfortably warm through spring snowstorms after the couple moved in April. They recently had to run it in reverse to cool the building when temperatures climbed into the 80s, a rarity for the Fraser Valley.  

When installing the system earlier this year, the CU team took added steps to further reduce the high efficiency heating and cooling device’s climate impact. First and foremost, the team used solar panels to provide electricity for the devices, thereby eliminating 80 percent of the emissions typically associated with heat pumps. 

The well-insulated and well-sealed home uses three small “mini-split” heat pumps, a system that is collectively slightly smaller, and therefore uses slightly less refrigerant than the average heat pump system. 

The group also paid close attention to the siting of the devices’ compressors so that they could reduce the distance the refrigerant had to travel between the outside and inside of the home, thus limiting the amount of refrigerant they needed. 

The team also made a conscious decision not to use spray foam insulation to insulate the home. Such insulation has historically used chemicals that are highly potent greenhouse gases. The amount of chemicals used in insulation are typically far greater than the amount circulating in a heat pump system. 

Instead of using spray foam, the team chose a rather novel material: sheeps wool. 

“I don’t think it’s really meant for housing, although it worked pretty perfectly,” said Nathaniel Hottenstein, who graduated in May from CU Boulder after working on the project. The sheeps’ wool insulation they used came from a company based in New Zealand that markets its product for use in vans. “It is warm in there,” Hottenstein said. “You set it at a temperature and it stays that temperature….Compared to any blown in insulation that I have ever seen, it is exceptionally better.”

Taddonio also ran the numbers and, despite the use of a highly potent greenhouse gas as a refrigerant, the climate benefits of using high efficiency heat pumps run on rooftop solar, far outweighed alternatives such as heating with natural gas. 

Still she wishes her home could have used a more climate friendly refrigerant and says that the additional years it will likely take building codes in most states to approve their use is time we don’t have. 

“With the Arctic recording triple digit heat and Americans and citizens worldwide increasingly suffering from climate induced heat wave catastrophes, we don’t have more time,” Taddonio said. “If I could wave my magic wand and clear all the barriers that we needed to overnight, in order to hasten the transition to climate friendly alternatives to these terrible super pollutants, I would do it. And I’d hope any policymakers paying attention would consider the same.”