We can expect material improvement of heat pump efficiency over the next few years. Improvements likely to reach the market within two years will involve a shift in refrigerants that will improve conventional air-to-air heat pump efficiency by 10% or more, while reducing the climate risks of refrigerant leaks. From an environmental perspective, it may make sense for some homeowners, especially those who heat with gas and who do not have an immediate need to replace their heating system, to wait for the next generation of heat pumps. During that pause, consumers could focus on improving the insulation and general energy efficiency of their homes. Longer term improvements are harder to project, but we can hope for air source heat pumps capable of heating water hot enough to use in existing home radiators.
Evolving regulation of refrigerants
Heat pumps cycle a refrigerant fluid through compression and expansion to move heat from a cooler place to a warmer place. The characteristics of an ideal refrigerant fluid include:
- Thermodynamic properties that favor efficient heat pump operation
- Not too harmful to the ozone layer
- Not too harmful as a greenhouse gas
Heat pumps have been around for a century in various applications — air conditioners, chillers, refrigerators, ice machines. The set of chemicals available to use as refrigerants has been well studied. The last few decades of changes in refrigerants has reflected evolving public policy more than chemical innovation.
Since the world banned ozone-depleting refrigerants, the refrigerant used most commonly in heat pumps in the United States has been a chemical mix known as R-410A. See this note for an explanation of the numbering system used to identify refrigerant chemicals in the hydrofluorocarbon or “HFC” family.
R-410A, like other HFCs is a very potent greenhouse gas. The potency of a greenhouse gas is commonly measured by comparing it to carbon dioxide which is deemed to have a “Global Warming Potential” of one. R-410A has a global warming potential of approximately 2,000, meaning that each pound of R-410A released to the atmosphere does roughly as much warming damage as a ton of carbon dioxide.
Refrigerant leaks are an important source of environmental damage and the EPA is engaged in rule-making which will effectively ban sales of heat pumps using R-410A after January 1, 2026 (banning new manufacture even sooner, by January 1, 2025). The specific terms of the ban will prohibit the use in heat pumps of HFCs that have a global warming potential (GWP) more than 700 fold greater than carbon dioxide. For mixes of HFCs, the GWP will be computed for the mix, not for the individual components. The rules will not apply to sales of used heat pumps (for example, those sold with a home). (For more technical background see Docket EPA-HQ-OAR-2021-0643-0066 at Regulations.gov.
EPA tailored its proposed limits to reflect the availability of substitutes. There are ready alternatives to R-410A, notably R-32, which has a GWP of 675 and so would remain legal under the new rule. R-32 is already heavily used elsewhere in the world (for roughly a decade in Japan) and many manufacturers see it as the successor to R-410. For examples, see these posts by Mitsubishi, LG, Carrier, Fujitsu, and Daikin North America. See also general discussions from achrnews.com (2019), achrnews.com (2020), and iccsafe.com (2022), all of which predate EPA’s latest rulemaking that would mandate new refrigerants — regulatory changes are moving rapidly.
The final rule is projected for release in September 2023.
Required building code changes
Ironically, R-32, the likely successor to R-410A is one of the two ingredients in R-410A. The other ingredient, R-125, is a so-so refrigerant and an especially bad GHG but a good flame retardant. The mixture of the two compounds makes an adequately-performing, non-flammable refrigerant which has an unacceptably high GWP.
Refrigerants are classified according to toxicity and flammability. For residential applications, we will want to continue to require non-toxicity (toxicity class A). But to allow pure R-32 and other lower-GWP alternatives to R-410A, we will need to accept refrigerants in a higher flammability class moving up from flammability class 1 (non-flammable) to flammability class 2L (mildly flammable). See also see this general discussion of refrigerant safety standards from the the EPA.
Moving to “A2L” — non-toxic but mildly flammable — refrigerants will require modifications to building codes.
EPA anticipates that states will adopt the 2021 revised versions of the International Building Code and the Residential Building Code that allows for use of several lower-GWP refrigerants that exhibit lower flammability (2L flammability classification). EPA understands that by 2025 building codes may be updated or updates will be under consideration which is relevant for some but not all of the potential lower-GWP HFC refrigerants and other non-HFC substitutes.Federal Register, 12/25/2022
These code changes are likely to involve requirements for leak detection, circulating fans, labeling and handling instructions. Additionally, the new rules may limit the ratio of heat pump refrigerant charge (weight of refrigerant contained) to the size of the space it could leak into — one wants to avoid the possibility that a full leak could create an air mix with a high enough concentration of mildly flammable refrigerant to ignite. For a survey of code changes, see this EPA technical support document on building codes.
Legislation may be necessary in some states, including Massachusetts, to assure that delays in state building code changes do not interrupt heat pump installations.
Consumer Consequences of Regulatory Changes
Overall, these regulatory changes and related industry adjustments to heat pump products appear to be good for consumers. As manufacturers move to A2L refrigerants like R-32, we can expect an improvement in heat pump performance, perhaps 10% greater efficiency, meaning less electricity use, lower operating costs, and greater net environmental benefits. A quick survey suggests a range of results — ultimately, the performance improvements achieved will depend on careful engineering by the major competing manufacturers.
|Source||Relative efficiency of R-32 heat pumps||Comparison Base|
|International Journal of Refrigeration||9% higher coefficient of performance||similar R-410A system|
|International Refrigeration and Air Conditioning Conference at Purdue||12% higher performance||R-410A in air to water heat pump|
|International Institute of Refrigeration||2% higher COP||R-410A|
|LG||more efficient||other refrigerants|
|Daikin||up to 12% more efficient||R-410A heat pumps|
|Ex-machinery (a Daikin affiliated installer)||20% more efficient||R-410A heat pumps|
|Building Safety Journal||more efficient (A2L generally)||refrigerants they replace|
|ACHR News||over 8% more efficient||refrigerants they replace|
The previous policy choice of less-efficient, non-flammable refrigerants was driven by safety considerations, but the consensus seems to be that we can address safety considerations with appropriate controls and rules. Consider that the new A2L refrigerants are much less flammable than methane and propane both of which we burn routinely within homes. Unlike methane and propane in homes, the refrigerants will not be routinely exposed to flame.
The new heat pumps will pose much smaller risk of leaks that harm the environment. They will contain less total refrigerant, they will likely be equipped with leak detection to comply with new safety codes, and the refrigerants used will have much lower GWP than current refrigerants.
For people who already have R-410A refrigerant pumps, the change does not create an imminent problem. However, eventually, R-410A may become scarcer and more expensive to acquire for the purpose of replacing leaked refrigerant. Note that one cannot retrofit R-32 into R410-A pumps. Ten years from now, if an R410-A pump leaks, an HVAC installer may suggest replacing the pump early rather than recharging the pump. HVAC industry observers see the switch as generating business for HVAC installers.
The likely 10% efficiency improvement expected from the heat pumps to be available soon will result in a considerably larger percentage increase in net greenhouse gas reductions in heat pump conversions from natural gas, where the difference between the carbon saved from reducing fossil fuel and the increased carbon costs of electricity generation is relatively small. For those conversions, the lifetime net GHG savings from a more efficient pump would likely exceed the GHG costs of a couple of years of delay. Those consumers who heat with gas who do not have an immediate need to replace a heating system may wish to wait for the better heat pumps. (For more math on this point, consider the simplistic pro forma at page 18 of this overview powerpoint: If, with a better refrigerant, the sample heat pump SCOP were increased to 2.5 from a currently typical 2.3 , this would pencil through to increase the net annual GHG benefits by 18%.)
The Longer View
Looking beyond the current generation and the next generation of heat pumps, there is room for further improvement. New propane-based pumps may offer even higher efficiency. See for example, Fraunhofer, Mitsubishi, Swedish Royal Institute of Technology. Of course, propane is highly flammable and it may not make sense to pressurize it indoors. However, we may eventually see safe, factory-sealed propane heat pumps that do not circulate refrigerant to indoor units. They will do all their heat pumping outside, heating a reserve of water or glycol which is then circulated indoors, conceivably through radiators. This would solve a huge problem for older New England homes that have hot water heat.