Image courtesy of Koolex
Charging heat pumps and air conditioners with propane refrigerant can help mitigate climate change and other environmental problems. That may seem scary, as propane is highly flammable, but with advanced technology we can use it safely.
Refrigerants flow through the innards of every air conditioner and heat pump. Their alternate evaporation and condensation move energy from cold to hot, thereby heating and cooling our homes and buildings. But as magical as the refrigeration cycle seems, refrigerants have a dark side. They leak into the atmosphere and contribute to climate change and other environmental problems.
We can mitigate those problems by charging heat pumps and air conditioners with propane. Unlike current refrigerants—which are composed of hydrogen, fluorine, and carbon—propane contains only hydrogen and carbon. It breaks down rapidly in the atmosphere into naturally occurring chemicals, and contributes little to climate change and not at all to fluorine pollution.
Other benefits include that propane is efficient, inexpensive, and widely available. Given all these benefits, we’re already moving, in the US and globally, down the path towards making propane the standard refrigerant for heat pumps and air conditioners.
There’s only one major problem with propane refrigerant, and it’s a doozy: it’s highly flammable. In the US, we haven’t used flammable refrigerants in homes for nearly a century. Using propane refrigerant sounds scary, and there will undoubtedly be people who stoke those fears by presenting out-of-context information. Don’t forget that climate change and fluorine pollution also expose us to uncertainty and hazards.
Even though propane’s safety record is not perfect, there are numerous reasons why you can be excited about its potential and confident it will be used safely. It’s already being used in a variety of market niches in the US and around the globe. US regulators are moving cautiously and incrementally. Documented incidents involving injuries and property damage are rare. Advanced technology, much of which already exists, will help to assure safety. Rigorous technician training and certification will help avoid some of the biggest risks.
If you want to support the transition to propane refrigerant, one thing you can’t currently do in the US is buy a propane-charged heat pump or air conditioner. But, you can support the Environmental Investigation Agency, a non-profit organization that advocates for propane by interacting with government agencies and informing the public.
Propane returns
In the late 19th and early 20th century, propane was a popular refrigerant, largely used in industrial ice making machines. At the time, the home refrigerator industry was in its infancy and heat pumps weren’t yet manufactured. The refrigeration systems back then were not especially leakproof. While there are documented cases from that era of fires being caused by leaking refrigerants, it’s not clear how many were caused by propane.
Noxious odors from refrigerant leaks may have been a more widespread problem, given that two of the main competing refrigerants were ammonia and sulphur dioxide. Unlike those refrigerants, propane doesn’t irritate eyes and bronchial passageways. Indeed, in 1922, an advertisement in Ice and Refrigeration magazine, touted propane as “The Odorless Safety Refrigerant.” That wasn’t enough to make propane a major player in the nascent home refrigerator market.
The Frigidaire company’s search for a nontoxic and nonflammable refrigerant was a major factor driving the invention of chlorofluorocarbon refrigerants in 1928. Sold under the trade name Freon, these chemicals—composed of chlorine, fluorine, and carbon—came to dominate the refrigeration and air conditioning industries for over half a century. During Freon’s reign, most US residential building codes prohibited propane refrigerant and it largely disappeared from the the market.
The damage Freon did to the ozone layer came to be its undoing. Its eventual banning began with the signing of the Montreal Protocol in 1987. Freon was the first refrigerant banned for environmental reasons, but certainly not the last. It took less than 30 years until the next international agreement on refrigerants, which resulted in the EPA banning the chemicals that replaced Freon because of their impact on the climate. It will likely take even fewer years for the latest refrigerant generation to be banned.
With each of these successive waves of refrigerant introduction and subsequent banning, propane has made its way into more applications. Eventually, after a few more rounds, it may be the only refrigerant left to be used in heat pumps and air conditioners.
Problem with current refrigerants #1: climate change
The main reason why current refrigerants will get banned is that they contribute too much to climate change. In theory, that shouldn’t be a problem. Heating and air conditioning technicians are supposed to install systems that don’t leak, and when those systems reach the end of their life, technicians are required by law to capture and recycle the refrigerants. In reality, these systems leak more, and technicians capture far less used refrigerant, than they should. Perhaps 3% to 4% of air conditioner and heat pump refrigerants leak annually, on average. When you consider that there are over a billion air conditioners worldwide, that adds up to a lot of leaked refrigerant.
Once they leak into the atmosphere, the impact refrigerants have on the climate is typically expressed using the term Global Warming Potential, or GWP for short. It expresses how much more a refrigerant warms the climate than does carbon dioxide. Carbon dioxide features a GWP of 1. A refrigerant with a GWP of 1,000 would, in theory, be about a thousand times more potent greenhouse gas than carbon dioxide.
Starting January 1, 2025, the EPA will require all air conditioner and heat pump refrigerants to feature a GWP of 700 or less. That’s about 1/3rd the GWP of the current standard refrigerant (R-410A). While the EPA’s new limit is certainly an improvement, it’s likely the Agency will eventually lower it. According to the EPA, propane’s GWP is 3, so it’s unlikely to ever be banned on that basis. By the way, the EPA’s regulations only apply to newly installed systems. The Agency doesn’t require the refrigerants in existing systems be changed out, and the new refrigerants can’t be used in older systems.
How much would switching to propane reduce refrigerant’s climate impact? A team of scientists, mostly from the International Institute for Applied Systems Analysis, an Austria based research organization that supports sustainable development public policies, explored this question. They modeled the global temperature impact of three different scenarios: 1) Continue using the current standard refrigerant, R-410A, in all newly manufactured split air conditioning units worldwide. 2) Switch over those same units to R-32, the most common low-GWP refrigerant replacing R-410A. 3) Switch over to propane. They concluded that switching to propane would avoid three times as much temperature rise by the end of the century, as switching to R-32. They also concluded that “propane as a refrigerant can play a key role in creating a more sustainable split AC sector.”
Problem with current refrigerants #2: fluorine
Refrigerants may get caught up in the international drive to ban some fluorine-bearing chemicals. The technical name for these chemicals is per- and polyfluoroalkyl substances, but they’re better known by their abbreviation, PFAS. They’re also known as forever chemicals, as they can persist for a long time in the environment. They’ve been linked to a lot of adverse health effects, including increased cancers and decreased fertility.
Some fluorine-bearing refrigerants are characterized as PFAS, and some of them break down in the environment into chemicals that some scientists characterize as PFAS. For example, one recently formulated refrigerant that will soon be used in heat pumps breaks down into trifluoroacetic acid, also known as TFA. This acid accumulates in waterways and gets into drinking water supplies. Even at low concentrations, TFA is potentially harmful to human health and aquatic life.
Several research teams recently found concerning levels of TFA in drinking water and elsewhere in the global environment. The chemical and HVAC industries don’t dispute these findings, but they do dispute the severity of the threat posed by TFA. One research team, financed by an industry trade group, concluded that “with the current knowledge of the effects of TFA on humans and ecosystems, the projected emissions through 2040 would not be detrimental.”
While the debate over TFA rages, it looks like the first regulatory body to take action on refrigerant-related fluorine pollution could be the European Union. The European Parliament recently forged an informal agreement that would ban all fluorine-bearing refrigerants from being used in newly-manufactured split air conditioners and heat pumps by 2035. It still needs to go through a formal approval process before it passes into law.
In the US, the EPA is developing its own PFAS Strategic Roadmap which will set timelines and specific actions the agency plans to take in this area. The Roadmap doesn’t include refrigerants, but lots of scientists and environmentalists are calling for the agency to expand its scope.
Seven reasons to expect a safe transition to propane
How concerned should you be if one day in the future you find yourself in a house or building with propane-charged equipment? Not much, and here’s why.
1. Propane refrigerant is already being used in equipment around the globe
Propane charged air conditioners are currently sold in the UK, Germany, India, and China. In the US, it’s used in small commercial units like beverage coolers, frozen drink machines, ice makers, and ice cream freezers. Most refrigerators now come charged with isobutane, which, like propane, is a hydrocarbon and highly flammable. You may have a flammable refrigerant in your home without knowing it. As all this equipment is being used, the industry is learning how to work with propane ever more safely.
2. US regulators are moving cautiously and incrementally
What makes refrigerants exhibit low GWP is that they react with oxygen and break down quickly, should they ever leak into the atmosphere. That same ability to react with oxygen also makes them flammable. When US regulators realized they couldn’t reduce the climate impact of refrigerants without increasing their flammability, they decided to compromise. The latest refrigerants, approved for US heat pumps and air conditioners starting in 2025, only moderately reduce GWP, and are only slightly flammable. These refrigerants can be ignited, but only under limited circumstances, and only in the presence of a high energy flame, like a candle or cigarette lighter. A hair dryer, toaster, or electric space heater won’t do it. Over time, as the domestic industry gains experience with these slightly-flammable refrigerants, regulators will likely decrease GWP limits, and increase flammability, incrementally.
3. Documented incidents involving injuries and property damage are rare
In the modern era, starting with the Montreal Protocol, there have been some incidents of fires and explosions caused by propane refrigerant. I’ve only seen less than a half-dozen incidents documented, although I’m sure there are more. My sense is that the number of incidents is minuscule compared to the overall number of systems installed.
The most serious accident took place at a refrigerated warehouse in New Zealand in 2008, when a propane refrigerant explosion killed one fireman and injured another 7. In another accident, a service technician working on a system designed for a different refrigerant, didn’t realize it contained propane, and set off an explosion. In nearly all the documented incidents I’ve seen, standard safety measures weren’t utilized, service personnel ignored labeling, or untrained personnel didn’t follow safety protocols. All of these factors seem preventable with rigorous training and certification.
4. Charge limits help control risk
The smaller the amount of propane contained within a heat pump, the less damage a leak can cause. To manage the risks associated with flammable refrigerants, regulatory bodies often limit the amount of refrigerant that a particular device can be charged with. Typically, such charge limits are based on the amount of occupied space and the equipment serving it.
In the US, charge limits are set by the EPA, usually in conjunction with other standard setting organizations. Currently, the Agency only approves propane for use in self-contained products, like window air conditioners. For these products, the EPA limits the amount of refrigerant they may contain by a complex formula. Allowable charge amounts vary depending on cooling capacity and equipment type, but the highest charge allowed is 1 kg. A typical gas grill tank holds as much as 9 kg.
To my knowledge, no one manufactures propane-charged self-contained units for the US market. Even if they did, it’s unlikely US consumers would heat and cool entire homes using them. In the US market, consumers prefer central and mini- split systems. For US manufacturers to produce propane charged versions of such equipment, the EPA would have to approve using propane for split systems and set charge limits. Indeed, environmental organizations are advocating for both of these actions.
One problem with charge limits is that because they’re based on safety, they can be set so low that they limit equipment efficiency. It doesn’t help to mitigate climate change if equipment consumes more electricity, which then releases more carbon dioxide emissions. It would make it more feasible for equipment manufacturers to meet charge limits with high efficiency if new technology enabled heat pumps to operate with less refrigerant.
Researchers working at the Fraunhofer Institute, a German organization focused on technological innovation, recently demonstrated in the lab a 3½ ton propane charged heat pump containing about one-fifth the amount of refrigerant of commercially available systems. To achieve these impressive results, the research team evaluated more than 20 different combinations of heat exchangers and compressors. It remains to be seen if the technology developed by this project is picked up by equipment manufacturers.
5. Some systems put the entire propane charge outdoors
One way to protect building occupants is to place the entire refrigerant charge outdoors, contained within an air-to-water heat pump. Heat or coolth is transferred from the heat pump to the building via pipes containing either water or an antifreeze solution. Energy is exchanged between the water (or antifreeze) and occupied space using radiant floors, radiators, or fan-coil units. This will probably be the first way propane heat pumps are approved for use in the US. The main drawback is that it adds an additional heat exchanger, which adds expense and reduces efficiency.
Even though this format seems safe enough, given the profusion of gas grills located outside US homes, it still isn’t allowed by the EPA, but that could soon change. In September 2023, ASHRAE, the standard setting association for HVAC engineers, issued a proposal for public review to allow outdoor heat pumps to be charged with up to 4.9 kg of propane. If ASHRAE approves this proposal, the EPA probably will enact a similar regulation.
6. Additional safety features may be required
For some systems, with equipment inside buildings, regulators may want more safety than they can get from charge limits alone. At a minimum, labels and signs will be required that make it clear to service technicians which equipment and pipes contain propane. Additionally, regulations often require propane equipment be located a specified distance from ignition sources or completely isolated from open flames and hot surfaces. They may also require special electrical boxes that isolate electrical circuits to ensure that leaking refrigerant doesn’t come in contact with sparks or hot wires.
Another safety feature typically installed with propane systems is a refrigerant leak detector. When indoor refrigerant concentrations get to one-fourth of the level at which they’re problematic, these sensors set off alarms and trigger safety systems. Even with these detectors, avoiding leaks is critical, so special materials and techniques are required for pipes and joints, as well as rigorous field testing for newly installed systems.
Some regulations require plastic blades for air distribution fans. These blades are typically made of metal but could potentially cause sparks if they rubbed against housing or cowling. Those sparks would be problematic if leaking refrigerant came in contact with them.
Research in this area is ongoing, and future systems may well require additional features yet to be developed.
7. Rigorous technician training and certification for technicians will be implemented
To ensure that safety protocols are followed impeccably, there will have to be a rigorous system of training and certifying installers and service personnel. Also, they’ll have to be well trained in service and decommissioning, both of which are especially vulnerable to accidents.
Looking at this list, two things are apparent. First, even though propane is inexpensive, installing safety features will drive up the cost of air conditioners and heat pumps. Second, even with the most rigorous training and inspections, all these features and protocols will not be perfectly enacted on every installation. Accidents will likely be rare, but they will not be completely eliminated. Then again, climate change and fluorine pollution aren’t risk free either.
Want to help?
If you like the idea of heat pumps and air conditioners charged with propane, and would like to help make them available in the US, there is a way for you to get involved. Support the Environmental Investigation Agency. Founded in 1982 by UK anti-whaling activists, it’s now a worldwide non-profit organization devoted to investigating and campaigning against environmental threats.
EIA’s climate division does a ton of work in the refrigerant area. To bring attention to how much grocery store refrigeration systems are leaking, the group sent activists into stores armed with refrigerant leak detectors. In addition to investigations, EIA produces reports on policies to mitigate refrigerant-related climate change and other pollution. It also lobbies the EPA and other major US standard organizations to support those policies. When the EPA was recently considering which policies to enact to transition to lower-GWP refrigerants, EIA organized a joint petition on behalf of several environmental groups that was influential within the agency.
EIA has long advocated for regulations accepting propane refrigerant. Its US division is a registered 501(c)(3), and accepts donations.
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