A2L Refrigerants: Safely Addressing Flammability Concerns
Refrigerants used in air conditioners are changing … again. The last round of changes took place between the mid-1990s through 2010, as chlorofluorocarbon (CFC) and hydrochlorofluorocarbon (HCFC) refrigerants were phased out and largely replaced by hydrofluorocarbons (HFCs). This transition was a global effort under an international treaty known as the Montreal Protocol that addressed the destructive effects these compounds had on the ozone layer. Periodic measurements have confirmed that, since implementing those refrigerant changes, there has been a steady decrease of ozone-depleting substances in the atmosphere. The current round of changes stems from the Kigali agreement to the Montreal Protocol, and it aims to reduce the greenhouse effect of refrigerants.
This article will focus on the class of refrigerants that will replace R-410A, the most commonly used refrigerant in air conditioners and heat pumps installed today. R-410A was the primary replacement for R-22 during the last transition. While not ozone-depleting, R-410A has a higher global warming potential (GWP) than R-22, so its environmental impact rules it out as a long-term refrigerant solution. Similar concerns apply to other refrigerants currently in use for applications involving cooling or refrigeration but are beyond the scope of this article.
Drivers of Change
Though the U.S. has not ratified the Kigali Amendment, and no federal policy or regulation exists at this time to reduce the usage of higher-GWP refrigerants, several states are moving ahead in the absence of nationwide action. Figure 1 summarizes the status as of July 2019, though it’s rapidly evolving. California is leading the activity with a rulemaking process already underway. Proposed is a limit of GWP 100 < 750 for refrigerants in new air conditioners, effective Jan. 1, 2023. Other states, like Washington, New York, Vermont, and Maryland, have indicated they will begin a rulemaking process through their state agencies in the near future. These states are all members of the U.S. Climate Alliance, a coalition of states that share a common goal of reducing greenhouse gas emissions. Of the 25 members, 14 states have committed to address short-lived climate pollutants (SLCPs) but have not yet begun the legislative or regulatory process.
Scientists and researchers have conducted exhaustive searches for substances suitable for use as refrigerants. Suitability is measured by many factors including:
• Air conditioner cycle energy efficiency;
• Total cost impact of both refrigerant and equipment;
• Safety classification considerations such as flammability and toxicity;
• Environmental considerations such as ozone depleting potential (ODP) and GWP;
• Design and reliability considerations, such as chemical compatibility and stability; and
• Life cycle considerations, including recycling and reclaiming of used refrigerant.
The conclusion of these studies is that there is no perfect refrigerant that meets all the desirable qualities, and tradeoffs must be made to find an acceptable balance. Part of the tradeoff problem is the chemistry of refrigerants. Refrigerant molecules with a reduced number of fluorine atoms generally have lower GWPs, but they have a higher proportion of hydrogen atoms and are therefore more flammable than refrigerants with more fluorine atoms. Many of the candidates for lower GWP refrigerants are blends of two or more refrigerant molecules — such as a hydrofluoroolefin (HFO) mixed with an HFC — a technique resulting from searching for the right combination of desirable qualities. R-410A itself is a blend of R-32 and R-125, which was intended to create a zero-ODP, nonflammable refrigerant with energy efficiency equal or better than R-22. However, with a GWP100 value of 2090, R-410A is no longer considered an acceptable compromise of qualities.
Transitioning to low-GWP refrigerants doesn’t mean compromising on safety. The leading lower GWP candidates to replace R-410A are flammable, but they fall into the relatively new lower-flammability classification of 2L. As depicted in Figure 2, Class 2L refrigerants have lower probability of ignition as well as lower severity of ignition events, significantly reducing the flammability risk relative to Class 2 or Class 3 refrigerants. Keep in mind that the classification system evaluates a refrigerant as a fuel source, whether it can sustain a flame or not. Due to the hydrocarbon (HC)-like molecule structure of all halogenated refrigerants, all of them can be combusted when put into a high-energy situation, such as a fire with another fuel source, or they can thermally decompose when exposed to sufficiently high temperature. This includes most of the Class 1 no flame-propagation refrigerants, such as R-12 and R-22, which have been in use since the late 1930s. All air conditioners are designed to avoid an explosive over-pressurization when exposed to heat from a structural fire, with either copper tubing braze joints that melt in a fire or a pressure relief device, to safely relieve the buildup of refrigerant pressure at high temperature. Thus, structural fires have occasionally exposed firefighters to refrigerant combustion byproducts for decades, such as hydrogen fluoride (HF) and/or hydrogen chloride (HCl), both of which form acids with exposure to water. These combustion byproducts are not considered as part of the toxicity classification for refrigerants, as combustion of a refrigerant is a rare event. The toxicity classification for refrigerants is either lower toxicity (Class A) or higher toxicity (Class B), based on the refrigerant itself as a direct hazard and not based upon the combustion byproducts.
Table 1 compares a representative refrigerant in four of the safety groups (combined toxicity and flammability classifications). Figures 3 and 4 provide further detail on flammability characteristics of several substances using the parameters lower flammability limit (LFL), heat of combustion (HOC), maximum laminar burning velocity (BV), and minimum ignition energy (MIE). Figure 5 shows the GWP100 of numerous refrigerants versus the flammability classification. The trend is clear: To shift the balance toward being more environmentally friendly, we must accept some degree of flammability. For small appliances, such as refrigerators, the risks of Class 3 refrigerants, like propane (R-290) and isobutene (R-600a), are manageable. For larger appliances and equipment, Class 2L refrigerants are a reasonable and prudent choice to mitigate the flammability risk.
Adoption of A2L Refrigerants
As of December 2018, more than 68 million air conditioners using A2L refrigerants have been installed around the world. The majority of these units are installed in Asia and Europe. Manufacturers have addressed the safety issues around using A2L refrigerants in air conditioning systems and managed to maintain or improve the energy efficiency versus existing systems. To date, no accidents or incidents with loss of life or limb have been reported. The U.S. is lagging behind the rest of the world because of the time needed to update building codes and product safety standards. Perhaps due to concerns with reported accidents involving higher flammability Class 3 refrigerants, though despite a lack of such incidents to date with lower-flammability safety group A2L refrigerants, many of the U.S. committees do not agree with the international safety requirements used around the world and would like to make them more stringent.
The overall risk of a hazard is determined by the combination of the probability of occurrence and the severity of the event. Approaches to risk management seek to reduce both the probability and the severity, to achieve a risk level that is as low as reasonably possible and acceptable within societal norms. For air conditioners, the first approach is to avoid the release of refrigerant, and the second approach is to restrict the maximum allowable quantity of refrigerant in the system. If one does not adhere to product safety design standards and correct practices for installation and servicing, even A1 refrigerants, when released, can cause death due to asphyxiation or injury, such as frostbite.
Product safety standards used to approve listed and labeled equipment impose design requirements that minimize the chance of component failure or damage causing a rapid release of the refrigerant that could create a flammable cloud. It is extremely unlikely that slow refrigerant leaks, such as those due to corrosion pin holes or permeability of seals, pose a fire hazard because the refrigerant has sufficient time to disperse and remain below the LFL. However, it is likely that in some air conditioner, somewhere, at some point in time, refrigerant will be rapidly released for one reason or another — whether due to defective component failures, accidental damage, or a structural fire from another cause. When that happens, following safety standards minimizes the probability of a refrigerant ignition event. These standards remove ignition sources in the equipment near the source of the refrigerant, require adequate room size to allow for dispersion and dilution, or require mechanical ventilation triggered by a refrigerant sensor when room size alone is not enough. The air conditioner safety standards recognize there are uncontrollable factors, such as potential ignition sources in the conditioned space. Many items commonly found inside buildings that are potential ignition sources for Class 2 or Class 3 refrigerants are not ignition sources for Class 2L refrigerants due to their higher MIE values, thereby significantly reducing the probability side of the equation.
Several organizations have collaborated to conduct peer-reviewed research on how to safely use A2L refrigerants, with the aim of providing technical justification for requirements in standards and codes. Air-Conditioning, Heating and Refrigeration Institute (AHRI); ASHRAE; and the U.S. Department of Energy (DOE) have collaborated to publish numerous reports with the results, and additional work continues to further explore more detailed issues. One focal area has been refrigerant charge quantity limits, which directly impact the available fuel in the event of a release and the potential worst-case event severity. The charge quantity limits are based on:
• The available space to which released refrigerant can disperse;
• The elevation from which refrigerant could be released;
• Presence of a fan in the equipment for recirculation air movement; and
• Presence of ventilation system to supply and exhaust air in the space.
Heavier than air refrigerant released near the ceiling will mix and dilute as it falls, while refrigerant released near the floor will pool if the air is still and take much longer to diffuse into the room. The risk mitigation concepts are to use recirculation fans to mix the refrigerant and air so that the concentration at all locations in the space is quickly brought below the LFL and cannot ignite. Above certain refrigerant charge limits, a refrigerant sensor is required to be part of the equipment design and used to trigger recirculation airflow and, in some cases, additional ventilation airflow is needed for further dilution.
Figure 6 shows the refrigerant charge limits and relationships to safely measure concepts common to ASHRAE Standard 15-2019 (published) and UL 60335 2-40 edition 3 (proposed changes to edition 2) — the two relevant consensus-based national standards. There are some differences in the details between these two standards, but the installation will always need to comply with the more restrictive of the applicable standards. The UL 60335-2-40 requirements will be both inherent to the product design and a mandatory part of the manufacturer’s installation instructions. The ASHRAE 15 requirements will typically be invoked by building code requirements for commercial and industrial applications but not for smaller residential applications with restrictions on the application and installation of the equipment.
There are clear limits for refrigerant charge quantity to ensure safety, with a maximum allowable quantity under any circumstances designated as m3 in the UL standard. For smaller scale equipment below a charge quantity designated as m1, no additional safety measures are required for the product or the installation. For larger-scale equipment with refrigerant charge quantity above m1, the basic concept is use of a refrigerant sensor to detect a leak and trigger recirculation of air to dilute the refrigerant below the LFL. Above a threshold defined in the safety standard, higher levels of refrigerant for a given size of the conditioned space will also require mechanical ventilation to further dilute released refrigerant. In both approaches, the use of airflow to mix and dilute the released refrigerant is designed to keep the average concentration below 25% of the LFL and to quickly disperse any local concentration near the source of the release that may be above 25% of LFL. Note that while smoke detectors and CO detectors sound an alarm to trigger occupant/emergency response, air conditioners using flammable refrigerants will proactively trigger equipment response to mitigate the hazard.
When implementing any change or transition, there is a need to communicate and educate. With new refrigerants, there are a few issues that require attention. Equipment installers, authorities having jurisdiction (AHJs), service technicians, and fire service personnel need to be aware of the changes. The installation process for air conditioners and heat pumps will remain largely the same but must follow the manufacturer’s instructions, including confirmation that the room size is adequate for the refrigerant charge quantity. Installers and service technicians will be trained on the new requirements through several programs to be offered by manufacturers and other associations, such as North American Technician Excellence (NATE) and ACCA. Fire officials will need to know when periodic inspection of refrigerant sensors is required. Firefighting procedures and personal protective equipment will remain the same, as will training on the hazards of combustion byproducts from halogenated refrigerants (CFCs, HCFCs, HFCs, and HFOs). Structural fires have generated dangerous combustion byproducts from refrigerants in the past, as air conditioners and heat pumps were consumed in the fire along with many other hazardous materials. Post-fire clean up measures will always need to be appropriate for the event.
Heating and cooling are essential parts of life, and due to environmental impact, existing A1 refrigerants will transition to A2L refrigerants in air conditioners and heat pumps. It is in the best interest of all stakeholders to understand how to safely work with A2L refrigerant as they are adopted into use. While there are some new considerations with A2L refrigerants, the changes are incremental in nature.