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Staying Frosty with A2L Refrigerants

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Article reposted with permission from RSES Journal.

A brief discussion on this class of lower-GWP refrigerants emerging from the AC/R market.

The world of refrigerants, as we know it, is changing once again. Regulatory drivers and increasing efficiency demands are helping to shape the course of our industry. Unlike previous refrigerant transitions, for many AC/R applications, a move to flammable refrigerants will be required. While this reality may give some in our industry pause, a quick overview of the new A2L refrigerants will help alleviate many of those considerations.

Regulatory considerations

Previous refrigerant transitions here in the US focused on the elimination of ozone depleting substances, namely CFCs and HCFCs like R-12, R-502 and R-22. This was accomplished, in large part, by migrating to the non- ozone depleting HFC refrigerants. Although significant environmental benefits were achieved during this move, many HFCs are still considered potent greenhouse gases. Therefore, our industry has now turned its focus towards addressing the issue of global warming.

At the international level, the Kigali Amendment to the Montreal Protocol establishes the framework for a global phasedown (Note: not a phaseout) of HFCs. While the U.S. has not yet ratified the Kigali Amendment, recent passage of the American Innovation and Manufacturing (AIM) Act gives the EPA the authority to enact a phasedown of HFCs, on a Global Warming Potential (GWP) weight-basis, over the next 15 years. Although all the details of the EPA’s implementation are not yet finalized, many believe that this will help us meet the requirements of the Kigali Amendment (see Figure 1).

Additionally, the EPA issued proposed SNAP Rule 23 last year. Part of this proposal would list six refrigerants as acceptable, subject to use conditions, for residential and light commercial air conditioning and heat pump end-uses. A quick review of the list of refrigerants included for these end- uses (R-452B, R-454A, R-454B, R-454C, R-457A, and R-32) reveals that all six refrigerants have the same ASHRAE safety group classification—namely that they are all A2Ls (Lower Toxicity, Lower Flammability). Given this, it appears that A2Ls will be a part of our future, and understanding their behaviors is vital to ensuring a safe and effective transition.

Getting to low GWP

To achieve the lower GWP levels required by the AIM Act, it will be necessary to shift our industry to lower GWP refrigerants in new AC/R applications, many of which are flammable. While there are some HFC refrigerants that have inherently lower GWP (e.g. R-152a), the use of hydrofluoroolefins (HFOs) will be required in many applications to reach lower GWP. HFOs have much shorter atmospheric lifetimes than their HFC counterparts, resulting in much lower GWPs (see Figure 2). HFOs, like R-1234yf, are commonly used with HFCs in A2L refrigerant blends to reduce GWP.

Stacking up

Given the impending move to A2Ls, it is only natural to ask how these refrigerants compare to HFCs. Well the fact is that A2Ls have much in common with the traditional A1 (No Flame Propagation) HFC refrigerants they are intended to replace (see Table 1). This includes similar pressure-temperature profiles, similar thermodynamic properties, similar oil compatibility/miscibility, and the potential use of similar system architectures. Table 2 shows a quick comparison of R-410A and R-454B, an A2L R-410A alternative, using standard thermodynamic cycle calculations. R-454B has similar capacity and efficiency (COP) to R-410A, with slightly lower operating pressures and slightly higher discharge temperatures, all while offering a 77% reduction in GWP. This level of closeness in operating characteristics helps OEMs minimize system design changes when transitioning to A2Ls.

Flammability classes

Figure 3. Refrigerant safety groups

The primary difference between A1s and A2Ls is their flammability characteristics. To understand these differences, it’s helpful to review the refrigerant flammability classes (see Figure 3). Class 3 refrigerants are the most flammable and are listed as “Higher Flammability”. Hydrocarbons, like propane and isobutane, are examples of Class 3 refrigerants. Class 1 refrigerants are the least flammable and are listed as “No Flame Propagation.”

It is important to note though that while Class 1 refrigerants are often described as “nonflammable” many can still combust and burn at higher pressures and temperatures. Traditional HFCs, like R-410A and R-134a, are examples of Class 1 refrigerants. Class 2L refrigerants are listed as “Lower Flammability” and are sometimes described in our industry as mildly flammable. An important property of Class 2L refrigerants is that they have lower burning velocities, meaning that they produce slower flame propagation than Class 2 or 3 refrigerants, making them inherently less flammable.

With the move to lower GWP, safety group A3 and A2L refrigerants are being considered for expanded use by the AC/R industry. Both refrigerant types are currently being used here in the U.S. in smaller self-contained systems. However, safety standards and codes are currently being updated to allow for larger charge size systems to begin using these products. To better appreciate the differences of how these safety groups will affect system design and service, it is important to examine their primary flammability parameters. These include the Lower Flammability Limit (LFL), Minimum Ignition Energy (MIE), Burning Velocity (Su), and Heat of Combustion (HOC). Table 3 pro- vides a quick comparison of the flammability parameters of three A2Ls compared to R-290 (Propane), an A3 refrigerant.

LFL describes the minimum concentration of a substance in air needed to exhibit flame propagation and is often shown as volume percent in air. However, it’s also useful to look at it on a concentration basis, meaning how much mass of a refrigerant must leak into a volume of air to form a flammable concentration. When compared to R-290, roughly eight times as much mass of the A2Ls is required to form a flammable concentration. Therefore, A2L refrigerants are less likely to form flammable concentrations from leaks than the higher flammability A3 refrigerants.

MIE is the minimum energy required to ignite a flammable gas/air mixture. Sources with energy levels below the MIE will not result in an ignition. Hydrocarbons, like R-290, can be easily ignited by relatively low energy sources, such as static electricity. However, the MIEs of A2Ls are much higher. Many common electrical components in use today in the AC/R industry are not strong enough to ignite A2Ls. Therefore, the risk of an ignition occurring with A2Ls should be less likely than with A3s.

Burning velocity (Su) describes the laminar speed of a flame for given values of composition, temperature and pressure.

The burning velocities of the A2Ls are much lower than those of A3s like R-290. In a quiescent environment, ignitions with A2Ls tend to produce weak flame fronts that propagate slowly and that may self-extinguish when the ignition source is removed. This is in contrast to higher flammability refrigerants, which produce rapid flame front propagation.

HOC describes the amount of heat released per unit mass by the combustion of a substance. From Table 3 we see that the HOCs of the A2Ls are much lower than that of R-290. Given the much lower burning velocities and HOCs of A2Ls, compared to A3s, ignition events with A2Ls should also be lower severity than those produced with A3s.

An example of MIE testing (ASTM E582) with A2Ls and an A3 is shown in Figure 4, with images taken a fraction of a second after ignition is attempted. R-290 (bottom image), an A3, produced an ignition at 1 mJ with flames rapidly filling the globe, violently ejecting the stopper, and spreading into the fume hood. Energy levels were increased to 100 mJ for the ignition with R-32 (A2L is shown on the top right). The flames spread more slowly throughout the globe before popping the stopper, producing an overall less energetic ignition event. With R-1234yf (A2L is shown on the top left), an ignition could not be produced using the test set up, even at an energy level of 1,000 mJ.

When putting all these parameters together, it is clear that A2Ls are less likely to form flammable concentrations, are harder to ignite, and produce lower severity ignition events than A3s.

Figure 1. Regulatory landscape

Figure 2. How HFOs work.

Table 1. A sampling of A2L refrigerants

Table 2. Thermodynamic comparison of R-410A & R-454B.

Table 3. Flammability parameters.


Working with A2Ls

When working with A2L refrigerants, it is worth- while emphasizing a few guiding principles. First, only use A2Ls in systems specifically designed for them. Flammable refrigerants should never be used as retrofits or in systems intended for A1 refrigerants. The use of these products must also be in compliance with relevant safety standards and building codes. It is also important to follow OEM installation, service, and use instructions. Revisiting “Best Practices” is also highly advisable, as many best practices in use today will be required when working with A2Ls (see Table 4).

Unlike natural gas or propane used in the home, flammable refrigerants will not use stenching (i.e. rotten egg odor) to alert you to the presence of a refrigerant leak. This is due to the fact that stenching agents in use today present corrosion or compatibility concerns for AC/R systems.

Additionally, stenching agents may be absorbed by the refrigerant oil or desiccant, rendering them ineffective. Therefore, technicians should only use appropriate leak detection methods when working on systems with A2Ls, such as hand-held sniffers or soap bubbles. Open flames should also never be used for leak checking.

Table 4. Revisiting best practices for A1s and A2Ls

Figure 5. Disposable cylinder band/PRV

Many tools used with A1 refrigerants can also be used with A2Ls. However, there are some items and areas where there will be differences. Cylinders for A2Ls will have left- handed threads and will also have a red band or cylinder top, indicating they contain a flammable refrigerant. Additionally, dis- posable cylinders for A2Ls will no longer use a rupture disc but will instead have a reset- table pressure relief valve (PRV) (see Figure 5). Electronic leak detectors, vacuum pumps, and recovery machines must also be compatible for use with A2L—verify with the manufacturer.

Systems designed for A2Ls may appear to be very similar to those used with A1 refrigerants. However, there will be differences. New labeling will be required to indicate a flammable refrigerant. Technicians will also need to mark the total system charge on the unit label for systems requiring field adjustments. A2L equipment will also use improved piping practices and joint requirements to help prevent leaks. Some units will also incorporate integrated onboard leak detection, which may activate indoor fan air circulation or other mitigation measures to prevent flammable concentrations from forming in the event of a leak. Units using A2Ls will also be prohibited from containing components that could be a source of ignition for leaked refrigerant or must place those components in a flame arrest enclosure.

OEMs will provide service and installation manuals with each unit. It is imperative that technicians take the time to review these materials, as these will help highlight differences in system design and service practices that may be required.

Be prepared

The move to lower GWP will require our industry to work with flammable refrigerants on a larger scale. Fortunately, our industry has developed new A2L refrigerants that help minimize the risks associated with flammability. Safety standards and procedures have been rigorously developed to help ensure that these products can be used safely. While the use of A2Ls will require some changes, many of the tools and best practices that the HVACR industry has used for decades will remain the same. Technicians and service providers should take the time to become familiar with A2Ls, as well as the newest tools and equipment designs that will be used with them. This is key to a successful transition to lower GWP. There is a wealth of training resources available or under development related to A2Ls. Now’s the time to get prepared, and “Stay Frosty My Friends.”

Stephen Spletzer is the Principle Engineer with Chemours. He started his career doing military research but has since spent more than two decades working in AC/R applications for both refrigerant and equipment manufacturers. Now with Chemours, his experience includes equipment testing, new product development, training, and codes and standards. For more information, visit

In January 2021, Spletzer presented a one-hour technical webinar also titled “Staying Frosty with A2L Refrigerants.” To view this webinar, visit


Posted In: Technical Tips, Technology

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