By Rajan Rajendran, Ph.D., Emerson Climate Technologies

Editor’s Note

As recently as five years ago, air-conditioning and refrigeration applications were dominated by a handful of refrigerants – ammonia in large industrial systems, HFCs like R404A and HFC-134a in stationary and mobile refrigeration and air-conditioning, and R410A in stationary air-conditioning applications. Meanwhile, the use of HCFC-22 was declining as it was being phased-out by the Montreal Protocol agreement. In Europe, especially in domestic refrigeration, hydrocarbons like isobutane and propane had already become quite common. Today, CO2 as a refrigerant in stationary refrigeration has grown in usage and acceptance in Europe and Australia and is making inroads in North America and elsewhere. The use of R290 in small charge systems and even some large units is increasing across Europe and is being tried out in other parts of the world. In response to this demand for lower GWP fluids, refrigerant manufacturers are introducing and will continue to release new fluids over the next few years. So what might we expect ten years from now? In this paper, author Rajan Rajendran poses a few answers to that question, examining the changes to the refrigerant landscape and the resurgence of natural refrigerants as one of the best options in addressing climate change.

In the years since the Montreal Protocol went into effect in 1989, the global refrigeration and air-conditioning industry has met the challenge of phasing out ozone depleting chemicals and adapted well to achieving increasing efficiency for their equipment using synthetic hydrofluorocarbons, HFC’s, in most common applications. In large industrial applications, ammonia has become the standard refrigerant of choice, even though HFCs are used in some installations. In recent years, the alarming growth and emissions of HFCs into atmosphere has caused concern that, if left unchecked, these HFCs could become a major contributor to global warming gases in the atmosphere. Starting with some European countries, and followed by Australia and eventually the rest of Europe, governments have started taking action to curb this growth and emission of large quantities of HFCs. More recently, the United States Environmental Protection Agency (EPA) announced proposed rules to remove some of the HFCs by application from their approved list of refrigerants. The refrigerant landscape has changed significantly and continues to evolve as these factors come into play. In response to a demand for refrigerants with lower climate impact, natural refrigerants have seen resurgence, made possible in large part due to the advent of electronics and software to make natural refrigerant systems more efficient. New synthetic refrigerants have also been developed, more efficient than existing HFCs and having less impact on the climate. We are looking at a future with more options than we have ever had before – which means that we have to be careful in how we make our choices to minimize or eliminate unintended negative consequences.


The Montreal Protocol was agreed to by all the countries in September 1987, and went into effect on January 1, 1989[1]. Since that original document was approved, the protocol has been adjusted several times to make allowance for new technical information that became available. The Montreal Protocol was born out of concern for the development of the “ozone hole” (the protective ozone layer is needed in the upper atmosphere to prevent harmful ultra violet rays from the sun reaching the earth’s surface) caused by chemicals in the stratosphere reacting with the ozone and depleting the useful protecting layer. One family of chemicals eliminated from use is the chlorine containing chloro-fluro carbon refrigerants or CFC as they are commonly known. These CFCs were used in nearly all air conditioning and refrigeration applications worldwide, due to their high efficiency and low toxicity and non flammable characteristics. Since 1989, first Europe and then the other non Article 5 countries [1], like the United States and later Article 5 countries like China and India, have banned the use of CFCs and embarked on the path of phasing out the use of hydro chloro-fluro carbon (HCFC) refrigerants like HCFC-22. At the time of writing this paper, HCFCs have been phased out in Europe, soon will be in the United States, and the process of phasing out of HCFCs in Article 5 countries has begun. Figure 1 below shows these various stages of the phase-out process of HCFC-22, used in nearly all refrigeration and air conditioning equipment until recently. The effect of the elimination of these gases that deplete the ozone layer has been the beginning of the process of “healing of the ozone hole”. According to a recent report on the effect of the Montreal Protocol, titled “Assessment for DecisionMakers – Scientific Assessment of Ozone Depletion: 2014”, the ozone depleting substances have started decreasing steadily in the last fifteen years. Figure 2 below shows the steady increase in equivalent chlorine in the stratosphere through the last two decades of the 20th century, followed by a stabilizing period and then a gradual decline, clearly due to the effect of the Montreal Protocol.

The report also documents that this stabilization and decline of ozone depleting substances has led to an increase in the ozone content as shown in Figure 3. The solid blue line is the measured data compared to the model, indicating that the ozone layer is indeed healing, thus leading to their conclusion that ozone levels will reach 1960 levels in the year 2100.

One of the benefits of the phasing out of CFCs is that the Montreal Protocol eliminated a whole family of gases that have high global warming potential (GWP). Global Warming Potential is a measure of the climate impact of a gas measured relative to that of carbon dioxide, which is given a reference value of 1. Global warming potential numbers are reported by the Intergovernmental Panel on Climate Change (IPCC), a United Nations organization and Table 1 shows the global warming potentials of some commonly used chemicals [3].

Commonly accepted values are those for the 100-year time horizon. On this scale, CFC-12 or R12 as it is also known, has a GWP of 10,900 and when phased out by the Montreal Protocol, was rapidly replaced by HCFC-22 or HCFC-22 at a GWP of 1810, an 83% decrease. The effect of this dramatic change was the positive impact that the Montreal Protocol had on climate change. A recent article in The Economist, titled “Curbing Climate Change”, dated September 20, 2014 [4], highlighted this “unintended consequence” of the Montreal Protocol through a dramatic comparison of the various efforts to mitigate global warming. The figure from the article that is reproduced as Figure 4, clearly shows that the elimination of CFCs had the most significant impact on global warming.

At 5.6 billion tones of equivalent CO2 , the phase-out of CFCs has been the single largest positive influence on reducing gases that impact climate change.


The replacement gases to R12 and HCFC-22, called hydrochlorofluorocarbons have a lower GWP than R12 (and some HFCs’ GWP are lower than HCFC-22 as well), yet, with rapid increase in economic activity worldwide, the contribution of these HFCs as global warming gases is expected to grow at a rate of 7% per year for the foreseeable future [2]. The effect of this continued growth in HFC consumption and emission could, if left unchecked, lead to significant increase in HFC contribution to global warming [5]. Figure 5 shows the possible growth in HFCs in developed and developing nations. In Figure 6, Velders et al show that the growth in HFCs, particularly in developing countries, could more than offset all the gains made in emissions through the Montreal Protocol.

These concerns for climate change impact due to growth in HFC consumption and emissions have prompted many countries to take actions on their own. Starting with countries in northern Europe that placed a GWP based tax on HFCs, Australia followed suit and, most recently, the rest of Europe approved the changes to the Fgas regulation. Since the F-gas regulation changes have the most impact in Europe, we will briefly review the key elements of those changes here.


The European Fluorinated Gas rule, called simply the F-gas rule, was first adopted in 2006 and regulates the use of refrigerants in various industries across Europe [6]. The F-gas rule was recently amended in March 2014 to include stricter rules on the use of HFCs. Table 2 summarizes the key elements of the revisions of the F-gas rule that affect the air-conditioning and refrigeration industries.

The most important impact of the F-gas directive is on the refrigeration industry and supermarket applications with R404A or R507A in particular. New equipment bans for supermarket applications that go into effect in January of 2022 will drive large stationary refrigeration systems to using less than 150 GWP fluids except for cascade or hybrid systems. A typical example of a cascade system is shown in Figure 7 where CO2 is used in the low temperature part of the refrigeration system which condenses using a HFC like HFC-134a (less than 1500 GWP) in the medium temperature primary circuit. This implies that the medium temperature cooling has to be carried out by a secondary pumped fluid like CO2 or glycol.

Domestic refrigerators and hermetically sealed commercial refrigeration systems will be restricted further to refrigerants with GWP less than 150, beginning in January 2015 and January 2022, respectively.

In addition to the bans, the F-gas rule also implements a phase-down on the GWP based consumption of HFCs. This HFC phase-down mechanism is shown in Figure 8 along with the key elements of the specific application bans. The net effect of this HFC phase down will be to force applications to move to a weighted average GWP that is significantly lower than what it is today.


The United States, Mexico and Canada, the three North American countries, have proposed the highly successful Montreal Protocol be amended to include a global HFC phase-down. This proposal shown in Figure 9 has been presented and debated for five years without success. The European Union’s F-gas HFC phase-down is also shown here for reference. The proposal envisions two phase-down paths, one for Article 5 countries like China and India and another more aggressive step down for developed countries like the United States. The Article 5 countries like China and India have been the main resistance to such an amendment, based on the grounds that it was not the intent of the Montreal Protocol to curb greenhouse gases but ozone depleting chemicals. However, since 2013, there has been a gradual shift in public comments made by both countries which indicate that perhaps such a global agreement might indeed be possible in the coming year or two [7,8].


As we have seen earlier, even as the United States was leading the effort (with support from many countries including those in the European Union) to implement a global phasedown of HFCs, Europe’s F-gas rule has gone into effect curbing the use of many high GWP HFCs. In the United States, on June 25th 2013, President Obama announced a Climate Action Plan [9] to curb the emission of many short-lived greenhouse gases. One of those gases listed were HFCs and executive action to reduce the impact of these gases was promised. In the subsequent months, the EPA held several industry stakeholder meetings and published two Notices of Public Rulemaking (NOPR) in 2014. The first of the two, published on July 9, 2014, listed new refrigerants, uses and revised venting prohibitions, and is summarized below [10]:

  • Stand-alone commercial refrigerators and freezers: R600a, R441A (150 g)
  • Household refrigerators and freezers: R290 (57 g)
  • Vending machines: R600a, R290 (150 g)
  • Self-contained room AC, PTACs, PTHPs, window AC and singleroom portable AC: R290, HFC-32, R441A (subject to UL 484 limits)

This rulemaking is important as it is a precursor to several more promised by the EPA in an accelerated process of approval.

The second NOPR that was published in the Federal Register on August 6, 2014 is a delisting of several highGWP HFCs in various applications [11]. This delisting proposal is summarized in Table 3 (some applications are not shown on the table).

Since this proposal was published, there have been several meetings within the industry and between various stakeholders and the EPA to discuss on comment on this rulemaking. By the time of publication of this paper, it is likely that the final rule would have already been published by the EPA after review of all the comments. In general, the timing is extremely aggressive and for the self contained application, for example, the lack of alternatives to meet all their needs will pose serious issues. We can expect that the comments will reflect upon all of these points and ask for fewer candidates to be delisted (or not at all) and where a refrigerant is delisted, the industry be given sufficient time to research, develop, test and market the alternatives.


The search for alternatives to high GWP refrigerants began several years ago with industry and other stakeholders engaging in research into different options. As will be seen in the next section, the known alternatives to HFCs and the new candidates being developed have some characteristic or another that makes them a challenge to be adopted universally in all applications everywhere. The principles to be followed in the search for alternatives, is shown in Figure 10.

Beginning with safety (toxicity, flammability, high pressure), the alternate’s performance in an actual system, the economics, which includes both first cost and cost of ownership, as well as the impact on the environment have to be taken into account. Not doing so will lead to unintended negative consequences that we will have to deal with at a later date.

Alternatives to high GWP HFCs raise the issue of safety to a very high level – ammonia has toxicity and flammability, CO2 has high pressure, and propane has flammability as risks that have to be mitigated. Standards are only now being developed around the world to accommodate these and other flammable refrigerants in various applications. Figure 11 shows a snapshot in time of a sampling of the standards and other work that needs to follow in order to fully absorb the alternatives to HFCs into the mainstream. Optimistic estimates place this date for completion of the safety work across the world and in all applications, somewhere beyond 2020.

Performance of an alternative refrigerant in an actual system, instead of just in a component is the next most important variable in the selection of a refrigerant. Often alternatives are evaluated in multiple ways and using different standards makes it difficult to compare one to another. In order to have a uniform method of comparison, the Air conditioning, Heating and Refrigeration Institute (AHRI) started and completed Phase 1 of a Low GWP Alternative Refrigerant Evaluation Program in 2012/2013 and a conference was held in New York in January, 2014 [13]. Twenty one companies participated in the testing, with six refrigerant producers supplying thirty eight different candidates resulting in forty one reports that are available to the public on the AHRI website. The outcome of the evaluation and the conference was an awareness that many new alternatives were available and with component and system optimization, it should be possible to achieve same or better performance than the current HFC refrigerants. Cost was not addressed in this study. A Phase II of the study is now in progress and is expected to be complete in late 2015. Performance in a system and the environment impact of a refrigerant are connected. When considering the impact of a system on the environment, it is common to think about the effect of the refrigerant leak, which is called the “direct” impact. There is another, called the “indirect” impact of the system on climate that is due to the energy consumed by the equipment and the source of energy. The indirect impact, which can be as much as 95% of the total, when taken together with the direct, is called total equivalent warming impact and often approximated to the life cycle climate impact or LCCP. Figure 12 is a simple representation of this life cycle approach to calculating climate impact. Unfortunately, there is no single model for LCCP that is widely accepted as a standard, though effort is underway to change this. Emerson Climate

Technologies has a web based software that enables a user to compare different refrigeration systems using an LCCP approach [14].

As one evaluates the different alternatives to replace high GWP HFCs in various applications, it is important to keep all of the above factors in mind in making comparisons. Even if alternate refrigerants are available, and standards for safety are in place, there is considerable effort required to research, develop and produce components that are compatible with these new refrigerants. Refrigerant and component availability precede equipment development, testing and qualification, where the qualification is for performance to minimum efficiency standards and reliability. Any effort to phase down HFC consumption has to take all of this into account to minimize negative impact on the industry, the economy and user.


The two most common refrigerants that are being targeted for delisting or bans are R404A and R507A. R404A is used mostly in low temperature and often in medium temperature refrigeration applications. It is what is classified as a “medium pressure” refrigerant in the Figure 13 at a GWP of around 4000. The non flammable, non toxic (ASHRAE classification A1) refrigerant that is available now to replace R404A is R407A (and R407F in some systems). These refrigerants are less than 50% of the GWP of R404A and perform less efficiently in low temperature systems, but slightly more efficiently in medium temperature. Over 50% of the new supermarket stores in the United States have already switched to R407A from R404A.

Replacing R507A is more of a challenge in some applications. R507A exhibits very low “glide”, a characteristic of the blend that causes the saturated evaporation temperature to vary as the composition of the refrigerant changes in the evaporator. Typically, flooded evaporator systems use refrigerants with little to no glide as refrigerants with glide will cause the refrigerant composition to be different than specified at the exit of the evaporator (and inlet of the compressor). Systems that use R507A because of its low glide cannot use R407A as an alternative.

To the left of R407A in Figure 13, at around 1300 GWP, two new refrigerants are listed, R448A and R449A. These are A1, lower GWP candidates that are being tested by component and equipment manufacturers to replace R404A, especially in Europe where a refrigerant less than 1500 GWP is required in the future. It is expected that these refrigerants would be available for use in 2015.

The other HFC that will come under pressure due to the HFC phase down is HFC-134a, first in Europe, and later elsewhere. Two new A1 refrigerants, R450A and R513A are being developed to replace HFC-134a, at a GWP less than 600. Mildly flammable refrigerants like HFO 1234yf and HFO 1234ze will also become options in the future for replacing HFC-134a.

The other A1 refrigerant that is available to the refrigeration industry is CO2 , at a GWP of 1. This is not a direct drop-in replacement, but one that involves complete redesign. It is becoming increasingly popular in supermarket applications and is expected to grow in the industrial applications as well.

In Figure 13, to the far left at a GWP of around 8, propane, a hydrocarbon refrigerant is growing in acceptance in small self contained applications and even in larger secondary applications. Ammonia, an excellent refrigerant that is both toxic and flammable (mildly flammable, B2L classification), is common in large industrial applications. In Europe, Ammonia is also finding application in commercial comfort cooling applications where smaller charges and safety methods make it possible for its use near populated areas. A couple of supermarket chains are trying this refrigerant in secondary systems and this too has potential for growth, especially in Europe. This is discussed in greater detail in the section on carbon dioxide systems.


Starting with northern Europe, and then Australia, carbon dioxide is making a comeback as a refrigerant in many large refrigeration applications, particularly in supermarkets. There are two major types of system architectures being employed in these applications. The more common system is called a hybrid or a cascade CO2 system and is shown in Figure 14.

CO2 is used as a direct expansion refrigerant in a low temperature system (heat transfer takes place directly from the refrigerant to the air in the refrigerated space) but condenses at a temperature that is closer to what is typically called a medium temperature system. The heat from the low temperature system is rejected into the medium temperature system, and hence the term “cascade” to describe this architecture. The medium temperature system is typically a HFC, mostly HFC-134a, which absorbs heat from the low temperature CO2 system condenser and the various medium temperature evaporators in the supermarket and typically, rejects it to the atmosphere. Because these systems are a combination of a natural refrigerant and an HFC, these are called “hybrid” systems. Cascade hybrid CO2 systems are more complex than a traditional supermarket system, but they can be equally efficient in any climate zone.

Another variation of the cascade system is one where the medium temperature primary refrigerant is an HFC like HFC-134a, but the actual medium temperature cooling is through a secondary fluid like CO2 that is pumped through the refrigerated cases. This type of a system is shown in Figure 7 and uses very low charge of HFC usually confined to a “machine room”, which makes this architecture ideal for different low GWP primary refrigerants, be they A2L, B2L or even A3.

The second type of CO2 system is a “trans-critical booster” CO2 system where both the medium and low temperature cooling is through direct expansion use of CO2 as a refrigerant. The compressors from the low temperature circuit discharge the compressed gas into the suction of the medium temperature part of the system. The discharge of the medium temperature compressors is into a condenser which, when the outdoor ambient is close to or above the critical temperature of CO2 , operates as a gas cooler in the trans-critical mode. This type of system is more complex than a cascade hybrid CO2 system, but, it is an “all natural” solution. Because a standard trans-critical CO2 system is not as efficient as a comparable HFC based system, trans-critical CO2 systems tend to be more common in those areas wherethe transcritical operation is limited or its impact is not significant enough to make it an issue. Research is ongoing to find competitive ways to improve the efficiency of the CO2 system in the transcritical region as well as reduce the impact of the system operating in this mode.

As mentioned earlier, both types of CO2 systems are increasing in popularity, mostly in Europe and Australia, but increasingly in Canada, the United States and elsewhere. While first cost and maintenance of the systems are important factors, the energy consumed and the LCCP of a refrigeration system need to be considered in making the right choice. Figure 16 shows the analysis for a basic standard R404A supermarket refrigeration system (both medium and low temperature) compared to a cascade CO2 system with HFC-134a in the medium temperature primary with pumped CO2 (similar to Figure 7) and a booster transcritical CO2 system for Boston, MA.

It is clear that from a climate impact point of view, a CO2 based system is better than a HFC R404A based system in a cool climate region like Boston, MA. While the same could be said for the annual energy consumption, the peak power consumed on those warm summer days, indicate that there is a penalty to pay when a simple trans-critical CO2 booster system is employed. Note however that the cascade CO2 system shows good performance all around.

A similar analysis done for a warmer region like Dallas, TX shows a different picture, as seen in Figure 17. While the CO2 emissions for the cascade CO2 and trans-critical booster CO2 system are much better than that for R404A, annual power is slightly higher while the peak power in the summer is significantly higher. Several laboratory and field trials are underway to develop novel ideas to improve the CO2 system efficiency without significantly increasing the first cost of the system but these will take a few years to see widespread use.


Synthetic fluids that are considered lower GWP can be classified as non flammable (A1) and mildly flammable (A2L). They can be further subdivided into three classes of refrigerants for the purposes of discussion – R410A-like, R404A/HCFC-22-like and HFC-134alike. There could be a lower pressure classification, but for the purposes of this paper, we will not be including that list of refrigerants. Table 4 lists some of the synthetic lower GWP alternates available or in development right now. It should be noted that this is not meant to be an exhaustive or complete list of all candidates being developed. For a more complete listing of alternates being offered by the various chemical manufacturers, the reader is referred to the Low GWP Alternative Refrigerant Evaluation Program by the AHRI [13].

For the most part, these synthetic blends try to mimic the performance characteristics of the refrigerant they are trying to replace. While they are close in theoretical capacity and power, many of the replacements (R446A, R447A, R407A, R407F, R448A, R449A, N20, ARM32, R444B, L40, DR7, HDR110, DR3, ARM20) exhibit the phenomenon called “glide” due to their blend characteristics [15]. In addition many of these replacements have higher heat of compression and could have discharge temperature issues at the compressor – which often is mitigated by lowering the superheat at the suction of the compressor or using liquid or vapor injection. These are not insurmountable issues, however, any timeline to eliminate the use of refrigerants like R404A and replace it with its alternates has to consider the fact that compressor and other component testing and development takes months and years to complete. Figure 18 shows some of the performance data that was presented by the refrigerant suppliers at a recent meeting [16].3


As recently as five years ago, the air-conditioning and refrigeration applications were dominated by a handful of refrigerants – ammonia in large industrial systems, HFCs like R404A and HFC-134a in stationary and mobile refrigeration and air-conditioning, and R410A in stationary air-conditioning applications and use of HCFC-22 declining as it was being phased-out by the Montreal Protocol agreement. In Europe, especially in domestic refrigeration, hydrocarbons like isobutane and propane had already become quite common. Today, CO2 as a refrigerant in stationary refrigeration has grown in usage and acceptance in Europe and Australia and is making inroads in North America and elsewhere. The use of R290 in small charge systems and even some large units is increasing across Europe and is being tried out in other parts of the world. In response to this demand for lower GWP fluids, the refrigerant manufacturers are introducing and will continue to release new fluids over the next few years. So what could we expect ten years from now?

CO2 will play a major role in supermarket refrigeration, especially in cold climates where the periods of trans-critical operation are limited to a few days in the year. The cascade refrigeration system will also become quite common, with synthetic fluids like HFC-134a and R407A yielding to lower GWP candidates like R448A, R449A and eventually candidates with less than 150 GWP as the HFC phase-down in Europe begins to take effect. Cascade refrigeration systems with CO2 as a pumped secondary fluid in medium temperature and CO2 as a compressed refrigerant in low temperature can be expected to be popular in southern Europe and warmer climates where use of HFCs will be restricted. Small refrigeration systems will see a mix of CO2 (vending machines), hydro carbons, and HFCs and HFO blends as regulations permit. In these applications, even more so than in larger systems, synthetic fluids with less than 150 GWP will become common. Ammonia will continue to play a dominant role in industrial refrigeration and in those applications where it could be used as a primary refrigerant in a large refrigeration system, the use of ammonia will grow. In industrial applications, the use of HFCs will decline following the trends in the supermarket industry, giving way to similar architectures and fluids. The growth of CO2 in supermarket applications will spill over into the industrial sector, and CO2 as a pumped fluid and as a direct refrigerant will increasingly be common, especially in smaller industrial systems. All of the different CO2 system architectures, but more importantly, the cascade system will grow in popularity in smaller industrial systems.

Air conditioning applications will continue to be dominated by R410A well into the foreseeable future, with refrigerants like HFC-32 and its blends with HFOs growing in use in Asia in the next few years. The shift from non flammable to flammable refrigerants in air conditioning systems will depend entirely on the safety standards and codes being developed around the world and their timeliness of adoption. Starting with Europe, other regions could evaluate and accept ammonia in limited commercial comfort cooling applications as secondary systems (chillers are already in use in these applications).

It is safe to say that we are in a period of flux right now, with several ideas competing with each other to establish themselves as mainstream. As applications move in the direction of lower GWP fluids, the choices confronting us are many – only the more efficient, safe and lower life cycle cost options can be expected to outlast the race. Not-in-kind technologies defined as non-vapor compression cycle systems for cooling are not discussed here, but are a growing area of interest worldwide. We have to be careful that attempts at regulations to pick winners and losers could force a solution that may not be the optimum. In an ideal world, we would set a goal for climate impact and let the market and applications find the best answer.


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    Depletion: 2014. 10th September 2014. World Meteorological Organization Global Ozone Research and Monitoring Project – Report Number:
  3. Intergovernmental Panel on Climate Change Fourth Assessment
  4. Curbing Climate Change. The Economist, September 20, 2014. http://
  5. The Large Contribution of Projected HFC Emissions to Future Climate
    Forcing. Velders, Guus J. M., David W. Fahey, John S. Daniel, Mack
    McFarland and Stephen O. Andersen. Proceedings of the National
    Academy of Sciences, July 7, 2009.
  6. European F-gas Regulation, 2014.
  7. United States and China Agree to Work Together on Phase-down
    of HFCs, June 08, 2013.
  8. India – US Joint Statement September 30, 2014. http://www.
  9. President Obama’s Climate Action Plan, June 25th 2013. http://www.
  10. Notice of Public Rulemaking listed in the Federal Register, July 9, 2014.
  11. Notice of Public Rulemaking listed in the Federal Register, August 6,
  12. Private communication, Mr. Richard Lord, Carrier Corporation.
  13. Low Global Warming Potential Alternative Refrigerant Evaluation Program, AHRI.