The safety of synthetic refrigerants versus anhydrous ammonia continues to provide a major point of debate within the refrigeration industry. Since ratification of the Montreal Protocol two decades ago, there has been a trend toward eliminating non-flammable synthetic refrigerants that have ozone depleting potential or global warming potential. At the same time, questions concerning the flammability and toxicity of replacement refrigerants have remained at the forefront.

From a performance perspective, ammonia has always been recognized as an exceptional refrigerant, but flammability and toxicity concerns have historically driven some refrigeration applications towards non-flammable, non-toxic synthetics. However, the flammability gap has been closing rapidly because new synthetic blends that are coming to market with low-ODP and low-GWP characteristics are flammable on a par with ammonia.

“Codes and standards have shied away from allowing flammable or toxic refrigerants in human comfort, which is air conditioning, and commercial applications where there is a risk of occupants being exposed to refrigerant in the event of a leak, but that’s changing.”

–Jeff Shapiro, IIAR code consultant.

New synthetic blends and ammonia fall into a class of refrigerants called “2L.” The class, established by ASHRAE Standard 34, applies to flammable refrigerants that are not easily ignited and will burn with a relatively low flame velocity if they are ignited. ASHRAE 34 further divides 2L refrigerants into two subcategories – A2L and B2L — based on toxicity, with A2L being the lower of the two.

“Codes and standards have shied away from allowing flammable or toxic refrigerants in human comfort, which is air conditioning, and commercial applications where there is a risk of occupants being exposed to refrigerant in the event of a leak, but that’s changing,” said Jeff Shapiro, IIAR code consultant. “Now that there are synthetic refrigerants in the 2L category, there’s been growing interest in mitigating the risk of low-flammability refrigerants, as opposed to outright prohibiting them.”

Changes being proposed so far to model codes and standards to permit 2L refrigerants have been divided into two areas. As mentioned above, one is human comfort, or air conditioning, applications. Some codes and standards will likely permit the use of A2L refrigerants for this purpose, but refrigerant quantities will be limited and engineered safety controls, such as leak detection and ventilation, will be provided to reduce risk. Likewise, A2L refrigerants will be recognized for commercial applications where a larger refrigerant charge is kept in a refrigeration machinery room or outdoors, with a non-flammable, non-toxic secondary refrigerant serving occupied spaces.

Less certain is what codes and standards will, or won’t, allow with respect to any systems that circulate liquid A2L refrigerant in occupied spaces. Detection and mitigation features needed to deal with leakage events in occupied spaces become much more complicated with field-assembled equipment located in occupied building areas.

Beyond direct ignition of a refrigerant leak, a secondary concern that should not be overlooked is toxicity. “Ammonia shares the characteristic with A2L refrigerants of being mildly flammable, but that is pretty much where the parallel ends,” Shapiro says.

ASHRAE 34 classifies ammonia as a Group B refrigerant based on toxicity of the chemical itself. However, ammonia is relatively clean burning fuel when it catches fire. In contrast, leaked synthetic refrigerants in pure form are less toxic than ammonia, but the consequences of decomposition due to heat or flame exposure are far worse than ammonia. If an A2L refrigerant is burned, either by ignition or by exposure to a flame, highly toxic and corrosive products of combustion may be produced, creating a life-threatening condition for any occupants who may be exposed.

Although A2L refrigerants are not as risky as pure hydrocarbon refrigerants, such as isobutane, flammability risk remains a concern that cannot be overlooked. “A recent study evaluated the ignitability of A2L refrigerants in scenarios such as a commercial kitchen, a residential hallway and rooftop units. Leak conditions were created in the presence of ignition sources to test what would happen, and significant fires with dangerous products of combustion were observed,” Shapiro said. “Given the known risks associated with synthetic A2L refrigerants, we need to get it right when it comes to requiring adequate safety measures in codes and standards.”

“Ammonia is much easier to deal with from a detection perspective because you’re only trying to detect a single gas with known characteristics, and even if detection fails, the odor of ammonia is self-alarming.

–Jeff Shapiro, IIAR code consultant.

Synthetic A2L refrigerants that are coming to market don’t have an odor. Therefore, absent detection devices, a leak may not become evident until there is a flammability or toxicity issue. Most applications will require some type of leak detection device or system, and these may have to be specific to an individual gas. However, even if these safety features are initially satisfactory, long-term reliability must be addressed to ensure public safety.

“Unfortunately, we currently have no standard for any kind of system to detect synthetic A2L gases to tell us how many detectors you need, where they should be placed, what the reliability is, how they will be connected, how often they will be tested or who is going to test them,” Shapiro pointed out.

In contrast, the International Institute of Ammonia Refrigeration has made a lot of progress with ammonia detection, and many new ammonia-detection requirements have been incorporated into IIAR 2 since 2014. “Ammonia is much easier to deal with from a detection perspective because you’re only trying to detect a single gas with known characteristics, and even if detection fails, the odor of ammonia is self-alarming. You will smell it and you’ll naturally want to move away, even at low concentrations that are below established safe-exposure thresholds,” Shapiro said.

With ammonia having similar flammability to synthetic A2L refrigerants that are likely to become commonplace in air conditioning and other refrigeration equipment, the only barrier to broader acceptance of ammonia in such applications is the question of toxicity. And given ammonia’s advantage of being self-alarming at non-hazardous concentration levels, versus odorless synthetics that must rely on detection equipment to identify a leak, ammonia is certainly positioned for increased consideration as a replacement for outgoing ODP and GWP refrigerants.