• Tag Archives: Refrigerant

New Refrigerants Transitional

The American Innovation and Manufacturing Act of 2020, or AIM was passed as part of the 2021 appropriation bill, passed on December 27, 2020. Although you probably would not know it from the title, the AIM Act is about phasing down HFCs. This is a phasedown, not a phase out. This table shows the phasedown schedule. Noite, we started the 60% of baseline period this year.  

AIM Act HFC Phasedown Schedule
Date% of Production Baseline% of Consumption Baseline
2020 – 202390%90%
2024 – 202860%60%
2029 – 203330%30%
2034 – 203520%20%
2036 –15%15%

Allowances

Because this is a phasedown and not a phaseout, some HFCs will still be available for a long time. However, ALL HFCs will not necessarily be available. The phasedown is regulated by assigning allocations for production and/or importing HFCs to producers and distributors. How quickly a company uses up their allocation is calculated by multiplying the quantity of HFC produced or used times its exchange value, which is basically its AR4 GWP. For example, R410A has an exchange value of 2088, R32 has an exchange value of 675, and R454B has an exchange value of 465. This means that a company can make 3 times as much R32 as 410A, or almost 4.5 times as much R454B as 410A.

Still Using HFCs?

One interesting situation is that the “new” refrigerants which will be replacing the current high GWP HFC refrigerants also contain HFCs. R32 is an HFC and it is on the list of refrigerants being phased down. Other alternatives being studied, such as R454B, are blends that contain R32. Phasing down R32 will eventually make both R32 and R454B not practical for equipment manufacturers. They will probably be replaced in ten years or less.

Making the Switch

HFC allowances are currently 60% of the baseline. If manufacturers continue to make exactly the same equipment with the same HFC refrigerants that they have been using, they will run out of their HFC refrigerant allowance before the end of the year. I believe most manufacturers will start producing R32 and R454B equipment this year. Just by switching from R410A to R32 a manufacturer can triple the amount of refrigerant they can use. Doing the math 60% x 3 = 180%. Suppose they used up half of their allowance on R410A before switching, that would be 30% x 3 = 90%. This means they could actually increase their overall production. The math is even better for R454B. 60% x 4.5 = 270%, or 135% if you had already used up half your allocation on R410A.

Down the Road 2029

Things get a bit tighter down the road in 2029 when HFCs are restricted to 30% of baseline. Now the R32 calculation is 30% x 3 = 90%, meaning you are restricted to less than the baseline for your entire production. The manufacturer will either have to use a refrigerant with a lower GWP or figure out a way to reduce the charge of the systems they produce. R454B looks a bit better with 30% x 4.5 = 135%, so you are still in business assuming you have not increased production a great deal and you are not using your HFC allocation for anything else.

Down the Road 2034

By 2034 manufacturers will definitely need to have more answers because the allocation drops to 20% of baseline. Now the R32 calculation is 20% x 3 = 60% and the R454B calculation is 20% x 4.5 = 90%: both fall short, and that is before accounting for any growth or using your HFC allocation for any other purpose.

Crystal Ball

In ten years or less I believe we will see systems using other refrigerants start to take over. At this point, I think the manufacturers are thinking HFOs. They are already widely used in car air conditioning and have very low GWPs. What remains to be seen is whether they will be widely accepted. The European Union is pushing back against fluorochemicals in general. They want “natural” refrigerants such as propane or carbon dioxide. Europe seems more comfortable with R290 in larger systems than we are in the US. I don’t think the added fire risk of R-290 is going to be accepted in larger systems in the US. I don’t believe CO2 is practical in smaller systems the size of residential air conditioners and heat pumps. Whatever the form, I believe in ten years we will be looking at very different equipment.  

A2L Guides, Standards, and Codes

The regulations regarding A2L refrigerant are found in a tapestry of Guides, Standards, and Codes which often refer to each other. Here is a list of the most consequential.

ICC International Mechanical Code 2024, IAPMO Uniform Mechanical Code 2024
These recently revised codes allow the use of A2L refrigerants in traditional HVAC systems. They specify conditions for use of A2L refrigerant and refer to ASHRAE Standard 15/34 2022 and UL 60335-2-40, 2022.

Amendments to ICC International Mechanical Code 2021, IAPMO Uniform Mechanical Code 2021
Some states have passed amendments to their existing 2021 codes to allow the use of A2L refrigerants in traditional HVAC systems and specifies conditions for use of A2L refrigerant. They refer to ASHRAE Standards 15/34 2019 and UL Standard 60335-2-40, 2022.

UL 60335-2-40, 2022
This is the latest standard from UL for HVAC systems. It is similar to the international IEC standard with the same name and number. The provisions in the IEC standard have already been in place in many other places around the world; including, Europe, Japan, and Australia. UL 60335-2-40 spells out in detail how A2L refrigerant may be safely applied. It refers to ASHRAE Standards 15/34 2022.

ASHRAE Standards 15 2022 “Safety Standard for Refrigeration Systems”
This standard describes how refrigeration systems may be safely installed and operated. The 2022 edition includes specific conditions for A2L refrigeration systems.

ASHRAE Standard 15.2 “Safety Standard for Refrigeration Systems in Residential Applications” Standard 15 addresses larger buildings while 15.2 describes how refrigeration systems may be safely installed and operated in residential low-rise applications, including specific conditions for A2L refrigeration systems.

ASHRAE Standard 34 – 2022 “Designation and Classification of Refrigerants” Standard 34 lists refrigerant safety ratings and important safety data for a long list of refrigerants, including several A2L refrigerants. Data listed in Standard 34 is used to determine specific system requirements detailed in Standards 15 and 15.2. Taken together, Standards 15 and 34 provide very clear guidance for application of A2L refrigerant.

EPA Final SNAP Ruling 23
This ruling specifically allows the use of A2L refrigerants R-32, R-452B, R-454A, R-454B, R-454C, and R-457A in new residential and light commercial air conditioners and heat pumps. The rule incorporates UL 60335-2-40 by reference.

EPA AIM Act Final Ruling, Sept 2021
This ruling establishes the HFC allocations for the phasedown of HFC refrigerants under the AIM Act. One unexpected significant component of this ruling is a ban on disposable refrigerant cylinders beginning in 2025.


AHRI Guideline M 2020 “Unique Fittings and Service Ports for Flammable Refrigerant Use”
AHRI Guideline M specifies that service connections for systems with flammable refrigerant. For cylinders holding less than 50 lbs. of A2L refrigerant, it specifies CGA 164 fittings which are 1/4 inch flare with left-hand threads. For equipment using A2L refrigerant it specifies 1/4 inch flare with right-hand threads, exactly the same as those used on systems with A1 refrigerant. Same size and same threads.


CGA Standard V-1 2019 “Standard for Compressed Gas Cylinder Valve Outlet and Inlet Connections” The CGA V-1 standard describes the connections on compressed gas cylinders. Standard CGA V-1 2019 introduces the CGA 164 connection specifically for A2L refrigerant cylinders. It describes the CGA 164 connection as a 1/4 inch flare with left hand threads. The latest edition of this standard is now CGA V-1 2021.

What is an HFO Refrigerant?

I have talked to many folks who wondered what exactly is the difference between HFC refrigerants, the refrigerants being phased down, and HFO refrigerants, the low GWP refrigerants that will be replacing HFCs in many applications. The puzzle is that HFOs are also HFCs. That is, they contain hydrogen, fluorine, and carbon. So why the different name?

Alphabet Soup

For many years we have used a sort of short-hand to describe a refrigerant based on the atoms in the molecule. CFC for the older chlorofluorocarbons containing chlorine, fluorine and carbon. HCFC for the Hydrochlorofluorocarbons containing hydrogen, chlorine, fluorine, and carbon. And more recently, HFC for the hydrofluorocarbons containing hydrogen, fluorine, and carbon. So it is natural to think that the O in HFO stands for a single chemical, but it doesn’t. Instead, the O stands for Olefin, which is a description of a hydrocarbon chain containing a double bond between two of the carbons. All our previous hydrocarbon-based refrigerants (CFCs, HCFCs, HFCs) were all built on hydrocarbon chains that used only single bonds. For example R-12 and R22 are methane molecules  while R-32, R-125, and R-134a are ethane molecules. HFOs (Hydrofluoro olefins) are based on carbon chains that contain a double bond between two of the carbons, in other words, an olefin. The most well known HFO R1234yf is based on propene. Notice the “ene” at the end. Hydrocarbons ending in “ane” are single bond molecules while hydrocarbons ending in “ene” have a double bond between two of the carbon atoms.

Why This Matters

So why is this important? The olefin based compounds break down much more rapidly in the air than their single bond cousins, which is how they achieve such low global warming numbers. Their calculated GWP is much lower because of their short atmospheric life. They don’t survive intact for long in the atmosphere. This reduced chemical stability is also why the HFO refrigerants are mildly flammable. So what is the difference between an HFC and an HFO? Basically the way they are put together.

A2L Refrigerant Standards and Regulations

The regulations regarding A2L refrigerant are found in a tapestry of Standards, Codes, and Rulings. I like to research using original documentation whenever possible. Not that I don’t trust the folks doing webinars, blogs, and videos, but when answering questions about new technology I want to be able to point to authoritative documents. So, I look for the actual standards, guidelines, and codes. I found it a bit confusing because there are so many agencies publishing many standards and regulations regarding A2L refrigerant. Furthermore, these standards often refer to each other. I have listed below some of the more important documents you should study if you like to do your own research.

ICC International Mechanical Code 2024, IAPMO Uniform Mechanical Code 2024
These completed but yet unpublished codes allow the use of A2L refrigerants in traditional HVAC systems and specify conditions for use of A2L refrigerant. They refer to AHRI Standards 15 and 34 2019 and UL 60335-2-40, 3rd edition.

Amendments to ICC International Mechanical Code 2021, IAPMO Uniform Mechanical Code 2021
Some states have passed amendments to their existing 2021 codes to allow the use of A2L refrigerants in traditional HVAC systems. Typically these amendments accomplish this by referring to AHRI Standards 15/34 2019 and UL Standard 60335-2-40, 3rd edition.

UL 60335-2-40, 3rd edition
This is the latest standard from UL for HVAC systems. It is similar to the international IEC standard with the same name and number. The provisions in it have already been in place in many other places around the world; including, Europe, Japan, and Australia. It spells out in detail how A2L refrigerant may be safely applied. It refers to ASHRAE Standards 15/34 2019.

ASHRAE Standard 15 – 2019 Safety Standard for Refrigeration Systems describes how refrigeration systems may be safely installed and operated. The 2019 edition includes specific conditions for A2L refrigeration systems.

ASHRAE Standard 34 – 2019 Designation and Classification of Refrigerants lists refrigerant safety ratings and important safety data for a long list of refrigerants, including several A2L refrigerants. Data listed in Standard 34 is used to determine specific system requirements detailed in Standard 15. Taken together, Standards 15 and 34 provide very clear guidance for application of A2L refrigerant.

ASHRAE Standard 15.2 – 2022 Safety Standard for Refrigeration Systems in Residential Applications is the low-rise residential companion to ASHRAE Standard 15. Standard 15 has historically primarily been applied to larger commercial buildings, not low-rise residential homes. Standard 15.2 describes in detail what must happen to safely use A2L refrigerant in a residential application.

EPA Final SNAP Ruling 23, April 2021
This ruling specifically allows the use of A2L refrigerants R-32, R-452B, R-454A, R-454B, R-454C, and R-457A in new residential and light commercial air conditioners and heat pumps. The rule incorporates UL 60335-2-40, 3rd edition by reference.

EPA AIM Act Final Ruling, Sept 2021
This ruling establishes the HFC allocations for the phasedown of HFC refrigerants under the AIM Act. One unexpected significant component of this ruling is a ban on disposable refrigerant cylinders beginning in 2025.

AHRI Guideline M 2020, Unique Fittings and Service Ports for Flammable Refrigerant Use specifies that service connections for systems with A2L refrigerant should be exactly the same as those used on systems with A1 refrigerant. The connection on A2L refrigerant cylinders is described as a CGA 164 connection. The CGA 164 connection is described in the CGA Standard V-1 2019 as a 1/4 inch flare with left hand threads.

CGA Standard V-1 2019 Standard for Compressed Gas Cylinder Valve Outlet and Inlet Connections introduces the CGA 164 connection. It is designed specifically for A2L refrigerant cylinders. The standard describes the CGA 14 connection as a 1/4 inch flare with left hand threads. While the CGA 164 connection is first introduced in the 2019 edition, the latest edition of the CGA V-1standard is now 2021.

UL Standard 207 Standard for Safety Refrigerant-Containing Components and Accessories, Nonelectrical covers nonelectrical, refrigerant-containing components and accessories in accordance with ASHRAE Standard 15. This standard is specifically referenced by ASHRAE Standard 15.2 when describing fittings, valves, and mechanical joints.

Decoding HFO Refrigerant Numbers

Undoubtedly you have seen news articles mentioning HFO refrigerants with names like 1234yf, 1234ze(Z), or 1234ze(E). Although these names look like a secret code, there is method in the madness. The good news is that technicians probably don’t need to know exactly how to read this secret code to do their job. However, telling me I don’t need to know what’s behind the curtain just encourages me to pull the curtain back. So here goes.

What is an HFO

First, you need to understand what an HFO is. An HFO is essentially an HFC with a double bond between two carbon atoms. You might remember from high school chemistry that carbon has a valence of 4. Think of the valence as the number of Velcro tabs on the atom. The carbon atoms in a normal hydrocarbon molecule are joined by single bonds, just one set of Velcro tabs joined between each carbon atom in the chain. They are called saturated because they are connected to largest number of atoms possible. Unsaturated hydrocarbon molecules, like HFOs, have a double bond between two of the carbon atoms. They use two sets of Velcro tabs between two of the carbon atoms. The double bond means there is one less atom in the molecule since two bonds are used between a pair of carbon atoms. Thus the designation as unsaturated.

Secret Code

The first four numbers of the secret refrigerant numbering code identify, in order: the number of double bonds, the number of carbon atoms, the number of hydrogen atoms, and the number of fluorine atoms in the molecule. However, there are many ways those atoms can be arranged, and different arrangements of the same components create different refrigerants with different physical properties. The last two or three letters describe how the atoms are arranged in the molecule.

First Number

The first number in the HFO numbering system describes the number of double bonds. At present, I am not aware of any HFO refrigerants that have more than one double bond. Currently all HFO refrigerants start with the number 1.  The 1 at the start of R1234ze(Z) indicates that the molecule has one double bond.  

Second Number

The second number is equal to the number of carbon atoms minus one.  The 2 in R1234ze(Z) indicates that the molecule has three carbon atoms: (#Carbons (3) – 1 = 2).

Third Number

The third number is equal to the number of hydrogen atoms plus one. The 3 in R1234ze(Z) indicates that the molecule has two hydrogen atoms (#Hydrogens (2) + 1 = 3).

Fourth Number

The fourth number is equal to the number of Fluorine atoms. The 4 in R1234ze(Z) indicates that the molecule has four Fluorine atoms.

First Lower Case Letter

HFO refrigerants are based on propylene, which has three carbon atoms. The first lower case letter identifies the atom connected to the middle carbon atom: x for chlorine, y for fluorine, and z for hydrogen. The lower case z in R1234ze(Z) indicates that the atom bonded to the middle carbon is hydrogen.

Second Lower Case Letter

The way the atoms are arranged on the ends of the molecule can vary. The second lower case letter describes the arrangement of the atoms on the end carbon containing the double bond. The letters are defined as 

a: 2 chlorine atoms

b: 1 chlorine atom and 1 fluorine atom

c: 2 fluorine atoms

d: 1 hydrogen atom and 1 chlorine atom

e: 1 hydrogen atom and 1 fluorine atom

f: 2 hydrogen atoms

The lower case e in R1234ze(Z) indicates that the end carbon with the double bond is connected to 1 hydrogen atom and 1 fluorine atom.

The Upper Case Letter in Parenthesis

In some instances, there are two ways to connect the remaining hydrogen atoms. (Z) indicates the hydrogen atoms are on the same side of the double carbon bond. Z stands for zusammen: German for together. (E) indicates the hydrogen atoms are on opposite sides of the double carbon bond. E stands for entgegen: German for opposite. The (Z) on the end of R1234ze(Z) indicates that the two hydrogen atoms are located on the same side of the carbon double bond.

Although R1234yf, R1234ze(E), and R1234ze(Z) are all built out of the exact same type and number of atoms, the difference in how the atoms are arranged makes them three different refrigerants with different physical properties.  

Zeotropic Refrigerant Terms

Zeotropic refrigerants, or blends, are now the most common refrigerants available. Any refrigerant whose ASHRAE number is 400 something is zeotropic. A whole list of somewhat confusing terms are used to describe the characteristics of these refrigerants. The first issue to tackle is simply what zeotropic means. I find remembering multisyllabic tehno-jargon much easier if I understand the derivation of the word. In ancient Greek zeo means “to boil” while tropo means “to change”.  Putting the two together zeotrope means that something changes when it boils. That is why refrigerant mixtures whose percentage composition charges when they boil are referred to as zeotropic. Also in Greek, the prefix a means “not”. It basically inverts the meaning. That is why refrigerant mixtures that do NOT change when they boil are known as azeotropic.

Dew-point, Bubble-point, and Glide

Because zeotropic refrigerants change percentage composition as they change state, the temperatures at which they start to evaporate and condense are different for any given pressure. Dew-point describes the temperature at which the first liquid droplets start to form in a saturated vapor, and bubble-point describes the temperature at which the first bubbles start to appear in a saturated liquid. For a “normal” refrigerant these are the same temperature. For zeotropic refrigerants, at any given pressure the dewpoint is a little higher than the bubble-point. Glide is the difference between the bubble-point temperature and the dew-point temperature at any given pressure. Low glide blends typically have a glide of less than 2°F, while high glide blends have glides as high as 10°F. You may also have seen the term near-azeotropic. Personally, I think near-azeotropic sounds like something coined by the marketing department. It literally means “nearly does not change when it boils”. I believe a more useful description is a low-glide zeotropic mixture.

Zeotropic PT Charts

When calculating evaporator and condenser pressures, superheat, or subcooling I admit I have to think a bit when figuring out if I need to know the dew-point, bubble-point, saturated liquid temperature, or saturated vapor temperature. I find the terminology can be confusing. I would like to offer a way to keep these terms straight. Refrigerant pressure temperature charts typically have one column for pressure and two columns for temperature. The temperature columns will either be the dew-point and bubble-point, or saturated vapor and saturated liquid.

Saturated Vapor and Saturated Liquid

Since superheat involves determining the temperature of a gas in excess of its saturation temperature it makes sense to use the saturated vapor temperature when calculating superheat. Similarly, since subcooling involves determining the temperature of a liquid below its saturation temperature, it makes sense to use the saturated liquid temperature when calculating subcooling.

Dew-point and Bubble-point

The terms here can lead you astray because they describe the beginning of condensation or evaporation. However, they also describe the very last stage of any amount of vapor in a saturated liquid (bubble-point) or liquid in a saturated vapor (dew-point). The key here is to pay attention to the state the bulk of the refrigerant. For example, at bubble-point, most of the refrigerant is a liquid because the first bubbles are just starting to form. So, the bubble-point temperature is used when calculating subcooling. Bubble-point and the saturated liquid temperature describe the same condition. At dew-point, most of the refrigerant is a vapor because the first liquid droplets are just starting to form. So, the dew-point temperature is used when calculating superheat. Dew-point and saturated vapor temperature describe the same condition. I hope this helps you sort out all the techno-babble surrounding 400 series refrigerants.

No More R-22 Production

No more manufacturing or importing R-22 beginning January 1, 2020. That day has come and gone. It is time to take a hard look at how you service R-22 equipment. If the equipment needs charging there is something else wrong. There are two common practices that should end: seasonal topping off and “diagnostic” refrigerant additions.   

Topping Off

Simply adding refrigerant has not fixed the problem. Adding refrigerant to a leaky system is like giving aspirin to someone with a fever: it temporarily decreases the pain but does nothing to solve the underlying problem, which will return soon enough. If a system that has been in service truly needs charging, it either has leak or a recreational refrigerant inhaler resides nearby. You need to locate and repair the leak and/or the recreational user. In cases where a leak cannot be found, the addition of dye might help with both issues. Place a large warning sticker on the equipment near the service valves notifying future service techs that “indelible dye” has been added. The recreational inhaler will most likely find another source. If there are leaks, they should show up by the time more refrigerant is needed. Then you can determine if repair is feasible or if the system needs replacing.

“Diagnostic” Refrigerant Charging

I use the term diagnostic loosely here. A “diagnostic” refrigerant charge involves adding refrigerant to see what happens. You are adding refrigerant because the unit is not cooling and you don’t know what else to do. This is a poor practice at best. It is an especially poor practice when servicing R-22 systems.   

R-22 Replacements

Although there are many R-22 replacement refrigerants available, there are no replacements that can legally simply be added on top of existing R-22. Fortunately, new and reclaimed R-22 is still readily available at fairly reasonable prices because the demand for it has decreased. I recommend staying with R-22 as long as it is available at reasonable prices, which it is. Converting an R-22 system to another refrigerant is time consuming, costly, and carries with it the possibility of killing the unit. The difference in the refrigerant cost just does not justify the time and expense required to correctly and legally perform the refrigerant conversion.  

A2L Refrigerants and Codes

In the last post I talked about what it really means for a refrigerant to be classified as an A2L refrigerant. One practical ramification in the United States in 2019 is that an A2L refrigerant cannot be used in a direct expansion system in most buildings in the United States. Most building codes refer to ASHRAE Standard 34 Designation and Classification of Refrigerants and Standard 15 Safety Standard for Refrigeration Systems. Until the most recent revision in 2019, Standard 15 forbid the use of flammable refrigerant in what it describes as “direct systems.” And until the most recent revision in 2019, Standard 15 made no distinction between levels of flammability. Flammable is flammable. Because nearly all mechanical, building, and fire safety codes use ASHRAE as their refrigerant safety reference, no codes presently allow A2L refrigerants in direct systems. A direct system is one in which the building air is directly exposed to the refrigeration components, as in a normal direct expansion evaporator coil. However, now that ASHRAE has revised Standard 15, look for states and code agencies to begin adopting the revised standard. Washington state has already done that. Beginning July 1, 2020 direct expansion A2L systems will be allowed in Washington State subject to the stipulations of the revised 2019 Standard 15. There will be industry pressure for adoption of the new standards. Major refrigerant manufacturers such as Honeywell and Chemours have invested heavily in developing lower GWP refrigerants, many of which are rated A2L. Equipment manufacturers have invested heavily in designing equipment that uses R32, a lower GWP A2L refrigerant. If you would like to read more details of the revised 2019 Standards 15 and 34, ASHRAE here is a link where you can view them on-line https://www.ashrae.org/technical-resources/standards-and-guidelines/read-only-versions-of-ashrae-standards

Of course, ASHRAE will also sell you a downloadable pdf which is really better for extenedreading and studying.

What Does Mildly Flammable Mean?

I confess that I have always thought of flammability as an either or question: it either burns or it doesn’t. So the concept of different levels of flammability was a hard one for me to grasp. I wondered: what is the difference between 3,2, and 2L refrigerant designations? What follows is a somewhat lengthy discussion of what I learned.

First off, I found that it is not all that simple. There are several flammability characteristics that can be compared: lower flammability limit, upper flammability limit, auto ignition temperature, minimum ignition energy, heat of combustion, and flame velocity. The table at the bottom of the article shows these different specifications for a small selection of flammable refrigerants. Note that pressure and temperature also play a part. For the ASHRAE safety tests, a temperature of 140°F at atmospheric pressure is specified. You get different results when applying higher pressures and temperatures.

The original three classifications (1,2,3) were determined by the lower flammability limit and the heat of combustion. Later, ASHRAE added a 2L category for refrigerants with burning velocities less than 10 centimeters per second. The table below summarizes the different flammability classifications.

Classification Lower Flammability Limit % by volume Heat of Combustion Burning Velocity
1 Does not support combustion at atmospheric pressure
2L Greater than 3.5% Less than 19 kj/g 10 cm/s or less
2 Greater than 3.5% Less than 19 kj/g Greater than 10 cm/s
3 3.5% or less 19 kj/g or more NA

Lower flammability limit (LFL) is the minimum percentage required in air to be combustible. For example propane (R290) has an LFL of 2.1% by volume while ammonia (R717) has an LFL of 15%. Notice that propane only requires 2.1% while ammonia requires 15%. So that is one difference – the amount that must build up before it can burn.

The upper flammability limit (UFL) describes the maximum concentration which will still burn. If the concentration of flammable vapors exceeds the UFL, it will not ignite. It is more difficult to draw a straight line comparison using the UFL. However, you can say that refrigerants whose LFL and UFL are closer together are generally a bit safer simply because the conditions for a flammable mixture are less likely to occur.

The auto ignition temperature is the temperature which the flammable mixture will ignite. With the exception of 1234yf, refrigerants with a lower flammability have higher auto ignition temperatures than the more flammable refrigerants.

The minimum ignition energy is a bit different than the auto ignition temperature. It is the amount of energy that must be used to ignite a flammable mixture, measured in megajoules. Note that in this case R1234yf stands out because the minimum ignition energy is so high compared to the other refrigerants. Also note that the class 2L refrigerants all have minimum ignition energy ratings in the hundreds of megajoules or higher while propane’s minimum ignition energy is a very small 0.25 megajoules. Basically, this means it takes a lot more energy to ignite a class 2L refrigerant than a highly flammable class 3 refrigerant such as propane. Again, this means that the chance of having the right condition for combustion is much lower for class 2L refrigerants.

The heat of combustion is a measure of the amount of heat created when the refrigerant burns. Note that the class 2L and class 2 refrigerants have a heat of combustion in the single digits per gram while propane jumps to 46 kilojoules per gram. This means that the heat produced by combustion of a class 2L or class 2 refrigerant is far less than a class 3 refrigerant. Indeed, it would be possible for a class 2L refrigerant to burn and not ignite other nearby flammable materials.

Burning velocity is the characteristic which distinguishes class 2 and 2L refrigerants. It is the speed with which the flame advances. Note that the 2L class refrigerants have a burning velocity in the single digits while 152a, a class 2 refrigerant, has a burning velocity of 23 cm/sec. Propane’s burning velocity is twice that of 152a. The take home point here is that the flames from higher flammability refrigerants spread faster.

So wrapping it up, my general impression is that

1. Lower flammability refrigerants (2L) are less likely to burn in the first place.

2. When class 2L refrigerants do burn, the flames are not as hot as higher flammability class 3 refrigerants.

3. The flames from burning  2L refrigerant do not spread as quickly as the flames from higher flammability class 3 refrigerants.

Refrigerant R1234yf R32 717 Ammonia 152a 290 Propane
Safety Group A2L A2L B2L A2 A3
Lower Flammability LImit 6.5% 14.4% 15% 3.9% 2.1%
Upper Flammability Limit 12.3% 33.3% 28% 16.9% 10%
Auto Ignition Temperature 405°C 648°C 651°C 440°C 455°C
Minimum Ignition Energy 5,000 – 10,000 mJ 30 – 100 mJ 100 – 300 mJ 0.38 mJ 0.25 mJ
Heat of Combustion 9.5 kJ/g 9 kJ/g 22.5 kJ/g 6.3 kJ/g 46.3 kj/g
Burning Velocity 1.5 cm/sec 6.7 cm/sec 7.2 cm/sec 23 cm/sec 46 cm/sec
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