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