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Non-flammable, slightly flammable, A2L, A1…What do these statements mean and how does that help you decide which refrigerant is best for your application? This article will cover all of this and how the flammability categories are determined.

Let’s start with a simple question. Are all refrigerants flammable?

Under normal circumstances, A1 refrigerants are not flammable while A2L, A2 and A3 refrigerants are flammable to a varying degree.

However, it’s important to recognize that A1 refrigerants can be made flammable if used improperly.  This is why safety data sheets (SDS) for refrigerants like R-22 (classified as non-flammable) warned against pressurizing a system that contained small amounts of refrigerant with shop compressed air.  Doing so could create a flammable condition, and if an ignition source is introduced, as in welding, an accident can occur.  A quick internet search should return multiple examples of these types of accidents.

So, how is a refrigerant classified as flammable?

Some of it’s the refrigerant properties are measured against a pre-determined set of criteria. Depending on where it falls, it will be labelled as non-flammable (which ASHRAE 34 refers to as “no flame propagation”) slightly flammable or highly flammable.  Using ASHRAE 34 terminology, A1/B1, A2/B2, A2L/B2L or A3/B3 (only the numbers in the code refers to flammability; the initial letter (A or B) indicates levels of toxicity, which we do not cover here, but we will use it here just for reference).

ASHRAE Standard 34 classifies flammability based on the answers to 3 questions:

1.     What is the minimum quantity of refrigerant needed in air to make it burn?

2.     How fast will it burn?

3.     How much heat is generated when it burns?

ASHRAE Standard 34 refers to these questions as Lower Flammability Limit (LFL), Burning Velocity (BV) and Heat of Combustion (HC).

Here is another explanation:

A refrigerant that requires a larger quantity of it in air to burn (high LFL), typically burns slowly (low BV) and generates low heat (low HC). It can then be considered less flammable than a refrigerant that requires small amounts to burn, burns quickly and generates a lot of heat.  ASHRAE Standard 34 has set very specific limits to each and guidelines as to how the values are tested and measured.  While it may sound somewhat arbitrary to set artificial limits, the limits are based on careful review of practical considerations. 

The Bottom Line

While all refrigerants should be handled with care, specifically in respect to their flammability, some will require special considerations to handle the added risks as in the case of those classified A2 or A3.  Their flammability classification will help determine any special precautions needed when handling them, and whether they could or should be used for a particular application at all.