HFO-1234yf is a Category 1 Flammable Gas. The manufacturers SDS(s) all say it, as well as 1910.1200 Appendix B. PLEASE see my article Refrigerant HFO-1234yf is a Flammable “4” in the NFPA 704 Diamond (Part 2) for the breakdown of these facts. With this Cat 1 Flammable Gas comes the hazard of flash fire(s)/explosions. Now there are some who like to say that this material is “mildly flammable”, a term that is used only when discussing flammable refrigerants. Refrigerants are classified by ASHRAE as A1 (nonflammable), A2L (mildly flammable), B2L (toxic, mildly flammable), and A3 (flammable). In ASHRAE Standard 34, the rank “2L” was set up for “mildly flammable refrigerants” with burning velocities lower than 10 cm/s. Together with ammonia, R1234yf is in the “2L” group for flammability; although ammonia is a B2L since it is highly toxic and HFO-1234yf is A2L since it is lower toxicity. These “safety groups” are based on Flammability (1, 2L, 2, or 3) and Toxicity (A or B). Here is where HFO-1234yf falls on the scale …