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.