Hydrofluoroolefin (HFO) and Hydrocarbon (HC) refrigerants are referred to as fourth-generation refrigerants for the 21st century following chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs).

HFO refrigerants are actually unsaturated HFC refrigerants and are widely recognized as the next generation of refrigerants because of their environmental friendliness, cost-effectiveness, and great energy efficiencies.

HFOs are distinguished from HFCs by being derivatives of olefins rather than alkanes (paraffins). Olefins have carbon atoms linked by a double bond where alkanes have single bonds between carbon atoms. Hydrofluorocarbon (HFC) refrigerants like R-134a, -125, -143a, and -152a, and HFC-based refrigerant blends like R-507, -407A, -407B, -407C, and -410A are all composed of hydrogen, fluorine, and carbon connected by single bonds between the atoms. Hydrofluoroolefin (HFO) refrigerants are also composed of hydrogen, fluorine, and carbon atoms, but contain at least one double bond between the carbon atoms.

HFO-1234yf and HFO-1234ze

Two popular HFO refrigerants are HFO-1234yf and HFO-1234ze (See table 1 here). Both HFO-1234yf and HFO-1234ze have a zero ozone depletion potential (ODP) with extremely low global warming potentials (GWP). Because of their lower GWP values, these refrigerants have a much shorter life cycle in the atmosphere.

These two HFO refrigerants have an A2L safety classification meaning they have low toxicity but are slightly flammable (See Table 2 here). A2L and B2L are low-flammability refrigerants with a maximum burning velocity equal to or smaller than 10 centimeter per second (cm/s) or 3.9 inches per second (in./s). The capital letter “L” refers to “lower” flammability. HFOs do exist that have A1 (nonflammable) safety ratings; however, further research and testing needs to be done with these refrigerants.

HFO-1234yf is a low-GWP replacement for R-134a intended for use in mobile air conditioning (MAC) systems in the automotive industry. Many European and some U.S. car manufacturers are currently using HFO-1234yf for mobile air conditioning applications.

HFO-1234yf has a GWP that’s 300 times less than that of HFC-134a.

HFO-1234yf has the lowest switching cost for automakers compared to other alternatives. However, the initial cost of the refrigerant is much higher than that of HFC-134a.

Although the refrigerant is classified as slightly flammable by ASHRAE, several years of testing by the Society of Automotive Engineers (SAE) proved that HFO-1234yf could not be ignited under conditions normally experienced by a vehicle.

HFO-1234ze is intended to replace R-410A in residential and light commercial air conditioning and heat pump applications while offering a 75 percent reduction in GWP. It can be used in both air- and water-cooled chillers in supermarkets and commercial buildings. Other applications include vending machines, fridges, beverage dispensers, air dryers, and carbon dioxide cascade systems in commercial refrigeration.

HFO-1234ze also offers excellent energy efficiency, is cost-effective, and can be used in existing equipment design with minimal changes. It has significant advantages over R-32, which has medium GWP, an A2 refrigerant safety rating, and high discharge temperatures. (A2 means low flammability and low toxicity.) High discharge temperatures can affect system performance and durability in hot climates. Also, oil changes may be needed with R-32. (Table 2 includes a comparison of atmospheric boiling points for some popular CFC, HCFC, HFC, HFO, HC, and natural refrigerants.)

Because of HFOs’ slight flammability, extensive studies have been conducted for its safety in automobiles, centrifugal chillers, and other stationary applications. A key consideration for widespread acceptance of HFO refrigerants is the development of standards and codes for safe use of these mildly flammable A2L refrigerants.


HFOs are miscible in Polyolester (POE)-type lubricating oils. The miscibility of HFOs with POE lubricants is comparable to that of R-134a. HFOs are not soluble in mineral-oil or alkylbenzene lubricants.

Hydrocarbon (HC) Refrigerants

HC refrigerants are classified by ASHRAE in the A3 safety group. Refrigerants in the A3 safety group are highly flammable. Pure HC refrigerants have no chlorine or fluorine in their molecules; they contain nothing but hydrogen and carbon, thus they have a zero ozone depletion potential (ODP).

Many CFC, HCFC, and HFC refrigerants originate from these two base molecules of ethane and methane. However, neither ethane nor methane is used as a stand-alone HC refrigerant today in the U.S. HC refrigerants do contribute to global warming, but their GWP is very low when compared to CFC, HCFC, and HFC refrigerants (See Table 1).

HC refrigerants have better energy efficiencies and reduced energy usage when compared to currently used HFC refrigerants. These refrigerants are created by nature, not a chemical company, and are often referred to as “natural” refrigerants. The most efficient and environmentally safe refrigerants that exist in the world today are all natural refrigerants. These refrigerants include HCs, ammonia, air, water, and carbon dioxide. HCs are used as stand-alone refrigerants in Europe for many applications. Common applications include commercial refrigerators and bottle coolers, split air conditioning units, and domestic refrigerators and freezers.

In the U.S., only some HCs have been approved by the U.S. Environmental Protection Agency (EPA) under its Significant New Alternatives Policy (SNAP). SNAP has approved HC use in the U.S. only in new equipment, in limited conditions, with limited refrigerant charge amounts because of their high flammability. For many years, small percentages of HCs have been used in many HCFC- and HFC-based refrigerant blends in the U.S. to assist in oil return to the compressor. However, refrigerant blends containing small amounts of HC refrigerants are not flammable. This is because the HC refrigerant constitutes only a small percentage (less than 3 percent) of the total refrigerant blend. Some popular hydrocarbons include propane, isobutene, isopentane, methane, ethane, and R-441A — an HC blend of ether, propane, isobutene, and n-butane.

Some refrigerant blends that incorporate HC refrigerants in small percentages for oil return purposes include R-402A, -402B, -403A, -406A, and -438A.

Applications Allowed and Charge Amounts

At the time of this writing (in early November 2014), only R-600a (isobutane), R-290 (propane), and R-441A (HC blend) had been approved by the EPA’s SNAP program for use in the U.S. Uses for these three HC refrigerants include new stand-alone retail refrigerators. Freezer equipment can use propane (R-290) as long as the refrigerant charge does not exceed 5.3 ounces (150 grams); and new domestic refrigerators and freezers or combination refrigerator/freezer can use isobutane (R-600a) or R-441A as long as the refrigerant charge size does not exceed 2.0 ounces (57 grams).

Notice the EPA’s SNAP program only allows HC use in the U.S. in limited applications and in new equipment only. Retrofitting equipment containing HC refrigerants is not allowed in the U.S. Only authorized HC refrigerants on the compressor and system’s nameplate may be used in each specific piece of equipment. Even replacement parts for equipment containing a flammable HC refrigerant must be compatible for that specific refrigerant. Local regulations governing HC usage as a refrigerant may also apply in certain municipalities. It is the responsibility of the servicing technician to stay abreast of local, national, and global regulations that govern HC refrigerants.

Boiling Points and Safety Classifications

HCs, like HCFC and HFC refrigerants, have boiling points that are desirable for refrigeration and/or air conditioning applications. However, because of these refrigerants’ low boiling points, safety precautions must be taken. (Table 2 lists some known CFC, HCFC, HFC, HFO, HC, and natural refrigerants with their safety classifications and boiling points at atmospheric pressure for comparison purposes.)

Publication date: 12/1/2014

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