HVAC standards, such as EN-378, use refrigerant flammability and toxicity properties safety properties to define refrigerant classifications and practical limits for systems for different access categories.  Refrigerant charge limits are defined based on the lowest charge determined from the flammability and toxicity data. A wide range of flammability properties (see below) have been investigated due to the introduction of mildly flammable refrigerants. The refrigerant safety data is generally developed by a refrigerant supplier and submitted for review by ASHRAE 34 or ISO 817 committees. Typically, ISO 5149 and EN378 use the same data sets.

Flammability Properties

International and European safety legislation and standards such as ISO 5149 and EN 378 define requirements to remain well below the lower flammable limit in case of accidental leakage. Other flammability properties determine the ease of ignition (minimum ignition energy) and the pressure rise for ignition in confined spaces.

If a refrigerant is flammable, the classification properties used are:

  • Lower flammability limit (LFL)
  • Heat of combustion (HoC)
  • Burning velocity (BV)

These are a subset of flammability properties which can also include:

  • Upper flammability limit (UFL),
  • Minimum ignition energy (MIE),
  • Peak constant volume pressure rise for an enclosed combustion
  • Deflagration index, measures the relative explosion severity, calculated from the maximum rate of pressure rise and the test vessel volume
  • Auto-ignition temperature (AIT)

Some of these properties are linked.  The charts show flammability data for refrigerants R-290 (A3), R-152a (A2), R-717 (B2L), R-32 (A2L), and R-1234yf (A2L). This data supports proposed links between flammability properties and helps explain why only three properties are used for refrigerant classification:

  • Heat of combustion (HoC), peak constant volume pressure and deflagration index all relate to how energetic combustion is.
  • Burning velocity [1]
    • inversely correlated with the minimum ignition energy (higher burning velocities were more easily ignited by lower ignition energies)
    • appears to be correlated to LFL and deflagration index
    • little to no correlation was found between BV and the auto-ignition temperature, with the reason likely being the nature of the standard test for measuring AIT, which is conducted by heating the mixture on all sides

Independent flammability testing is also carried out, for example by SAE CRPs (Co-operative Research Programs) and the Japan Society of Refrigerating and Air Conditioning Engineers as part of risk assessments for the use of flammable refrigerants.

Flammability References

[1] Journal of Loss Prevention in the Process Industries 49 (2017) 662 – 674, Flammability and explosion characteristics of mildly flammable
Refrigerants, S.G. Davis, J.L. Pagliaro T.F. Debold, M. van Wingerden, K. van Wingerden. Note, some of the flammability data in this reference is different to that usually quoted for refrigerants, but different test methods can result in different results although with the same trends
[2] Risk Assessment of Mildly Flammable Refrigerants Final Report 2016, March 2017, The Japan Society of Refrigerating and Air Conditioning Engineers
[3] Summary of SAE CRP1234 Refrigerant Evaluation and Risk Assessment – July 15, 2010
[4] IoR Guidance note 29: BS EN378:2016 – Summary of Changes


The refrigerant toxicity classification has 400 ppm Occupational Exposure Limit (OEL) as the divide between higher and lower toxicity refrigerants. All widely used HFCs (except HFC-245fa), HFOs and HCFOs are classed as low toxicity, with OELs in the range 400 to 1000 ppm. The hydrocarbon refrigerants are also classified as low toxicity. ASHRAE 34 also cites, in its refrigerant data and safety classification table, the toxicity category of the International Fire Code and Uniform Fire Code regulations.  The toxicity category is either highly toxic (LC50 ≤200 ppm), toxic (LC50 ≤ 2000 ppm) or neither (lesser toxicity than either of these groups). All the widely used HFCs, HFOs and HCFOs are in the ‘neither’ category. Ammonia has a OEL (time weighted average TWA) of 25 ppm and a short-term exposure limit (STEL) of 35 ppm, and according to ISO817:2014 an acute toxicity exposure level of 320ppm. The acute toxicity exposure level (ATEL) of refrigerants is used to determine toxicity based practical limits, with the maximum value being set by the limit of oxygen deprivation (ODL), which is 140,000ppm (14% v/v).