Low-GWP Flammable Refrigerants and Fire Risk: The Importance of Leakage-Induced vs Reaction-Influenced Scenarios

HFCs became the predominant refrigerant type after the Montreal Protocol ban on CFCs and HCFCs due to ozone depletion characteristics. Whilst HFCs do not affect the ozone layer, they have significant global warming potential. Thus, the Kigali Amendment established a global phase-down of high-GWP HFCs. Low-GWP alternatives for applications in residential environments include hydrocarbons, HCs, such as R290, propane. The HCs present, however, higher flammability characteristics. Hence, international standards limit their allowable charge in residential applications. In 2022 these standards eased certain limits focusing on leakage control within closed spaces. Leakage of flammable refrigerants has been considered the primary fire hazard in research and regulations on this topic. Conversely, the reaction of components filled with refrigerants to an independent fire has not received the same attention, despite their key role in the energy transition. This study reviews the current research on the fire risk of flammable refrigerants and the relevant regulatory approach. The review and the analysis of relevant empirical evidence from past accidents highlight the importance of fire reaction compared to leakage hazards. The yearly probability of a leaked refrigerant ignition is seven orders of magnitude less likely than an independent fire. The heat release rate from components engulfed in fire is not negligible and refrigerants can increase it. Data show that external refrigerant units may support fires on building façades and involve combustible thermal insulation. Finally, novel event trees for quantitative fire risk assessment and possible mitigation measures are provided.