R-410A is currently the primary refrigerant used in residential AC systems and heat pumps. It is a greenhouse gas, and possible replacements include ASHRAE Class 2L refrigerants, which have reduced global warming potential but are mildly flammable. Accidental releases due to equipment faults or fatigue could potentially result in refrigerant ignition if a sufficient ignition source is present. A risk assessment of R-32, R-1234yf, and R-1234ze(E) in residential split heat pump systems was conducted. The work included CFD modeling, experimental measurements, and a fault tree analysis (FTA) to quantify ignition risks. The assessment indicated that large releases of R-32, R-1234yf and R-1234ze(E) (i.e., 170 g/s for R-32, 78 g/s for R-1234yf and R-1234ze(E)) from units installed in basements, garages or attics would not reach flammable concentrations in areas where a sufficient ignition source might be present. Large releases of all three refrigerants from a unit located in a utility closet can generate flammable concentrations in the utility closet, although the time the refrigerant concentration was flammable was brief (70 s for R-32 and R-1234yf and 45 s for R-1234ze(E)). Flammable concentrations did not occur with smaller leaks of R-1234ze(E) and R-1234yf (e.g., 1.5 g/s or less) in utility closets. The FTA estimated that the risks of refrigerant ignition due to an accidental leak of R-32, R-1234yf and R-1234ze(E) were 9 x 10-5, 2 x 10-5 and 2 x 10-5 ignition events per unit per year, respectively. For comparison, the risk of a significant home fire in the US is 1 x 10-3 per home per year. The FTA in this study considered refrigerant ignition and did not determine whether a fire would ensue due to the ignition of surrounding materials. The analysis also did not include potential mitigation factors that would further reduce the probability of refrigerant ignition.