One of the recent global trends in the air conditioning industry is to replace the high global warming potential (GWP) refrigerants such as R410A and R134A with alternative refrigerants that have lower GWP. However, most of these alternative refrigerants are classified as slightly flammable (classified by ASHRAE as A2L) which impose a potential fire risk. Therefore, research efforts have focused on the study of flammable refrigerants and their potential risk. In this paper, we present a numerical investigation to study one R32 (ASHRAE A2L) leak scenario from a 5 ton rooftop unit into a conference room with the dimensions of 40 × 30 × 8 ft (12.2 × 9.1 × 2.4 m). A catastrophic rupture of a ¼-inch refrigerant line was simulated. A total charge of 15lb (6.8 kg) was leaked in 31.5 seconds. A mixture of air and refrigerant is diffused from four air diffusers mounted in the ceiling. The ducting system was simulated first to predict the air/refrigerant mixure entering the room from the four air diffusers.The total simulation time was 10 minutes to study the refrigerant diffusion and dispersion of the flammable refrigerant in the room after the inception of the leakage. The volume fraction is presented as a function of time at different monitoring points inside the room. The total room volume where the R32 concentration exceeds its LFL is quantified and presented as an indication of the risk of ignition.