In a heating, ventilating, and air conditioning (HVAC) system using a flammable refrigerant, a concentration sensor is used to check concentrations in the conditioned air or other locations. If concentrations are detected indicative a catastrophic or large leak, a purge valve is activated to immediately purge the refrigerant from the closed-conduit circuit of the system to a safe location. With lesser leaks, the blower may be activated and the leak monitored.