Over the past several years, many Northeast states have begun implementing measures to transition the electric grid to clean energy and increase the adoption of technologies that use electricity as a source of energy. This has fueled rapid growth of the heat pump market in the region. More recently, however, concerns have been raised about the impact these systems’ refrigerants may have on the environment. The story of HVAC refrigerants is complicated, but this paper attempts to explain the basics. The history, chemistry, safety concerns, environmental impact, and regulatory landscape all help inform the promotion and regulation of HVAC systems. If you're curious about the next generation of refrigerants, this paper will prepare you to participate in the HVAC industry’s fourth refrigerant phase down, now focused on reducing Global Warming Potential.