Refrigerant is the working fluid in an HVAC or refrigeration system. It absorbs heat when it evaporates (changes from liquid to gas) and releases heat when it condenses (changes from gas back to liquid) — this refrigeration cycle is what makes cooling possible.
Different refrigerants have different properties including operating pressure, cooling capacity, flammability, and environmental impact (measured by GWP and ODP). Common refrigerants include R-410A (being phased out of new equipment), R-454B (new residential standard), R-32 (used in many mini-splits), R-22 (legacy, banned for new equipment since 2010), and R-134a (automotive AC).
Handling refrigerants requires EPA 608 certification. Refrigerant leaks are both environmentally harmful and illegal to knowingly release. Technicians must recover refrigerant before servicing or disposing of equipment.