What is a refrigerant?

Refrigerant absorbs heat by boiling at low pressure inside the evaporator and releases it by condensing at high pressure in the condenser. Older domestic units use R-134a; many current ones use R-600a (isobutane), which is efficient and environmentally benign but flammable, requiring specific handling procedures during service. Refrigerant is not consumed — a system low on charge has a leak, and topping it up without finding that leak simply sells the same failure twice.