What type of liquid is used in a heat pump?

A heat pump typically uses a refrigerant as its working fluid. This refrigerant circulates through the system, absorbing heat from one environment and releasing it in another. Common refrigerants include hydrofluorocarbons (HFCs), although alternatives like propane and ammonia are also used for their lower environmental impact. The choice of refrigerant can affect the efficiency and environmental footprint of the heat pump system.