Which of the following gases is primarily used as a refrigerant in residential HVAC systems?

R-410A is primarily used as a refrigerant in residential HVAC systems due to its favorable properties and compliance with current environmental standards. It is a hydrofluorocarbon (HFC) that replaces older refrigerants like R-22, which has been phased out due to its ozone-depleting potential.

The use of R-410A has several advantages. It operates at a higher pressure than R-22, which allows for more efficient performance in air conditioning systems. Additionally, R-410A does not contribute to ozone depletion, making it more environmentally friendly. Its thermodynamic properties also enable it to achieve higher efficiency levels in both cooling and heating applications.

Although other options like R-12, R-404A, and propane might be associated with refrigerants in various contexts, R-410A is specifically designed for modern residential systems, especially in new installations adhering to energy regulations. R-12 is an older refrigerant that has been phased out due to environmental concerns, and R-404A is more commonly used in commercial refrigeration. Propane, while it can be a refrigerant, is not typically used in standard residential HVAC systems due to safety and performance considerations.