Which refrigerant is commonly used in HVAC systems?

The commonly used refrigerant in HVAC systems is R-22. R-22, also known as HCFC-22, has been widely utilized in residential and commercial air conditioning systems because of its effective thermodynamic properties and relatively low cost. However, due to its ozone-depleting potential, production of R-22 has been phased out under the Montreal Protocol, with a complete ban aimed for 2020 for new units.

R-134a is primarily used in automotive air conditioning systems and has different properties that do not make it suitable for use in standard residential and commercial HVAC units.

Freon is a trademarked term that refers to multiple types of refrigerants, including R-22; however, it's important to specify the actual refrigerant number to avoid misunderstandings.

Ammonia is a refrigerant used in industrial applications due to its high efficiency and low cost, but it is not commonly used in residential HVAC systems because it is toxic and can be hazardous if not properly managed.

The correct choice highlights the importance of understanding different refrigerants and their specific applications in HVAC systems.