What commonly serves as the refrigerant in older systems?

• A. R-134A
• B. R-22
• C. R-12
• D. R-410A

Answer: (C)

CFCs, particularly R-12, were widely used as refrigerants in older HVAC systems due to their effectiveness in refrigeration cycles and low toxicity. R-12 was commonly found in residential air conditioning units and automotive applications prior to the 1990s. However, due to its environmental impact, specifically its contribution to ozone depletion, R-12 has been phased out under the Montreal Protocol.

In contrast, R-134A, which is a hydrofluorocarbon (HFC), became popular in the 1990s as a replacement for R-12 in newer systems due to its much lower ozone depletion potential. R-22, which is another refrigerant used in older systems, is also being phased out but has a different chemical structure and properties compared to R-12. R-410A, a blend of HFCs, represents a newer generation of refrigerants and is not used in older systems.

Thus, R-12 is the correct answer because it was the predominant refrigerant in older systems before regulations prompted a shift to alternatives with lower environmental impacts.