R-12 and R-134a _____.

The correct choice is that R-12 and R-134a should never be mixed. This is essential because R-12 and R-134a are two different refrigerants with distinct chemical compositions and properties. Mixing them can lead to increased pressure, inefficient cooling, and potential damage to the refrigeration system. Each refrigerant operates under different temperature and pressure characteristics, which means their performance standards are not compatible when mixed.

Using the appropriate refrigerant specified for a system ensures efficiency and longevity of the components involved. When converting from R-12 to R-134a, it is critical to evacuate the system and ensure it is properly retrofitted and cleaned before the new refrigerant is introduced. This rigorous approach keeps the system safe and functioning optimally.

Furthermore, while R-134a may use similar manifold gauge sets as R-12 in some cases, it is essential to understand the differences in handling and servicing each refrigerant. Overall, the principle of not mixing these refrigerants underscores the importance of using the correct materials and procedures in HVAC systems.