In which operational condition is the weigh-in method specifically required?

The weigh-in method is specifically required in situations where precise measurements of refrigerant charge are critical for optimal system performance, particularly in heating mode below 65°F. At lower temperatures, the performance and efficiency of heating systems can be more sensitive to the amount of refrigerant present. Insufficient or excessive refrigerant can lead to inefficiencies, suboptimal heating, or even system damage. Therefore, using the weigh-in method ensures that the correct amount of refrigerant is accurately charged, which is especially important in colder conditions to maintain system functionality.

In contrast, the other options either refer to contexts where the weigh-in method is less critical or not mentioned as a requirement, such as during routine maintenance, in cooling modes across all temperatures, or high external temperatures, where different charging methods might suffice based on the operational dynamics. The specificity of the weigh-in method's requirement under lower temperature conditions highlights its importance for ensuring reliability and efficiency in heating applications.