Final answer:
When a unit is opened for servicing, it is important to replace the filter-drier to maintain system efficiency and prevent damage to other components.
Step-by-step explanation:
The filter-drier is a component in HVAC systems that removes contaminants and moisture from the refrigerant. It is important to replace the filter-drier when a unit is opened for servicing because it can become clogged or saturated over time, reducing the efficiency and performance of the system. By replacing the filter-drier, the system can maintain proper refrigerant flow and prevent damage to other components. Additionally, a new filter-drier helps ensure that the refrigerant is free of contaminants and moisture, which can prolong the life of the system.