What does it indicate when the filter drier is half frosted up?

When the filter drier is half frosted up, it typically indicates a plugged filter drier. A filter drier is designed to remove moisture and contaminants from the refrigerant system. Over time, it can become saturated or clogged with debris, which restricts the flow of refrigerant. This restriction can lead to localized cooling in the filter drier, causing it to frost up as the refrigerant passing through it is unable to fully absorb heat due to the blockage.

A plugged filter drier can create various inefficiencies in the system, such as increased pressure drop, reduced refrigerant flow, and potential system damage if not addressed. It's vital for the overall health of a refrigeration system to monitor the condition of the filter drier and replace it when necessary to maintain optimal performance.

The other choices relate to different conditions that do not accurately explain the phenomenon of a filter drier being half frosted up. For example, a low refrigerant charge would typically cause evaporator coil issues instead of localized frosting at the filter drier, while insufficient airflow would lead to system-wide cooling problems rather than just affecting the filter drier. Normal operation is also inconsistent with this observation, as the presence of frost indicates a problem that requires attention.