Final answer:
A filter drier that is half frosted up likely suggests a low refrigerant charge in the air conditioning or refrigeration system. It requires inspection and servicing by a qualified technician.
Step-by-step explanation:
When a filter drier is half frosted up, it could indicate that there may be a low refrigerant charge in the system. A filter drier's function is to remove moisture and contaminants from the refrigerant. If there isn't enough refrigerant, the pressure inside the system drops, and the temperature can fall below the freezing point of water, causing the exterior of the filter drier to frost up. This condition is not indicative of normal operation, and neither a clogged air filter nor a malfunctioning thermostat would directly cause half of the filter drier to frost up. It's specifically related to the refrigeration cycle and indicates a potential issue with the refrigerant level. In such cases, it's essential to have the system inspected and serviced by a professional to avoid further damage or decreased efficiency.