An air conditioning filter drier needs to replaced any time the refrigeration system is open to the atmosphere, typically when the system is repaired. The most common time the filter drier needs to be changed is after replacement of a compressor.
Filter driers trap moisture and contaminates to prevent them from circulating throughout the air conditioner. Filter driers are especially useful in removing moisture during a compressor burnout and preventing damage to the system. Filter driers are also important in preventing damage in side piping leaks. The filter drier is typically located on the liquid line, at the coolest point in the air conditioning unit.