Filter driers are one of the basic parts of an HVAC system.  They exist to absorb excess moisture in the refrigerant without blocking the refrigerant’s normal flow through the system.  Although moisture can cause significant damage to various parts of the HVAC system, most often the compressor will be the affected component.  Filter driers are found in the liquid line and sometimes also found in the suction line of the compressor.

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Filter driers contain a hygroscopic substance (desiccant) which is responsible for inducing or sustaining a state of dryness in its vicinity.  There is a mesh screen on one end of the drier that helps keep the drying agent and other solids in the filter drier.  

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Some filter driers must be replaced entirely, while others are installed in a position that enables a technician to easily change the filter. With larger units this is important because of the increased build-up on the screens.

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How long does a filter drier last?  The desiccant can last throughout the HVAC system’s lifespan, but the filter drier’s housing may need replacing if subjected to an acidic environment.  A clogged screen may also make it necessary to repair or replace the filter drier.  Filter drier testing is done by measuring the temperature of the refrigerant entering and exiting the drier filter. The temperature should not be more than a few degrees different on each side or the drier is deemed ineffective and requires replacement.  Also, filter driers should always be replaced when a compressor, condenser, expansion valve or evaporator is replaced.

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