How Do HVAC Filters Work?

To understand the role of a filter, you must first understand how a furnace or air handler works. A furnace is an air handler with a burner inside. Indoor air is sucked into return vents and flows through ductwork into the air handler where it is conditioned–heated, cooled or circulated–and blown through supply vents around the home. The air filter is placed in between the return air duct and the air handler assembly so it can capture particles from the passing air.

Air filters are composed of fine fibers that are tangled together to both block and capture particles in the air. The fast-moving particles’ inertia causes them to crash and get trapped in the air filter’s fibers. The relationship between how much dirt the filter traps and airflow determine the filter's efficiency. The tighter the fibers are strung together, the more effectively the air filter will capture indoor particles, and less air will get through. This effectiveness is rated with a Minimum Efficiency Reporting Value (MERV).

What Does MERV Mean?

MERV ratings were developed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) to rate filter effectiveness. A higher MERV suggests more particles will be captured, and airflow will be more restricted. A lower MERV means fewer particles will be captured, and airflow will be less restricted. MERV scores are based on the filter’s ability to capture particles of varying sizes.

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