No single parameter impacts heating performance and energy efficiency like adequate airflow and nothing reduces airflow like a dirty furnace filter. The system blower circulates the entire air volume of your house through the filter multiple times every day. Carried into the filter is an abundance of dust, dirt, lint and other airborne particulates. And that’s just the stuff you can actually see: invisible contaminants including mold spores, pollen and microorganisms can actually breed inside the filter media.