Keep your equipment operating at its full potential with these Flanders Precisionaire Nested Glass 20″ X 20″ X 1″ Air Filters. They’re made of the same standard-grade materials as those protecting furnaces and central air conditioners around the world for over 50 years. These Flanders air filters have a filtering medium made of a continuous filament spun glass that allows for maximum airflow and features a self-retaining media that adheres directly to the frame for added resistance to wear. For peak equipment protection and energy efficiency . change them monthly.