The Aerostar 21x23x1 MERV 11 pleated air filter is a replacement furnace filter, AC filter, and HVAC return air filter for home heating, cooling, and central air conditioning systems. Each pleated furnace filter has a nominal size of 21x23x1 and an actual size of 20.875x22.875x0.75. Measure your existing filter before ordering to confirm fit. The 4-pack of replacement filters provides up to months of home air filter coverage at the recommended 90-day change interval. MERV 11 filtration captures household dust, pollen, pet dander, mold spores, lint, and fine dust. It does not capture bacteria, smoke, or virus-sized particles. For higher filtration of those particles, select a MERV 13 filter. Designed for furnace air, AC, heater, and house air return applications. Pleated electrostatically charged synthetic media captures more particles than flat fiberglass filters while maintaining low airflow resistance to support HVAC system efficiency. The reinforced moisture-resistant frame prevents filter collapse and air bypass over the filter's life. Compatible with most major brands including Carrier, Bryant, Trane, Lennox, Rheem, Goodman, York, Ruud, Honeywell, and Aprilaire furnace and air conditioner systems. Replace every 90 days under typical conditions, or every 30 days in homes with pets, smokers, or high dust levels for allergy and asthma relief. Made in USA.