The arrow on an HVAC filter should point in the direction of airflow, toward the blower and into the equipment. The physical direction varies depending on whether the filter sits at a return grille or beside the air handler or furnace.
Find the arrow printed on the filter frame
The arrow marks airflow, not simply which side should face outward. If the old filter was installed correctly, take a photo before removing it so you have a reference for the replacement.
At a return, air travels toward the equipment
At a wall or ceiling return grille, the arrow usually points into the duct. At a filter slot beside the unit, it points from the return duct toward the blower.
The support grid usually sits downstream
Many pleated filters have a reinforced side that resists suction. That support generally faces the blower, but the manufacturer’s airflow arrow is the authority if the design differs.
A backward filter can deform or perform poorly
If the filter bows, whistles, or fits poorly, turn the system off and check size, direction, and seating. Do not force an incorrectly sized filter into the rack.
Sources
- U.S. Environmental Protection Agency — Air Cleaners and Air Filters in the Home — Home air filtration, filter efficiency, particles, and system airflow considerations.