What the arrow means
Most disposable HVAC filters have an arrow printed along the cardboard frame. That arrow indicates airflow direction, not which side should face the room.
The arrow should point toward the blower or air handler—the equipment pulling return air through the filter.
At a furnace or air-handler slot
If the filter sits in a slot next to the furnace or air handler, the arrow usually points from the return duct toward the equipment.
Do not rely only on how the old filter was installed. It may have been installed backward. Look at the return-air path and the location of the blower.
At a return-air grille
When a filter sits behind a return grille in a wall or ceiling, air moves from the room through the grille and into the ductwork. The arrow should therefore point away from the room and into the duct.
If you are not sure
Turn the system off and check the equipment layout or documentation. If airflow direction is still unclear, an HVAC technician can confirm it quickly.
Correct direction matters because many pleated filters are built with downstream support intended to face the blower side.