Air flows through a furnace from the return air path into the furnace, is heated or conditioned there, and then moves out to the building’s supply ducts. In terms of filter orientation and airflow direction inside the unit:
- General airflow: Return air enters the furnace, is processed (heated or cooled), and exits toward the building through the supply vents. This means the filter should be placed so that air flows through it in that same direction. The arrow on most furnace filters points toward the furnace/blower, indicating the direction air should flow (from return duct into the furnace).
- Filter orientation: The arrow on the filter frame should point into the furnace, i.e., toward the blower, not toward the room or away from the furnace. This ensures air is filtered before it reaches the blower and ductwork. If the slot is vertical, arrows typically point downward; if horizontal, arrows point inward toward the blower.
- Common configurations to know:
- Upflow furnaces: Air is drawn from the bottom and exits upward; the general principle of air moving from return to furnace remains the same.
- Downflow furnaces: Air is drawn from the top and exits at the bottom, with heat distributed through ducts; the same arrow convention applies for the filter orientation.
If you want, I can tailor this to your specific furnace model or help you verify the filter orientation on your unit.
