• The arrow on a furnace air filter should point in the direction of the air flow. Since a filter is designed to clean the air before it reaches the blower on a furnace or A/C unit, the arrow should point toward the system.

    Finding Your Filter

    Most forced air heating and cooling systems have filters in or immediately adjacent to the furnace. Some filters are behind grills of air return ducts in hallways or ceilings.

    Identifying the Air Return

    If you are unsure which direction the air flows within your system, remove the old filter and look to see which side is the dirtiest. The air is coming from that direction. The arrow on the new filter therefore should point in the direction of the cleaner side of the old filter.

    Still Not Sure?

    Turn on your fan at the thermostat. Remove the filter. Hold a large piece of paper in the slot where the filter usually sits and observe the direction it bends. That is the direction in which the arrow should point.

    If You Get It Wrong

    Don't worry, if you place a new filter into a system with the arrow pointing in the wrong direction it will still filter the air, but may reduce the efficiency of your system. Check the filter after 15 days. If the side closest to the tip of the arrow is cleaner than the other side, then you got it right.

    Helpful Hint

    Once you determine the correct direction of airflow, use a marker or paint and place an arrow next to the filter holder so you don't have to think about airflow the next time you change the filter.

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