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I replaced my furnace filter (every 30 days) but upon Googling, I've found it has to go certain way. It's suppose to go the way where the arrow points for the airflow. But my filters have no arrows on them.

How would I tell which way it faces?

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What type of filter are you using? Posting an image of the filter, and/or a link to the product website would be helpful. –  Tester101 Jan 26 at 12:54
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Better quality filters generally will be directional. This is due to the filter containing a coarse and fine media or one side being reinforced with wire or mesh to support the filter against the flow of air. It may also be designed to flow air better in one direction. If you can feel or see a density difference between sides the least dense side should face out If the filter is not labeled it should install in either direction as there is likely no difference from one side to the other

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