Furnace air filter — tape a faded airflow arrow before you install

A furnace air filter with a rubbed-off arrow is easy to install backward. Before you discard the old filter, note which way its arrow pointed, then mark the new one with tape so the MERV media faces airflow for your furnace filter size slot.

Tape a faded airflow arrow on the furnace air filter before you install

If you are not sure which way your system pulls air, see which way the arrow goes first.

1. With power safe and the blower off, pull the old filter and note arrow direction toward the furnace or blower (airflow into the equipment).
2. On the new filter, if the printed arrow is faint, place a piece of tape with an arrow matching that same direction before you slide it in.
3. Seat the filter fully in the slot with the taped arrow pointing the correct way, then close the door or grille.
4. Listen for a new whistle; if airflow sounds wrong, pull and recheck arrow vs the old photo — not a higher MERV guess.

Honest stop: if you never saw the old arrow and the slot has no airflow mark, check the furnace manual or a clear photo of the prior install before guessing.

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