Which way does a furnace filter go? Reading the airflow arrow

Short answer: The arrow on the filter frame points in the direction the air moves: toward the furnace or air handler and away from the return duct. At a wall or ceiling return grille, the arrow points into the wall or ceiling, toward the duct.

Step by step

1. Turn the system off. Switch the thermostat off so the blower does not pull dust in while the filter is out.
2. Find the slot. Filters sit in the return grille, or in a slot or cabinet between the return duct and the furnace.
3. Note the size. Read the nominal size printed on the old filter, such as 16x25x1.
4. Point the arrow toward the blower. Slide the new filter in with the arrow toward the furnace or air handler.
5. Write the date. Mark the date on the frame so you know when it is due.

Popular sizes

• 3M Filtrete 600 Dust & Pollen Reduction Air Filter - 16x25x1 (4 Pack) — $60.37
• 16x25x1 Merv 11 Universal Air Filter By Tier1 (Single Filter) — $45.97
• 20x20x1 Merv 11 Universal Air Filter By Tier1 (Single Filter) — $45.97
• 16x20x1 Merv 11 Universal Air Filter By Tier1 (Single Filter) — $45.97
• 20x25x4 Merv 11 Universal Air Filter By Tier1 (Single Filter) — $95.16

Common questions

What happens if the filter is in backwards?

Filters are designed with airflow in one direction; backwards they may filter less well and add resistance. Turn it around at the next check.

Is there a front and back on a filter?

Yes, the side the arrow points away from faces the incoming air.

Can I run the furnace without a filter?

Avoid it; dust builds up on the blower and coil.

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Related categories: HVAC Air Filters. General information only; follow the equipment maker's instructions and local codes. Updated 2026-11-14.