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How to Replace a Furnace or AC Filter

A dirty air filter makes your furnace and AC work harder, raises energy bills and can cause the system to overheat or freeze up. Most 1 in. filters need changing every 1–3 months.

DifficultyEasy
Time5–10 min
DIY cost$5–$25
Pro would charge$75–$125

Safety first

Tools

  • None, or a screwdriver for some return grilles
  • Marker

Parts

  • Air filter in the exact size printed on the old filter frame (e.g. 16x25x1)

Step by step

  1. Turn the system off

    Set the thermostat to Off so the blower doesn't pull in dust while the filter is out.

  2. Find the filter

    It's in a slot on the furnace or air handler, or behind a large return-air grille in a wall or ceiling.

  3. Read the size

    Pull the old filter and read the size on its frame. Take a photo so you buy the right one next time.

  4. Install the new filter

    Slide it in with the arrow on the frame pointing toward the furnace (in the direction of airflow).

  5. Date it

    Write today's date on the frame, close the cover or grille, and turn the system back on.

When to call a pro

Call an HVAC tech if the system still runs short cycles, ices up or blows warm air after a clean filter.

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