How to Install a 20x23x1 Air Filter | Six Steps in Five Minutes

Installing a 20x23x1 air filter takes about five minutes and requires pointing the arrow toward the blower or fan.

Whether you slipped a fresh one into a return grille, a furnace slot, or a filter cabinet, getting the airflow arrow right does most of the work. The one thing people get wrong is the direction — the rest is simple handiwork.

Where the Filter Lives

The filter sits in one of three typical spots in a U.S. forced-air system: a return air grille mounted on a wall or ceiling, a narrow slot cut into the furnace near the blower compartment, or a dedicated filter cabinet between the return duct and the furnace. All three use the same basic procedure.

Turn off the HVAC system before touching anything. If you can’t reach the thermostat, flip the furnace room’s switch or the breaker labeled for the air handler. Powering down prevents the blower from kicking on mid-swap and pulling debris past the new filter.

The Step Order That Works

Follow this sequence from Filter King’s installation guidance and you’ll finish before the dust settles.

1. Turn off the system at the thermostat, the furnace switch, or the breaker.
2. Open the return grille, slide out the filter slot, or unclip the filter cabinet door.
3. Note the arrow printed on the old filter’s edge before pulling it out — that marks the airflow direction you need to match.
4. Remove the old filter and check the dimensions printed on its frame.
5. Slide the new filter into the slot with the arrow pointing toward the blower or fan, snug against the frame with no gaps.
6. Close the grille or door, restore power, and run the system.

You’ll know it worked when the system runs at the same volume as before and you don’t hear air whistling around the filter’s edges.

Which Way Does the Arrow Point?

On a furnace slot or dedicated filter cabinet, the arrow points toward the blower, the furnace’s fan, or the air handler — whichever sits downstream in airflow. That side of the arrow reads “toward the fan” or “airflow” on most filters.

Checking Fit and Measurements

The “20x23x1” on the box is the nominal size, not the exact cut. Most 20x23x1 filters actually measure around 19.5 x 22.5 x 0.75 inches, which is normal — manufacturers design the media slightly undersized so it seats cleanly without buckling. Filter King’s 20x23x1 size chart lists the exact specs for its MERV 8, MERV 11, and MERV 13 variants if you want to compare before buying.

Ready to grab replacements? Our tested roundup of the best 20x23x1 air filters compares MERV ratings, pack sizes, and pricing across the top brands.