How to Replace AC Filter

Replacing your AC filter is one of the simplest maintenance tasks you can do — and one of the most impactful. A fresh filter restores full airflow, improves cooling efficiency, and keeps the evaporator coils cleaner for longer. Here is how to do it correctly for both split AC units and central HVAC systems.

When to Replace vs When to Clean

Not all AC filters are the same:

• Reusable mesh filters (common in split AC indoor units): wash and reuse. Replace only when torn, warped, or no longer cleanable.
• Disposable filters (common in central HVAC, window units, and some portable ACs): replace on schedule. Do not wash these — washing damages the filtration media.

This guide covers replacing disposable filters and replacing worn-out reusable filters.

How Often to Replace a Disposable Filter

• MERV 1–4 (basic fiberglass): Every 30 days
• MERV 5–8 (pleated): Every 60 to 90 days
• MERV 9–12 (high efficiency): Every 6 to 12 months
• Homes with pets or allergies: Replace more frequently regardless of MERV rating

What You Need

• Replacement filter in the correct size
• Pen or marker to note the replacement date
• Trash bag for the old filter

Step 1: Find the Correct Filter Size

The filter size is printed on the frame of your current filter — for example, 14x18x1 or 20x25x1. The three numbers are length, width, and thickness in inches. Buy a replacement in the exact same size. If the size is not printed on the filter, measure the filter slot opening.

👉 Simply 14x18x1 Air Filter MERV 8, 6-Pack – For Home AC & HVAC

Step 2: Turn Off the AC

Turn the AC off at the thermostat or remote before replacing the filter. For central HVAC systems, turn off at the thermostat.

Step 3: Locate the Filter Slot

• Split AC indoor unit: Open the front panel — the filter slides out from behind the panel.
• Central HVAC: The filter is usually located in the return air vent (a large grille on the wall or ceiling) or in the air handler unit itself.
• Window AC: The filter is typically behind the front grille, which unclips or slides off.
• Portable AC: The filter is usually on the back or side of the unit behind a removable panel.

Step 4: Remove the Old Filter

Slide or lift the old filter out carefully. Fold it inward on itself to contain the dust and place it directly into a trash bag to avoid spreading dust.

Step 5: Check the Airflow Direction Arrow

Disposable filters have an arrow printed on the frame indicating the direction of airflow. The arrow must point toward the AC unit (toward the blower), not away from it. Installing the filter backward significantly reduces its effectiveness.

Step 6: Insert the New Filter

Slide the new filter into the slot with the arrow pointing in the correct direction. Make sure it fits snugly with no gaps around the edges — gaps allow unfiltered air to bypass the filter entirely.

Step 7: Note the Replacement Date

Write the installation date on the filter frame with a marker. This makes it easy to track when the next replacement is due without relying on memory.

Step 8: Restore Power and Test

Close the panel or grille, restore power, and turn the AC back on. You should notice improved airflow immediately if the old filter was significantly clogged.

Signs the Filter Needs Replacing Now

• Visible gray or brown discoloration across the entire filter surface
• Reduced airflow from the vents
• The AC runs longer than usual to reach the set temperature
• Increased dust on surfaces in the room
• It has been longer than the recommended replacement interval

Final Thoughts

Replacing a disposable AC filter takes less than five minutes and costs very little relative to the electricity savings and equipment protection it provides. Keep a stock of replacement filters on hand so you are never tempted to delay a replacement because you do not have a spare ready.