Quick answer
MERV (Minimum Efficiency Reporting Value) rates how well a filter captures particles of different sizes, based on ASHRAE Standard 52.2. For most homes, MERV 8 is a basic option and MERV 11–13 captures much more fine dust, pollen, and smoke. The best filter is the highest rating your system can move air through without restriction.
Filter thickness matters
A 1-inch MERV 13 filter can restrict airflow in some systems. A 4- or 5-inch media filter cabinet offers far more surface area, so it captures more while keeping airflow healthy, and usually needs changing less often.
How often to change it
Check 1-inch filters monthly and change them every one to three months. Media filters often last longer. During wildfire smoke events, check more often because filters load up quickly.
Signs the filter is too restrictive
Whistling returns, weak airflow at registers, a frozen coil in summer, or a furnace that overheats and shuts off can all point to a filter that restricts too much. A technician can measure static pressure to confirm.
