MERV measures how much of the small stuff a filter captures. Higher numbers capture more — and restrict more airflow, which is where the trouble starts.
The slot matters more than the number
Air takes the easiest path. If the filter slot leaks around the edges — and most do — a large share of your return air bypasses the filter entirely, whatever it is rated. Sealing the slot usually does more than upgrading the filter.
For most residential systems MERV 11 to 13 is the sensible ceiling, fitted in a slot that actually seals, and changed on a schedule you will keep.
