It is a two-minute job that most people get slightly wrong, and the errors matter more than you would expect.
Direction
The arrow printed on the filter frame points in the direction of airflow — toward the blower, away from the return. Backwards, the filter media collapses against its support and performs poorly.
Size
Use the size printed on the old filter, and note that nominal size and actual size differ slightly by design. A filter that is genuinely loose in the rack is the wrong size, not a tolerance issue.
The bypass problem
This is the important one. If air can travel around the filter frame instead of through the media, the filter's rating is irrelevant. Gaps at the edges, a warped rack, or a door that does not close properly all cause it. Check all four edges seat tightly. Foam weatherstrip on the rack is a cheap and effective fix.
Choosing a MERV rating
Higher is not automatically better. A filter too restrictive for your blower reduces airflow, which costs capacity and can cause coil problems. MERV 8 to 11 suits most residential equipment. Going higher is worth discussing against your specific system rather than assuming.
How often
Every one to three months for a one-inch filter, depending on pets, occupancy and season. In Florida, where systems run most of the year, err toward the shorter interval. Four and five-inch media filters run considerably longer — follow the manufacturer's guidance.
A note on what filters do
A filter's primary design job is protecting the equipment, with air quality benefit secondary. That is worth knowing when someone is selling a filter upgrade as an air quality solution. See filtration and purification options.
