The short answer
This Old House quotes Richard Trethewey: "Replace your furnace filter every two to three months or whenever it looks dirty." The same article says manufacturers commonly suggest every 90 days, with a typical range of 60 to 90 days depending on the age of the system and how much it runs. It also says the EPA advises checking the filter monthly, especially in winter, and replacing it if it looks dirty.
When to change it sooner
- Winter. The furnace runs the most, so filters get dirty fastest. Check more often during the heating season.
- Pets. If a pet sheds heavily, check the filter more often than the minimum. Regular grooming and cleaning reduce the hair and dander that clog it.
- Allergies or asthma. The article suggests considering a HEPA filter rated MERV 13 or higher, and changing the filter more often, especially in pollen season, if symptoms flare.
Where the filter is
The article says the filter is "located just inside the furnace or inside the air return vent." Pull the old one out first and note the size printed on its frame so you buy the right replacement.
Picking a filter
MERV is the rating for how fine a particle a filter catches. This Old House says most residential filters are MERV 7 to 12, that 13 or higher counts as high-efficiency, and that you should avoid MERV 1 filters. Basic fiberglass filters are the cheapest but catch the least fine particulate, while polyester and pleated filters do better. Check your furnace's manual before moving to a much denser filter, since some systems are not built for the extra resistance to airflow.
A habit that helps
Set a phone reminder for the first of each month to look at the filter. If you can't see light through it, or it looks gray and furry, replace it. If your filter is hard to reach, or you are not sure which size or type your furnace takes, a technician can show you once.
Sources
Every figure above comes from one of these. Where they disagree, the post says so rather than averaging them into a single number.