The general recommendation is to change a furnace filter every 2 to 3 months or whenever it looks dirty. During winter when the furnace is used heavily, checking the filter once a month and changing it if dirty is advised. The replacement frequency also depends on filter type, household conditions, and factors like pets or allergies. For example, 1-inch filters typically need replacement every 1 to 3 months, thicker filters less frequently (up to 9 to 12 months for 5- to 6-inch filters). HEPA filters may only need replacement once per year. Regular checks and visual inspections every 1 to 2 months help determine when a change is needed to maintain good airflow and furnace efficiency.
