What filter does your furnace take?

Where the model number is

Look for the rating plate, a printed label inside the furnace cabinet, usually on the inside of the blower or burner door. Carrier's manuals call it the unit rating plate and Goodman's call it the furnace rating plate. Copy the line marked Model or M/N. The serial number is a separate line and will not match anything here. More on finding the label.

Nominal size and actual size

Filters are sold by a rounded nominal size, such as 16 x 25 x 1, and the frame often measures a little less. Carrier's own papers show the gap: the 58MVC installation manual lists its washable filter as 16 x 25 x 3/4, while the 58MVC product data calls the same filter 16 x 25 x 1 nominal. Rheem goes the other way and prints rack sizes with fractions, like 15 3/4 x 25, without saying which kind of size it is. When a maker gives two numbers for one filter, the model page shows both.

How often makers say to check it

There is no single schedule. Carrier's 58TN0A manual says to check and clean the filter each month or more often if needed (Carrier 58TN0A-02SI Installation, Start-up, Operating and Service and Maintenance Instructions, PDF page 40). Lennox's ML180UHV instructions say monthly (ML180UHV Installation Instructions 507945-02 (04/2024), PDF page 33). Goodman's IOG-2009Q says every two months or as required (IOG-2009Q Installation Instructions *MSS9* & *CSS9* Single-Stage Gas Furnace, PDF page 40). Trane's S8V2 manual says monthly, and more often in midsummer and midwinter (Installation, Operation, and Maintenance FNR-SVX003A-EN (S8V2) (Oct 2025), PDF page 86). None of these give a separate timetable for 4- or 5-inch media filters; a media cabinet's own instructions set that.