The most common place you will find furnace filters are either in the unit or in the return air ductwork right next to the unit. Situations may arise where you want or need to install the filter at the return air grille. This comes in handy if your furnace is in an attic or otherwise hard to access area. Since you should replace furnace filters at least every 90 days, climbing into an attic just to change the filter might not be feasible. Proper sizing of the return air filter grille is required for the furnace to function correctly.
Open up the cabinet of your furnace to gain access to the service plate. Look on the plate to see how many BTU, or British Thermal Units, your furnace puts out.
Divide the BTU by 50 to see how much CFM, or cubic feet per minute, your furnace blows. For example, if your furnace is 40,000 BTU, it would put out 800 CFM.
Calculate the area in square inches you need for your filter grille. For proper operation, plan on 2 CFM per square inch of filter grille area. So if your furnace puts out 800 CFM, you need 400 square inches of filter grille area.
Multiply the lengths and widths of common filter grille sizes to calculate the square inches of each. Once you find one that meets or exceeds the above calculated needed number, you can use it as your filter grille size. Some of the common sizes include 20-by-14, 25-by-16 and 20-by-20. Using those examples, you would see their square inches are 280, 400 and 400 respectively. Since from this example, the furnace requires 400 square inches for your filter grille, you may use either the 25-by-16 filter grille or the 20-by-20 filter grille.