Look at unit where the fan is should be a filter frame and if theirs an old filter in there slide out and get a new one if no filter measure the frame --if no frame look for an access slot around and or inline with duct for fan it s usually narrow if no access panel look around area for a return register some registers are filter units unsrew tabs open and should have a filter this link has a variety of different filter systems How to Change your Home Furnace Air Filter
This is probably not a factory specified size. You need to look at the return to your furnace. There should be a frame or housing for the filter. Measure it to see what size you need. These are often customized by the installer to meet the configuration of your ductwork. You need to measure how tal and how wide, but also how deep (thick). If possible always get a pleated filter, and not one of those "horse hair" cheap ones.
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Look at unit where the fan is should be a filter frame and if theirs an old filter in there slide out and get a new one if no filter measure the frame --if no frame look for an access slot around and or inline with duct for fan it s usually narrow if no access panel look around area for a return register some registers are filter units unsrew tabs open and should have a filter this link has a variety of different filter systems
How to Change your Home Furnace Air Filter
This is probably not a factory specified size. You need to look at the return to your furnace. There should be a frame or housing for the filter. Measure it to see what size you need. These are often customized by the installer to meet the configuration of your ductwork. You need to measure how tal and how wide, but also how deep (thick). If possible always get a pleated filter, and not one of those "horse hair" cheap ones.