I can not find anywhere on this unit the size of the air filter. Some had put a 20" filter in the unit but you can see it was forced in the it's location. After removeal I could see an upper and lower rail for a filter to slide into. But can not deturemen is size. And after attempting to locate a heating services that might advise me, all I get is that they want to send some out to tell it's size for a price. Can any one advise m?

Thank you

Frank

It the slot on the outside of your unit, or inside the unit?