Why does the filter I buy sometimes fit and sometimes it does not?
It is very confusing. The grill manufacturers actually use two styles of grills for bascially the same sizes. For example a 20x20 Return Grill may be designed with the filter being 19 3/4" x 19/34" the filter will be called a "20x20 Grill Filter" or it could be based on the "filter size being 20"x20" and called a 20x20- Filter or 20x20 "exact". What happens is the hardware stores will carry both. Our recommendation is to switch to an Air Cleaner which usually only needs to be replaced every six months or a year. No more trips to the store to find that last size. Get rid of all your filters and get better quality air.