American Standard Perfect Fit Media Filters for Retrofit Frame Kit. - Original Perfect Fit Accordion type filters. - Approximately 21(L). - 2 filters per case. - High efficiency filter media collects small dust particles before they can collect on your coils. - For retrofit kit model numbers that begin with BAYFTAH.These filters are designed to work with the Trane Retrofit Frame Kit. You must already have the Retrofit Frame Kit for these filters to work in your unit. - Replace every 6 months.