• Bag or cartridge filters (MERV 11 to 15 based on ASHRAE Standard 52.2). The high efficiency MERV 8 or MERV 13 filters are very effective at removing small, respirable particles from the airstream.Typically, filters are labelled with their nominal sizing - which is the approximate size of the filter, rounded up to the nearest whole inch (example 16 x 14 x 4). Nominal size is sometimes also called the advertised size. The actual size is the exact size of the filter (example 15 ¾ x 13 ½ x 3 ¾). Change your filters frequently