Important Note:  Almost ALL furnace filters have a different \"actual size\" than the nominal size printed on the air filter. It's best if you measure your air filter for the EXACT size and compare it to our \"Actual Size\". With the 4\" and 5\" air filters there is a wider variance between the nominal and actual size depending on the manufacturer. If you are replacing a 4 3/8 depth 20x20x4 filter in your Honeywell unit you should order our 20x20x5 Honeywell Replacement air filter that measures 19 5/8 x 19 7/8 x 4 3/8 (19.63x19.88x4.38). Nordic Pure 20x20x4 Pleated Air Filters MERV 10 plus carbon are made of a dual material where the MERV rated material has been fused together with a carbon layer. This results in less flow restriction and 33% more surface area for cleaner smelling air. All Nordic Pure pleated air filters are manufactured using a heavy duty, moisture resistant beverage board frame. There is a metal mesh support grid bonded to the pleats for extra support. MERV 10 carbon furnace air filters are comparable to HVAC filters with the rating of MPR 900-1000. This high-performance HVAC replacement filter will help keep your air conditioning and heating systems running efficiently, saving money on utilities and costly maintenance.  It is important to regularly change your furnace air filters for fresh, clean air in your home or office. We recommend that you change your air filter every 3-6 months to maintain maximum efficiency and economy.