How to Install a Filter in Cold Air Return Ductwork


Modern heating/cooling units have two elements. A supply side pushes conditioned -- heated or cooled -- air into ducts to spread throughout the house. A return side collects that air and returns it to the unit to be conditioned again. The return, commonly called a cold air return even when it's collecting used cool air, has a filter system, to remove dust and dirt before the air re-enters the unit. This is to prevent damage to the unit. Filters must be installed appropriately and changed regularly to properly remove debris which could damage the heating/cooling unit.

Things You'll Need

  • New filter
  • Locate the return air duct; it will be on one side of the heating/cooling unit, next to the supply plenum or opening. It usually will be a large sheet metal opening to the side of or below the large supply opening. Find the return openings in the house, on inside walls, either in spaces with supply vents or in a central area like a hallway. Return openings should be covered with a removable grille or grate. Follow the route of the return duct if you cannot readily locate the filter opening in the house.

  • Open the grille or grate and remove the old filter; it normally pulls out easily. There are many styles; some use fiberglass, some have pleated paper, some have tiny metallic collectors. There also are many sizes. Check the size and type of the old filter and get one the same size. Filters range in size from 12 inches by 14 inches to 30 inches by 30 inches or larger. You must install the same size filter, although types can vary; fiberglass and paper filters generally last 30 days, other varieties may last longer. Some can be cleaned and used over and over.

  • Put the new filter in place, making sure the airflow arrows on the filter point toward the blower or inside of the filter cavity. Discard the old filter. Close the grille or grate and secure it once the new filter is properly placed.

  • Put an opening in a central location close to the heating/cooling unit for a new location. Size the filter and opening to the return opening on the unit. Install a metal frame between two studs (you may have to cut a center stud to make room, then add a cross brace at the top of the opening between the two studs). Screw the frame to the studs with a screw gun. Check that the grille opens and closes properly and the filter fits easily inside.

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