How to Tell Which Way Air Flows in an Air Conditioning Filter

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Air conditioning filters trap dirt, dust and pollen to help keep air cleaner.

Furnace and air conditioner filters are important devices for helping to remove dust and pollutants from the air in your home, and are often designed to have air flow through them in a particular direction. You need to determine the proper direction in order to obtain optimum filtering and have the longest service life possible for your filter. Improper installation can shorten the life of the filter or make it ineffective altogether.


Determining Air Flow

Step 1

Determine the direction of the air flow. Different systems put the filter in different places, but most commonly it is located in the air return duct. This means that air will be sucked through the filter before entering the mechanical equipment. To determine air flow direction hold a thin strip of paper in front of the duct while it is operating and see which direction the paper is pulled or blown.

Step 2

Examine the filter. Generally the edge of the filter is marked with an arrow which points in the direction of the airflow. In other words, the air travels from the tail of the arrow toward its point. If there is no arrow, the side that is stronger or reinforced by cardboard or wire mesh is the side the arrow should be pointing to. Air flows from the weaker side to the stronger side.


Step 3

Install media filters in either direction. Window air conditioning units often have a media filter that is simply a loosely woven mesh. The mesh fits inside a plastic housing on the inlet side of the unit to prevent dirt from collecting on the cooling coils. Air is drawn through the mesh, then passes through cold coils and is blown back into the room. If one side of the mesh is denser, it should face toward the coils. If both sides are the same it does not matter which direction it faces.


Be sure the filter fits snugly so air cannot pass around it rather than passing through it.


Improper installation can make the filter worthless or cause it to fail prematurely.