How to Determine Air Flow Direction for a Furnance Filter

When first servicing a new or unfamiliar furnace filter it is not always obvious the direction in which air flow passes through the filter. As filters are designed for and require proper orientation based on the direction of air flow it is critical that the filter be oriented in the proper direction both to provide proper functioning of the filter and to prevent damage to and failure of the filter. A few simple steps can help the homeowner be certain of knowing the proper orientation for replacement filters. The process is simple and can be performed in ten minutes or less.

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Remove the existing filter.

Step 1

Locate the furnace air filter housing remove the panel cover and existing filter. When removing the filter note the position of each face side of the filter as it was installed in the furnace.

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The face of the filter will be dirty.

Step 2

Examine the existing filter for lint and dust deposits. If the filter has been in service for 30 days or more during the operation of the furnace, dust and lint will accumulate on one side of the filter. There will also be a noticeable contrast between the discolored, fuzzy side of the filter and the clean side will be absent of any lint deposits.

Step 3

Determine which side of the filter was facing away from the furnace. The direction of air flow through the filter is from the lint covered side to the clean side of the filter.

Step 4

Determine the direction of air flow through a furnace when there is no pre-existing filter (new furnace) by cutting the 1/2-inch tacky strip from a sticky note and then cutting this strip into two 1/4-by-1/4-inch strips.

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Place a sticky tab in the center of both sides of filter.

Step 5

Stick one 1/4-inch strip on each side and centered in the middle of a new filter. Insert the filter into the furnace filter slot being careful to not knock off the sticky tab.

Step 6

Replace the filter slot cover and start the furnace.

Step 7

Allow the furnace to run for two to three minutes with the furnace blower fan running and then stop the furnace and remove the filter.

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A missing tab indicates air exit side of the filter.

Step 8

Examine the filter and observe which side still has an embedded sticky tab. The side with the tab remaining is the side from which the air flow enters the filter. The side without the tab is the side from which the air flow exits the filter.

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The arrow indicates correct direction of air flow.

Step 9

Orient the air direction flow arrow on the filter properly during installation.