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How to Choose the Right Furnace Air Filter - This Old House

I would add two rather important points to this information.

  1. The main purpose of the furnace air filter is to keep dust and debris from contaminating and fouling the furnace itself. Filtering the air for the home occupants is secondary, and generally only effective if you opt to have the furnace (or air handler) fan run continuously.

  2. Be careful not to use overly restrictive filters. It may be tempting to choose the highest MERV rated filters in an attempt to filter out more and smaller particles. You will want to check with your furnace manufacturer, or an HVAC tech, to determine what type of filter your furnace can tolerate without undue strain that can shorten the life of your equipment. Also, refer to point #1. If you are not running the fan continuously, a higher rated filter will likely provide little benefit. Certainly, you do not want the cheapest filter, a quality pleated filter is the minimum.

And I will restate, don't forget to change the filter at regular intervals.