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At the store you can find standard inexpensive filters, expensive filters designed for allergy removal, and several in between. In the air conditioning industry we rate the efficiency of a filter by its MERV (Minimum Efficiency Reporting Value) rating. The higher the MERV rating the better the filtration and the result is less particulates in the air. Good filtration serves a couple functions:
So going with a high efficiency filter seems obvious, right? Not necessarily. If there is not a lot of filter surface area, you can do more harm than good. Here is an example, if you have a 3 ton system and you have two (2) 20″ x 20″ filters, more than likely you can get away with using two (2) of the 1″ allergy filters that you can purchase at any home improvement store, however if that same 3 ton system were to only have one (1) 20″ x 20″ filter than a 1″ allergy filter would be too restrictive. The restriction of air flow can cause the same issues as a dirty coil, which are inefficiencies and an environment for mold and mildew growth. A sign of restriction is noise at the filter. It is not the only symptom, but the most common. If your filters are too restrictive you can always look into adding another return to allow for proper air flow.
Another type of filter that is been around for sometime is a cartridge filter. These filters are 4″ t0 5″ thick (ie. 20″ x 20″ x 5″) and are designed to create more surface area for air to pass across. The increased surface area reduces air restriction. These filters require a filter cabinet installed in the return duct system, most commonly found at the air handler. If you are interested in the high efficiency filters that are MERV 11-15, this is the type of filter you must use. There are many types of high efficiency filters available on the market which are great at removing tiny particulates like smoke and bacteria from the air. It is always best to speak to your air conditioning professional to find out which one is best for you.