Searching google on an answer for this topic will show no specific standard as the answer is relative to so many factors. The number of pets you have, the number of people in a home, the amount of overall dust in a home, smokers, etc. However you should inspect your filter at least once a month. If you have pets in your house (dogs, cats) it will require a more frequent filter change than if you did not. People with asthma and/or allergies would benefit of keeping the air filter up to date.
If you do have asthma and/or allergies Trane has a wonderful product called “Clean Effect” (you can see on their webpage here) This filter can remove up to 99.98% of all airborne allergens that passes through the filter. No need to buy a new filter and even has a light on the front to indicate if it needs to be cleaned.
For the rest of us, who don’t mind buying the cheapest filter at your local shopping center, here’s what you can do: Hold the used filter up to the light and compare it to a clean “spare.” When light is obscured by captured dust and dirt particles, the old filter should be changed. Keep a record for one year and then replace the filter on that basis.
At a minimum, it is always a idea to change filters at the start of the heating and cooling seasons and then in-between according to your need. Also, it is a good idea to have your heating and air system checked at the beginning of heating and cooling season to insure proper operation. You will actually find that all manufacturers of air conditioning and heating units in the United States recommend that you do preventive maintenance once or twice per year by having your unit serviced by a professional.
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