As a Home Inspector I often recommend the air filters be replaced or cleaned due to dust / dirt build-up on the filter. The biggest problem with having a dirty air filter is the operating efficiency of your heating and cooling equipment drops dramatically. With less air flowing through your system it will require longer run times to maintain proper temperatures and your operating costs increase. I have seen ice forming on the air conditioner due to the low air flow through the evaporator coils. Simply changing the air filter would have prevented this problem.
How often you should change your air filter depends on the type of filter you are using. Here are a of the most common types of filters and how often they should be changed;
Traditional fiberglass or polyester filter:
It is recommended these inexpensive and disposable filters be replaced monthly. This type of filter is not very effective at capturing smaller particles in the air. Their purpose is to protect the heating and cooling system from damage.
Washable or Reusable filter:
Reusable filters need to be washed out monthly. The synthetic material in this type of filter works about the same as the above fiberglass / polyester disposable filter.
Cotton or Polyester pleated filter:
Pleated filters should be changed every 2 to 3 months. The more and/or deeper the pleats are, the more surface area they have and the less often the filter will need to be replaced. If you have allergies, this type of filter helps by capturing pollen and smaller particles than the fiberglass or washable filters due to their denser media. I have been told by A/C service men that some HVAC manufactures do not recommend pleated filters due to the denser media restricting proper air flow.
Electrostatic air filter:
These washable and reusable filters should be cleaned out monthly and can last up to 10 years. Electrostatically charged filters trap microscopic particles such as fine dust, pollen and smoke, yet have little effect on the air flow. The combination of mechanical air filtration and electrostatic filtration are making this type of filter more popular with home owners who want to achieve better air quality in their homes without sacrificing the operating efficiency of their heating and cooling equipment.
When shopping for air filters remember the cheapest filter is not always a bargain in the long run. You may save a couple of dollars now, but may have higher operating and maintenance cost over time. In my opinion, the best air filter is one that removes the most dust with the least restriction of air flow. That is why I choose to use electrostatic air filters in my home.