The three factors most important to improving HVAC performance for filtration is the grade of MERV filters used, the fit of filters in the filter housing, and the frequency of filter changes.

One Energy Star Heat & Cool Efficiently recommendation is to perform regular air conditioner filter changes:

"Check your filter every month, especially during heavy use months (winter and summer). If the filter looks dirty after a month, change it. At a minimum, change the filter every 3 months. A dirty filter will slow down air flow and make the system work harder to keep you warm or cool — wasting energy. A clean filter will also prevent dust and dirt from building up in the system — leading to expensive maintenance and/or early system failure."

The better the air conditioner filter (the higher the MERV Rating), the less often it will need to be changed.  Higher rated air conditioner / furnace filters can be changed every three months, but may need to be changed more often depending on your indoor environment.

Do you live in an urban or rural environment?  Are there children and/or pets coming in and out all day?  Does anyone in your house suffer from allergies?  Does your ventilation system run mostly, or not very often?

If you are still unsure as to how often to change your air conditioner / furnace filter, you should check it every month.  If you hold a new air conditioner filter next to your existing air conditioner filter and notice that your filter is darker, or dirtier than a new one, it is time to change it.  This is a good test to determine the best schedule for changing your air conditioner / furnace filter.

Of course, if it looks dirty, it probably needs to be changed, particularly after any improvement projects that stir up dust in your home.  Use the chart below to help you determine the right air conditioner filter change schedule that meets your requirements.  The chart can be used for all MERV Ratings.

MERV stands for Minimum Efficiency Reporting Values:

• Reports a filter's ability to capture larger particles between 0.3 and 10 microns. 
• This value is helpful in comparing the performance of different filters.
• The rating is derived from a test method developed by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE).
• The higher the MERV rating the better the allergy air filter is at trapping specific types of particles.

If you have good general health and do not have poor indoor air quality, MERV 7 and 8 rated air conditioner filters will suffice.

If you suffer from allergies or other respiratory illnesses, it would improve indoor air quality substantially to upgrade to a MERV 11 rated furnace filter. Keeping in mind that the higher the MERV rating, the smaller the particles that are filtered out of the air.

If you have poor indoor air quality and want to improve it, a MERV 11 rated furnace filter is likely best.

Use the chart below to help you decide which filter is best.

Best Efficiency MERV 11 Air Filters	These pleated air conditioner filters provide high efficiency and provides better indoor air quality for: Allergy or other respiratory illness sufferers People who live where there is poor outdoor air quality, such as near factories Owners of multiple pets
High Efficiency MERV 8 Air Filters	These pleated air conditioner filters provide high efficiency for HVAC systems that do not run efficiently on higher MERV rated furnace / air conditioner filters, and better indoor air quality for: Allergy sufferers Pet owners