You should first determine how thick of a filter your forced air heating or heating and air conditioning system can accommodate. Many residential systems have space for only a one inch thick filter. The see-through flat panel filters often used in these locations are not recommended because they remove very few of the small particles that pose health risks.
Various rating systems are used to indicate the particle removal efficiency of filters sold for use in residential heating and cooling systems. In the U.S., these filters may have a MERV, MPR, or FPR rating. We recommend use of a filter with a particle removal efficiency rating of MERV 11, MPR 1000, or FPR 7 or higher; however; even a MERV 8 rated filter which is roughly equivalent to a MPR 600 or FPR 5 filter is substantially better than the common see-through flat panel filter. The use of more efficient filters is more important when indoor particle levels are high because of high outdoor air particle levels or strong indoor particle sources, and indoor particle sources should be reduced when possible. Also, high efficiency filters are more important when the occupants are highly susceptible to adverse health effects from particles, for example, when occupants have respiratory and cardiac disease.
Normally, the filters with higher particle removal efficiency ratings are pleated, i.e., they have a folded or accordion-like filter media, to maintain a low resistance to airflow. Sometimes, a higher efficiency filter will have too much resistance to airflow for the forced air heating and cooling system of a home. You might want to engage a heating and air conditioning contractor who can determine what type of filter you can use in your home. Filters that are more highly pleated will often have a lower airflow resistance than a less pleated filter with the same efficiency rating. Some heating and cooling systems can accommodate a pleated filter that is more than one inch thick, for example a two inch thick pleated filter. A thicker filter provides more space for pleated filter media to maintain a low airflow resistance. A heating and air conditioning contractor can usually modify your forced air heating and cooling system so that it can accommodate thicker filters. The airflow resistance of a filter increases over time during use, so frequent changing of filters will help to avoid periods of high airflow resistance.
When installed in your forced air heating and cooling system, there should be minimal gaps between the filter and the filter’s housing. Gaps allow unfiltered air to flow around the filter, sometimes substantially reducing the overall particle removal by the filter system.
The filter in a residential forced air heating and cooling system only removes particles when there is airflow through the system. When providing heating or cooling, many systems run only 10% to 20% of the time. In mild climates, operation times can be even less. Also, in homes without air conditioning, months can pass without system operation. Often a heating and air conditioning contractor can add controls so that the fan of your forced air system heating and cooling system can be programmed to operate a portion of each hour, or even continuously, regardless of the need for heat or air conditioning. However, use of portable air cleaners will, in some cases, be a lower cost option.
For more information, see the section of this web site on air cleaning.