Are Higher Rated Furnace Filters Worth the Money?

Efficiency and Effectiveness

• Furnace filters can be evaluated in terms of their efficiency and their effectiveness. A filter's efficiency, which is usually expressed as a percentage, refers to its ability to stop particles from moving through the filter; a particular filter may claim to be 99 percent efficient, meaning that it stops 99 percent of the particles in the air that passes through the filter. A filter's effectiveness refers to its ability to remove airborne particles from the indoor space that it serves. A filter may be highly efficient but still allow particles to remain in the space because particles tend to settle rather than remain airborne, so they may never reach the filter at all.

MERV

• The American Society of Heating, Refrigerating and Air-Conditioning Engineers has developed a standard for comparing the efficiency of filters called the Minimum Efficiency Reporting Value. A filter's MERV rating indicates the smallest particles that the filter is capable of stopping. A filter with a MERV rating of 1 can block pollen, dust mites and carpet fibers. A filter with a MERV rating of 20, the highest number on the scale, can stop viruses, carbon dust and smoke.

Filter Ratings

• The most inexpensive flat filters have a MERV rating of between 1 and 4; they are useful primarily for protecting fan motors and heat exchangers in the furnace. Pleated filters typically have a MERV rating between 5 and 13, and high-efficiency pleated filters have a rating between 14 and 16. True HEPA filters with a MERV rating between 17 and 19 remove up to 99.999 percent of 0.3-micron particles from the air that passes through them; HEPA filters with a MERV of 20 block particles as small as 0.1 micron in diameter.

Usefulness Data

• Studies cited by the U.S. Environmental Protection Agency suggest that the most expensive HEPA filters are not necessarily the most effective filters available. Data indicates that filters with a MERV between 7 and 13 are nearly as effective as HEPA filters at removing small particles from indoor air, and increasing MERV beyond 13 results in only a modest increase in indoor air quality. Medium-efficiency filters with a MERV of 12 are typically less expensive than HEPA filters and perform comparably.