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Replacing Heater Filters: About Hepa, Ashrae, And Merv

by Gabby Hyman
Repair-Home Columnist

Many homeowners pay little attention to replacing their heater or furnace filters. For some, it's a simple matter of visiting the home-improvement store every six months to buy a simple, honeycombed filter to install between their HVAC air intake and their furnace. But for those who suffer asthma or severe allergies from dust mites, pollen, smoke, pet dander, and bacteria, changing out the filter on schedule is a vital enterprise.

You can spend hundreds of dollars a year on top-quality heater filters, or as little as pennies day. The differences in filter performance -- any good HVAC contractor will tell you -- are legion. The main job of a furnace filter is to keep the heat exchanges and coils in your HVAC system clean to extend unit life and energy efficiency. For those on the receiving end of the heated or cooled air, a high-efficiency filter's ability to capture particulates is the real value proposition.

ASHRAE, and MERV Ratings

The American Society for Healthcare Engineering (ASHRAE) was founded in 1894 to create standards for advancing heating, ventilation, air conditioning and refrigeration. In recent years, ASHRAE released what is called the MERV rating system for air filters. The system evaluates HVAC filters based on average efficiency in microns, water resistance, and ability of "arrestance" - to trap particulates.

Home HVAC filters come with MERV ratings between 1 and 11(printed on the package). Typical use-and-toss-out fiberglass air filters have MERV ratings from MERV 1 - MERV 4. For comparison, a MERV 7 filter is 50% - 70% efficient at capturing particles between 3.0 and10 microns. A MERV 11 has an 85% minimum efficiency. You'll find that hospitals use MERV 14 rated filters.

Depending on brand and workmanship, a MERV 11-rated HVAC filter can work efficiently up to 90 days. Better types often employ ring panel filters rather than fiberglass pleats, and have self-sealing edges.

For those with health concerns, HEPA filters are the products of choice. A high-efficiency particulate arrester (HEPA) filter receives its rating at levels established by the United States Department of Energy (DOE), judged by thickness, fiber density, and diameter. The first such filters were created by the DOE to prevent workers on the Manhattan Project from inhaling radioactive particulates. By definition, HEPA filters must meet a minimum efficiency level in blocking and trapping particulates at 99.97% at 0.3 microns.


About the Author
Gabby Hyman has created online strategies and written content for Fortune 500 companies including eToys, GoTo.com, Siebel Systems, Microsoft Encarta, Avaya, and Nissan UK.