MERV

Minimum Efficiency Reporting Value is a rating for mechanical air filters. MERV is detailed in a standard developed by the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE).

MERV ratings are determined in a standard laboratory test measuring how well mechanical heating and/or air conditioning filters capture and hold airborne particles of a wide range of sizes. Dust, soot, pollen, smoke, and pet and human dander are a few of the types of particulate pollutants that may be found in indoor air. These particles can range in size from less than 1 micron (or millionth of a meter) to more than 10 microns. Simply put, MERV ratings are determined by what comes out compared to what went into the filter during standardized testing.

MERV ratings are useful in comparing different mechanical filtering products. In general, the higher the MERV rating, the more efficiently the filter initially performs in removing smaller particles. The EPA guidance for indoor air quality in schools, for example, recommends filters between MERV 8 and MERV 13. By comparison, the typical fiberglass throwaway filter used in most homes has a rating of MERV 4 or less.

All mechanical filters become more efficient as they load up since the caked dust and lint itself increases the filter’s ability to hold smaller particles. The higher the MERV rating, the more efficient the filter is when first installed. All filters become increasingly more efficient until they clog and restrict air flow, causing stress on the blower motor or compressor and creating system performance problems. Choice of a filter’s MERV rating, then, must take into consideration the HVAC system’s design capability. Can it handle increased air flow resistance without adding energy costs?

Therefore, there is a trade-off for high filter efficiency: The more efficient the filter, the harder the blower in a ventilation system must work to force air through the filter. This means that frequent cleaning or replacement of the filtration device is in order if top system operation is to be maintained. A MERV 8 pleated filter in the home might require replacement every quarter whereas a MERV 4 may last a year or longer before seriously impeding the air flow.