Minimum Efficiency Reporting Value (MERV)
To assist consumers of HVAC air filters select the filter that meets their clean air efficiency requirements, the American society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) developed a testing and reporting Standard that rates air filters based on their ability to remove contaminants by particle size. The ASHRAE Standard is the 52.2, “Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size”. The rating method developed under this standard is called the Minimum Efficiency Reporting Value (MERV). The MERV is a single number that is used, along with the air velocity at which the test was performed, to simplify the extensive data generated by the method of testing. MERV is expressed on a 20 point scale and is derived from the Particle Size Efficiency (PSE).
As shown in the chart below, filters that have a MERV rating of 1 to 4 are the least efficient and most manufacturers are phasing them out of production due to the increase awareness of consumers for indoor air quality. The next group, 4 to 8, include pleated panels that include the MERV 8 rated pleated fi lters. This is the filter now being used in most commercial buildings and residences. The filters to receive the highest MERV ratings in the 17 to 20 range are the High Efficiency Particulate Arrestors (HEPA). These filters are used primarily in very special applications and require high capacity blowers and fans to overcome their resistance to air flow.