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What is Furnace Filter Efficiency or a MERV Rating?


What is MERV?

MERV or Minimum Efficiency Reporting Value is the airborne particulate filtering efficiency rating of an air/furnace filter/air cleaning device that is arrived at through an efficiency evaluation process by applying the ASHRAE Standard 52.2 Test procedure. The filtering capability is determined through a comparative count of airborne particulate matter by taking measurements both upstream and downstream of the filter being tested. The MERV rating is based upon the worst case performance of the filter recorded through six different stages of dust loading with 0.3 to 10 micron size particles to assure the best performance over the life of the air filter.

The Testing Process

The ASHRAE Standard 52.2 test is the procedure used for determining the MERV rating of any air-cleaning device, including a furnace filter/air filter. The test is conducted by passing an air stream injected with an aerosol challenge, containing particulates in a dozen different known ‘size ranges’, through the filter being tested. A record for each particulate size is made both upstream and downstream to ascertain its efficiency level in filtering out airborne particles of various sizes.

The test is performed repeatedly six times for each of the twelve different particle sizes and the results are expressed as filtration efficiency ratio of the ‘downstream to upstream’ particle count recorded during the test. A Composite Minimum Efficiency Curve for the test filter is then determined using the lowest values recorded over the six test cycles. After compiling the values derived through these tests, the twelve ‘size ranges’ are segregated into three larger groups denoted as E1, E2 and E3.

The averages of each larger group i.e. E1, E2 and E3 are used for calculating the PSE or Particle Size Efficiency of the tested filter and resultant percentage(s) used to arrive at the MERV rating.

MERV Rating

A number between 1 and 16 that is expressed together with the air velocity under which the ASHRAE Standard 52.2 test was performed denotes the MERV rating of a filter. In simple terms, the higher the MERV rating of a filter, the better is its airborne particulate filtration capability. Furnace filters as part of the air intake system of a furnace unit are designed for trapping airborne particulate matter not only for motor, fan and heating coil protection, but also more importantly for improving indoor air quality. Different types of filters have different MERV ratings. For example, fiberglass and synthetic filters, disposable panel filters, electrostatic filters and washable metal foam filters have MERV ratings between 1-4, pleated and extended surface filters between 5-8 and non-supported bag filters, rigid cell/ cartridge filters and rigid box filters between 9 - 16 MERV. 1 -4 MERV filters trap particle sizes over 10 microns, those between 5-8 trap particles between 3 to 10 microns and filters rated between 9 to 16 filter particulate matter between 3 to 0.3 microns.