Indoor Air Resources

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Understanding the Minimum Efficiency Reporting Value (MERV)

By:   |   02.11.13

Selecting the right type of filter for your HVAC system is a critical component of delivering a healthy IAQ experience in your building.

Once problematic pollutants are identified, it’s time to look at how efficiently the filters you use or are considering using are able to remove those pollutants from indoor air. Because indoor air pollutants including lung-damaging dust have various particle sizes it’s important to consider the Particle Size Efficiency (PSE) of the filters you select. Most of the respirable dust and particles people breathe into their lungs are 3.0 microns and smaller. Lung-damaging dust, for example, can be as small as 0.5 microns, while some bacteria can be as small as 0.3 microns.

The Minimum Efficiency Reporting Value (MERV) is assigned to filters based on their minimum fractional-particle size efficiency, as determined by ASHRAE Standard 52.2. This standard test measures the efficiency of the filter media over three particle size ranges: E1 (very fine particles in the 0.3–1.0 micrometer range), E2 (fine particles in the 1.0–3.0 micrometer range), and E3 (coarse particles in the 3.0–10.0 micrometer range). E1, E2 and E3 efficiencies represent the measure of filter-media particle-capture performance and give users a more complete picture of  the filter’s ability to clean the air across various pollutant particle sizes.  A rating of five is least efficient, while a rating of 16 is most efficient.

However, to make the right filter choice you need to focus on more than MERV, as the MERV scale is non-linear. To make the best choice, review the Fractional Particle Size vs. Particle Diameter curve that is included with the ASHRAE test report. The curve will tell you the efficiency of the filter for the specific particle size you’re interested in. High E1 and E2 efficiencies are critical to providing healthier IAQ. That is why it is so important to consider filters with mechano-electret filter media and to look beyond a filter’s MERV to include a review of the Fine Particle Efficiency rating.

Many pleated filters today (especially commonly used MERV 8) have very low E1 and E2 efficiencies. In fact, under ASHRAE Standard 52.2, there is no minimum requirement threshold for E1 particulate capture below a MERV 13 rating and no minimum requirement threshold for E2 particulate capture below a MERV 9 rating. Be wary when attention is diverted away from poor E1 and E2 performance and instead is focused on MERV and the non-standard “MERV-A” test.

Providing a healthier IAQ experience for your building’s occupants is a smart choice.  Done right, it can lead to improved business results via higher productivity and employee engagement. 

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