HVACRDB

Who Is MERV Anyway?

Way back in 1998 when the ASHRAE 52.2P Standards Project Committee (SPC) was wrestling with how to show the particle-size efficiency for the new method of testing of air filters, the group expressed a desire to be able to show — with just one number (like the European Std. EN 779) — the efficiency of the filter at its lowest point.

Everyone knew (or thought they knew at that time) that an air filter increased in efficiency as it loaded over time, and the previous ASHRAE Standard, 52.1, used an Average Atmospheric Dust Spot Efficiency to show the efficiency percentage using outdoor air. The SPC wanted to show the number not as an average but as its minimum efficiency right out of the box.

After several attempts, someone suggested the committee use the “Minimum Efficiency Rating Value.” While everyone thought this was a good idea, its mission statement did not allow ASHRAE to “rate” anything. With this in mind, we changed the terminology to “Minimum Efficiency Reporting Value.” This terminology fit better because the Standard was not a rating function of the filter but rather a reporting of the results of the filter efficiency tested on various sizes of particles. This is how we conceived the Minimum Efficiency Reporting Value, or MERV.

What MERV allows engineers, architects, building owners and managers, and HVACR technicians to do is select the filter with a particle-size efficiency best suited for their specific building or application. This works especially well with those buildings that have the performance of a special process in them because the facility person can have a bank of lower efficient filters in the HVACR system and have a process that generates particles smaller than the efficiency of the filter to capture. In other words, the filters are out of design for what the building needs.

MERV to the rescue! Now, with a one-number reporting value, the HVACR person can quickly determine which efficiency they need for their building. All they have to do is grab the ASHRAE Standard 52.2-1999 and follow the MERV chart until they arrive at a MERV number showing a filter capable of capturing the specific size of particles they need removed from the airstream. For example, ASHRAE Standard 62.1 requires a minimum of MERV 6 in commercial buildings. This equates to at least 35 percent on particles in the range of three to 10 micrometers. While this is just a minimum, and most buildings should use even higher MERV filters (LEED® criteria is MERV 13), one must remember that many buildings and HVACR units are designed for less than 20 percent on this particle size.

The Standard works equally well for residential use since ANSI/ASHRAE 62.2 specifies a MERV 6 for residences also. The challenge here becomes one of selecting the correct MERV filter with the pressure drop (resistance to flow) to meet the requirements of the Standard, a “pressure drop no greater than 0.1 inches w.g” and the residential unit. Since, as a general rule, the more media in a filter, the lower the pressure drop — to a point — the idea for residential is to install a media filter that has enough surface area to reduce the flow through any one square foot to the point that the resistance is low enough to meet the unit specifications.

ASHRAE Technical Committee (TC) 2.4, the Committee concerned with “particulate contaminants and particulate contaminant removal equipment…,” asked the National Air Filtration Association (NAFA) to write a user guide that would be a quick explanation and reference for end-users, which NAFA has done. This guide is a handy trifold pamphlet that everyone can easily carry and refer to on the job site.

As an association partner of NAFA, HARDI members can obtain copies of the NAFA ANSI/ASHRAE 52.2 NAFA User Guides at member prices at www.nafahq.org or 757/313-7400.

Al Veeck is executive director of the National Air Filtration Association (NAFA), the trade group for air filter manufacturers and distributors worldwide. He is a Certified Air Filter Specialist, has been a member of ASHRAE and TC2.4 since 1985, and served as its chair for three years. He attended all but one meeting concerning 52.2 from its inception with the research to develop the MOT in 1989 through its passage in 1999. He is vice chair of the Society Program Committee. Contact Al at 757/313-7400 or [email protected].

MERV Table
Standard 52.2 Minimum Efficiency Reporting Value (MERV) Composite Average Particle Size Efficiency, % in Size Range µm Average ASHRAE Arrestance, %, by Standard 52.1 Method Minimum Final Resistance
Range 1
(0.3 - 1.0)
Range 2
(1.0 - 3.0)
Range 3
(3.0 - 10.0)
PA Inches of Water
1 n/a n/a E3 < 20 Aavg < 65 75 .3
2 n/a n/a E3 < 20 65 ≤ Aavg < 70 75 .3
3 n/a n/a E3 < 20 70 ≤ Aavg < 75 75 .3
4 n/a n/a E3 < 20 75 ≤ Aavg 75 .3
5 n/a n/a 20 ≤ E3 < 35 n/a 150 .6
6 n/a n/a 35 ≤ E3 < 50 n/a 150 .6
7 n/a n/a 50 ≤ E3 < 70 n/a 150 .6
8 n/a n/a 70 ≤ E3 n/a 150 .6
9 n/a E2 < 50 85 ≤ E3 n/a 250 1.0
10 n/a 50 ≤ E2 < 65 85 ≤ E3 n/a 250 1.0
11 n/a 65 ≤ E2 < 80 85 ≤ E3 n/a 250 1.0
12 n/a 80 ≤ E2 90 ≤ E3 n/a 250 1.0
13 E1 < 75 90 ≤ E2 90 ≤ E3 n/a 350 1.4
14 75 ≤ E1 < 85 90 ≤ E2 90 ≤ E3 n/a 350 1.4
15 85 ≤ E1 < 95 90 ≤ E2 90 ≤ E3 n/a 350 1.4
16 95 ≤ E1 95 ≤ E2 95 ≤ E3 n/a 350 1.4
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