MERV

A figure of merit for air filters (Minimum Efficiency Reporting Value), introduced by the American Society of Heating, Refrigerating and Air-Conditioninf Engineers (ASHRAE) in 1999 to address the shortcomings of earlier filter rating systems, such as dust-spot efficiency, that were not sufficiently focused on particle size. The higher the MERV number, the smaller the particles the filter will remove.

How is the MERV number determined?

The filter is tested using a generator which produces particles of potassium chloride of the desired diameter. Upstream and downstream from the filter are optical particle counters that count the number of particles entering the filter and the number exiting it. From this, the percentage of particles getting through the filter can be calculated.

In testing, particles are generated in 12 different groups of sizes, covering a range from 0.3 to 10 micrometers, which are grouped into three “efficiency groups.” The coarsest particles are in efficiency group E3 and the finest in group E1.

Particle group 1 2 3 4 5 6 7 8 9 10 11 12
Lower limit, μm 0.30 0.40 0.55 0.70 1.00 1.30 1.60 2.20 3.00 4.00 5.50 7.00
Upper limit, μm 0.40 0.55 0.70 1.00 1.30 1.60 2.20 3.00 4.00 5.50 7.00 10.00
Geometric mean, μm 0.35 0.47 0.62 0.64 1.14 1.44 1.88 2.57 3.46 4.69 6.20 8.37
Efficiency group E1 E2 E3

The filter is tested for each particle group; first as a fresh filter and then in five additional, progressive passes (generally the effectiveness of a filter increases as it becomes loaded with dirt, since that makes the sizes of its holes smaller). Each of the six tests yields a number which is the percentage of particles captured. The lowest value from these six results is selected; it is called the minimum composite efficiency for that particle group.

The four minimum composite efficiencies for groups 1 to 4 are averaged (call the result E₁), the four for groups 5 to 8 (E₂), and the four for groups 9 to 12 (E₃), resulting in three values. These are compared with the table below to find the highest MERV number for which the filter qualifies.

MERV numbers 1 through 4 are special cases. For these coarse filters an earlier technique is used, which is based on the average percentage by weight of the dust fed that was captured by the filter.²

MERV Composite Average Particle Size Efficiency Average
arrestance
0.30 to 1.0 μm 1.0 to 3.0 μm 3.0 to 10.0 μm
1 E₃ < 20% Aavg < 65%
2 E₃ < 20% 65% ≤ Aavg < 70%
3 E₃ < 20% 70% ≤ Aavg < 75%
4 E₃ < 20% 75 ≤ Aavg
5 20% ≤ E₃ < 35% n/a
6 35% ≤ E₃ < 50% n/a
7 50% ≤ E₃ < 70% n/a
8 70% ≤ E₃ n/a
9 E₂ < 50% 86% ≤ E₃ n/a
10 50% ≤ E₂ < 65% 85% ≤ E₃ n/a
11 65% ≤ E₂ < 80% 85% ≤ E₃ n/a
12 80% ≤ E₂ 90% ≤ E₃ n/a
13 E1 < 75% 90% ≤ E₂ 90% ≤ E₃ n/a
14 75% ≤ E1 < 85% 90% ≤ E₂ 90% ≤ E₃ n/a
15 85% ≤ E1 < 95% 90% ≤ E₂ 90% ≤ E₃ n/a
16 95% ≤ E1 95% ≤ E₂ 95% ≤ E₃ n/a

1. A description of a particle generator may be found in:
Robert T. Drew and David M. Bernstein.
The Laskin aerosol generator.
Journal of Toxicology and Environmental Health, Part A, vol. 4, no. 4 pages 661-670 (1978).

2. The first standard to describe the current type of MERV rating is
ANSI/ASHRAE 52.2-1999.
Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size.

The earlier standard, containing the procedure used for MERV 1 through 4, is
ANSI/ASHRAE 52.1-1992.
Gravimetric and Dust-Spot Procedures for Testing Air-Cleaning Devices Used in General Ventilation for Removing Particulate Matter.

In 2007, ASHRAE issued a revision of 52.2. The following year it issued Addendum b to 52.2-2007, which incorporated the method from 52.1 used to test for MERV 1 -4. It also, for example, added a method of conditioning the filter prior to the test, producing results that more nearly approximated real world performance. Ultimately 52.1 will be withdrawn.

ANSI/ASHRAE Addendum b to ANSI/ASHRAE Standard 52.2-2007 (appeared in 2008).

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