Air filter testing is a complicated and sometimes confusing subject. This article provides the definition of MERV or the Minimum Efficiency Reporting Value for air filters. It also provides information on how the MERV should be used to select air filters and the limitations of this testing system. Understanding MERV helps the air filter buyer make the best decision on a product that will provide the most economical and effective solution without being caught up in the marketing hype.
Question: Which of the following is the definition of MERV?
- Game show host, television personality and media mogul
- Ancient city in Central Asia located in present day Turkmenistan that was briefly the largest city in the world
- Minimum Efficiency Reporting Value is a method of stating the fractional efficiency by particle size of air filters
- All of the above
The correct answer is (d) all of the above. But in the world of air filters, there is only one MERV that counts and that is c) Minimum Efficiency Reporting Value. When I started this article I was tempted to assume that since people reading this publication are sophisticated and knowledgeable on IAQ and HVAC, they would know how a MERV for an air filter is determined and be able to understand a discussion of some of its advantages and disadvantages. Then I read the following definition from the website of one of the major HVAC equipment manufacturers.
“MERV Rating
The MERV (Minimum Efficiency Reporting Value) rating of a filter describes the size of the holes in the filter that allow air to pass through. The higher the MERV rating, the smaller the holes in the filter, the higher the efficiency.”
This definition is incorrect on so many levels that it is difficult to know where to begin. First, the MERV is not a rating. The assumption that the higher the MERV, the better the filter, is wrong. A MERV is intended to enable users to select filters based on their ability to remove target particles.
Secondly, the reference in the definition to the holes in the filter refers to the filtration mechanism called “straining.” This is where the particles are captured by the filter because they are too large to pass through the holes in the filter. In actual fact only about one percent of the particles captured by air filters are the result of straining. The other mechanisms of filtration including impingement, interception, diffusion and, in some cases, electrostatic attraction account for the remaining 99%. In other words, the definition not only is wrong about the meaning of MERV, it lacks a basic understanding of how air filters work.
So, let’s assume nothing and review the test method by which a MERV is determined. For those who know this standard please, bear with me. For those who don’t, here is your opportunity to understand what these air filter MERV numbers mean.
For more in depth information about air filter testing and all other aspects of air filtration the best resource is the NAFA Guide to Air Filtration – Fourth Edition published by and available from the National Air Filtration Association (www.nafahq.org). This helpful and comprehensive book includes chapters on the principles of air flow, air pressure and air filtration, the types of air filters, air cleaners, filter testing, procedures for dealing with controlled environments, avoiding molecular contaminants, indoor air quality, calculating owning and operating costs of air filters, Ultraviolet Germicidal Irradiation (UV lights) and photocatalytic oxidation (PCO). The NAFA Guide to Air Filtration is an invaluable reference manual and tool for those involved in air filtration, HVAC and Indoor Air Quality. The cost for the Guide is $79 (US).
Air Filter Testing Standards
The testing standard for air filters has been developed by committees of the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE). For many years, tests for air filters were done according to ANSI/ASHRAE Standard 52.1-1992, Gravimetric and Dust Spot Procedures for Testing Air Cleaning Devices Used in General Ventilation for Removing Particulate Matter. This test produced an Atmospheric Dust Spot Efficiency such as a 25%, 65%, 85%, 90% using outdoor air as a challenge, a Dust Arrestance (the percentage of the test dust captured by the filter by weight using ASHRAE Test Dust), dust holding capacity and pressure drop. ASHRAE Test Dust used is a combination of cotton linters, carbon black and Arizona Road Dust.
The ANSI/ASHRAE 52.1 Standard was relatively easy to understand. However, it had some major deficiencies that did not provide a very good guide for selecting filters for particular contaminants. In order to do that effectively you need to know two things – one, the particle size of the contaminant you are trying to remove and two, the level of efficiency of the filter on that particle size. Thus the ANSI/ASHRAE Standard 52.2 – 1999, Method of Testing General Ventilation Air Cleaning Devices for Removal Efficiency by Particle Size was developed to provide this data.
The ANSI/ASHRAE 52.2 Standard emphasizes the standardization and calibration of components. The testing apparatus must pass 13 qualification tests before filter testing begins. Theoretically, a filter tested by one ASHRAE 52.2 laboratory should have the same results as it would in any other correctly operating laboratory.
Obtaining the Data
An air filter’s performance is determined by measuring the particle counts upstream and downstream of the filter being tested. Particle counts are taken over the range of particles six times. One begins with a clean filter, measures the particle size efficiency counts, loads or “conditions” the filter with ASHRAE 52.1 synthetic test dust and measures the particle efficiency counts again. This loading with test dust and measuring is done 4 more times.
The particle generator creates particles of a known size in the air stream. The objective is to create particles of sufficient numbers to obtain meaningful counts in all of the measured particle ranges. These particle size ranges are listed in Table 1.
Table 1 – ASHRAE 52.2 Particle Size Ranges
| Range | Size (in microns) | Group |
| 1 | 0.30 to 0.40 | E1 |
| 2 | 0.40 to 0.55 | E1 |
| 3 | 0.55 to 0.70 | E1 |
| 4 | 0.70 to 1.00 | E1 |
| 5 | 1.00 to 1.30 | E2 |
| 6 | 1.30 to 1.60 | E2 |
| 7 | 1.60 to 2.20 | E2 |
| 8 | 2.20 to 3.00 | E2 |
| 9 | 3.00 to 4.00 | E3 |
| 10 | 4.00 to 5.50 | E3 |
| 11 | 5.50 to 7.00 | E3 |
| 12 | 7.00 to 10.00 | E3 |
This procedure results in efficiency numbers for the 12 particle size ranges for each of the 6 cycles. Efficiency is measured as the percentage of particles captured by the filter. The lowest of the 6 readings is then taken to determine the Composite Minimum Efficiency Curve.
The efficiency numbers in the twelve size ranges are then placed in three larger groups (E1, E2, E3) and the percentages in each group are averaged. This percentage is the Average Particle Size Efficiency and these are used to determine the Minimum Efficiency Reporting Value (MERV).
Table 2 – MERV Parameters
| MERV Value | Group 1 Av. Eff. % (0.30 to 1.00) |
Group 2 Av. Eff. % (1.00 to 3.00) |
Group 3 Av. Eff. % (3.00 to 10.00) |
| 1 | n/a | n/a | E3<20 |
| 2 | n/a | n/a | E3<20 |
| 3 | n/a | n/a | E3<20 |
| 4 | n/a | n/a | E3<20 |
| 5 | n/a | n/a | 20<35 |
| 6 | n/a | n/a | 35<50 |
| 7 | n/a | n/a | 50<70 |
| 8 | n/a | n/a | 70 |
| 9 | n/a | E2<50 | 85 |
| 10 | n/a | 50<65 | 85 |
| 11 | n/a | 65<80 | 85 |
| 12 | n/a | 80 | 90 |
| 13 | E1<75 | 90 | 90 |
| 14 | 75<85 | 90 | 90 |
| 15 | 85<95 | 90 | 90 |
| 16 | 95 | 95 | 95 |
Table 2 gives a breakdown of MERV parameters. When using this table you Start with E3 and move down each Group until you arrive at a true statement. This will correspond with a MERV number that represents the Minimum Efficiency Reporting Value for the filter.
Another important consideration is the speed of the airflow during the test. Table 3 provides the seven acceptable airflow speeds.
Table 3 – ASHRAE 52.2 Airflow Speeds
| 1. | 118 Feet Per Minute (FPM) | 0.60 meters/second (m/s) |
| 2. | 246 FPM | 1.25 m/s |
| 3. | 295 FPM | 1.50 m/s |
| 4. | 374 FPM | 1.90 m/s |
| 5. | 492 FPM | 2.50 m/s |
| 6. | 630 FPM | 3.20 m/s |
| 7. | 748 FPM | 3.80 m/s |
The Minimum Efficiency Reporting Value (MERV) must be stated with the speed of the airflow at which the filter was tested. Most commercial filters are tested at 492 fpm (2.5 m/s). Certain types of filters perform differently at different airflow velocities.
Let’s go over a few examples just to make sure you understand how a MERV is calculated. (All test results were obtained at 492 fpm.)
Example 1: Filter tests on a pleated filter give us the following Average Particle Size Efficiencies- E1=16.7, E2=44.7, E3=66.1 What is the MERV? Answer: MERV 7
Example 2: Filter tests on another pleated filter give us the following Average Particle Size efficiencies – E1=28.4, E2= 66.9, E3=85.9 What is the MERV? Answer: MERV 11
Example 3: Filter tests on a bag filter give us the following Average Particle Size efficiencies – E1=61.0, E2=90.8, E3=99.1 What is the MERV? Answer: MERV13
One has to admit that this Standard provides a pretty ingenious way of evaluating the particle efficiency of an air filter. The single number is an easy way to express the results of some pretty complex testing procedures. But from its adoption in 1999 to the present, ANSI/ASHRAE 52.2 has been the center of discussion and controversy in the air filter industry.
First, the ANSI/ASHRAE 52.2 Standard did not totally replace the old 52.1 Standard. Certain tests (Arrestance) had to be brought over from 52.1 and incorporated into 52.2 for filters with a MERV of 4 or less. In addition, the 52.2 Standard did not provide for a measurement of “dust holding capacity.” Many believe that dust holding capacity is a measure of the service life of a filter.
Secondly, there has been concern that the test results do not correspond to how the filters perform in real world situations. Many filters use an electrostatically charged media (electret) which can occur naturally or be the result of charging the media during manufacturing. This enhances the ability of these filters to capture particles. Laboratory and field studies have shown that as some electret media filters are exposed to particles (load) their “charge” is reduced and they drop in efficiency. (See the paper entitled, “The Long Term Performance of Electrically Charged Filters in a Ventilation System” by Raynor and Chae in a 2004 edition of the Journal of Occupational and Environmental Hygiene.)
However, not all electret media filters drop in efficiency as they load. Likewise non-electret media filters have also been shown to drop in efficiency both in testing and in actual field use situations. (See the paper entitled “Real Time Evaluation of Ventilation Filter-Bank Systems” by Moyer, et al. of the US Dept. of Health and Human Services in the January 2007 edition of the Journal of Occupational and Environmental Hygiene.)
In January of this year the ASHRAE committee responsible for the 52.2 Standard approved two changes:
Addendum B provides for testing of arrestance to determine MERVs for filters of MERV 4 and under and dust holding capacity for filters with MERVs 5 to 16. Public review has just been completed that retires the ASHRAE 52.1 Standard.
Appendix J is an optional test including a different conditioning step using nanometer-sized potassium chloride (KCl) particles sprayed on the filter. The tests run with this conditioning step would produce a separate test result called a MERV-A. Those manufacturers that choose to conduct this test would state the MERV and the MERV-A of their filters.
The forward to Appendix J makes clear that it is “not part of the standard.” It is merely informative and does not contain requirements necessary for conformance to the standard. It has not been processed according to the ANSI requirements for a standard and may contain materials that have not been subjected to public review or the consensus process.”
Some air filter manufacturers have stated that they will be using the MERV-A procedures in their testing and will show both a MERV and a MERV-A on their filters. Other filter manufacturers have concerns that the Appendix J procedures dramatically increase the cost of filter testing without sufficient proof that the new methods are reliable, reproducible, and truly represent performance in real life situations.
In a recent paper given at the National Air Filtration Association Technical Conference in 2008 and published in Air Media magazine (Fall 2008) Monroe Britt presented results of ASHRAE 52.2 tests in different laboratories. The filters used in these tests were all pleated filters produced in the same factory, using the same materials, by the same people with the same specifications (a size of 24”X24”X2” and a total of 19 pleats). The tests were conducted for initial resistance and for initial efficiency only. The media used in the filters was not electrostatically charged (electret). A total of 8 laboratories were used – six of the laboratories were independent third-party labs and two were “in-house.” The filters were then prepared using the new optional ASHRAE Conditioning step (Appendix J). Laboratory C used a modified conditioning step that involved spraying the filter with an isopropyl alcohol (IPA) spray much like is used in the EN779 European test method.
Since one of the claims is that the ASHRAE test dust does not perform like actual dust in real life situations, there have been studies to determine the composition of dust in indoor and outdoor environments. It appears that the most frequently occurring dust outdoors is in the 4 nanometer (.04 micrometers) to 8 nanometer (.08 micrometers) range. The Conditioning Step is done by bombarding the filter with millions of particles of these sizes produced by particle generators using potassium chloride (KCL). Parameters are established for determining when a filter has been sufficiently “conditioned.”
Table 4 provides a summary of the test results by different labs.
Table 4 – Comparing ASHRAE 52.2 Filter Tests from Different Laboratories
|
Lab |
Pleats |
Filter |
Initial Resistance |
E1 |
E2 |
E3 |
MERV |
Comments |
|
Lab A |
19 |
1 |
0.37” wg |
7% |
61% |
80% |
8 |
As received |
|
Lab A |
19 |
1 |
0.37” wg |
10% |
62% |
80% |
8 |
After 52.2 Conditioning |
|
Lab B |
19 |
1 |
0.37 “ wg |
20% |
57% |
83% |
8 |
As received |
|
Lab C |
19 |
1 |
0.27” wg |
19% |
58% |
79% |
8 |
As received |
|
Lab C |
19 |
1 |
0.27” wg |
10% |
47% |
77% |
8 |
After IPA Treatment |
|
Lab C |
19 |
2 |
0.30” wg |
24% |
66% |
82% |
8 |
As received |
|
Lab D |
19 |
1 |
0.37” wg |
16% |
58% |
76% |
8 |
As received |
|
Lab D |
19 |
1 |
0.37” wg |
13% |
51% |
64% |
7 |
After 52.2 Conditioning |
|
Lab D |
19 |
2 |
0.37” wg |
13% |
54% |
73% |
8 |
As received |
|
Lab D |
19 |
2 |
0.37” wg |
10% |
48% |
70.3% |
8 |
After 52.2 Conditioning |
|
Lab E |
19 |
1 |
0.40” wg |
11% |
51% |
60% |
7 |
As received |
|
Lab E |
19 |
1 |
0.40” wg |
9% |
42% |
48% |
6 |
After 52.2 Conditioning |
|
Lab F |
19 |
1 |
0.38” wg |
22% |
60% |
73% |
8 |
As received |
|
Lab F |
19 |
1 |
0.38” wg |
13% |
49% |
52% |
7 |
After 52.2 Conditioning |
|
Lab F |
19 |
2 |
0.38” wg |
23% |
62% |
77% |
8 |
As received |
|
Lab F |
19 |
2 |
0.38” wg |
13% |
48% |
65% |
7 |
After 52.2 Conditioning |
|
Lab G |
19 |
1 |
0.40” wg |
11% |
63% |
80% |
8 |
As received |
|
Lab H |
19 |
1 |
0.58” wg |
|
48% |
67% |
7 |
As received |
|
Lab H |
19 |
1 |
0.65” wg |
|
38% |
63% |
7 |
After 52.2 Conditioning |
This data from eight different laboratories gives us some very interesting information on the variations in test results and the reliability of the Appendix J Conditioning Step. One would think that with the controls over equipment and test procedures in the ASHRAE 52.2 Standard that you would receive the same initial MERV number for the same filter. As it turns out 6 of the 8 laboratories found that the filter was a MERV 8 while two of the laboratories found that it was a MERV 7. A more careful look at the test results gives us some indication of why that is the case and illustrates why there are some limitations to the MERV system. Take a look at the results for Lab H. The E3 efficiency is 67% or a MERV 7. Just 3% more and it becomes a MERV 8. In looking at the variation in the other test results, it would not be surprising that another test of the same filter in the same laboratory would produce a MERV 8. It could be within the margin of error in the testing methods.
If you were the filter manufacturer producing the filters used in these tests, which test would you use? Would you say your filter is a MERV 7 or a MERV 8? Possibly this explains some of the variation in claims between manufacturers for what appears to be basically the same filter made from the same materials.
In every case, except one, the efficiency of the filter decreased after conditioning. In half of the cases the drop was sufficient to reduce the filter by a full MERV number. Generally, it is believed that the new conditioning step in Appendix J would drop the MERV on filters made with electret media. It is interesting that we also see inconsistencies for mechanical filters used in these tests.
Although the ANSI/ASHRAE 52.2 Standard is a useful tool in helping to select air filters, it is not an exact science. Despite the best efforts to standardize and certify test procedures, there is going to be some variation in test results. Given this variation, some in the filter industry have suggested that customers look at a MERV number in terms of (+) or (-) 1. Filter purchasers should keep this in mind. Be wary of purchasing a better test result – rather than a better filter.
Filters need to be changed regularly to ensure they perform with minimal pressure drop and consistent filtration efficiency. Lengthening filter change intervals to “save money” is false economy.
Some manufacturers have their own in-house ANSI/ASHRAE 52.2 testing facilities. These have their place in the development of products. However, with the possibility of different test results, it would seem that a third-party laboratory should be used to maintain impartiality. The National Air Filtration Association (NAFA) Product Certification program could also play a role in this regard.
Proper filter installation is an important component in filter efficiency. Just small gaps between filters or around the perimeter of the filter housing can dramatically reduce the effectiveness of filtration. Especially in the case of high efficiency filters, proper sizing and the use of gasketing and spacers are essential. For particularly critical applications such as in certain areas of health care facilities, there should be on-site (in situ) testing. This is the only way to ensure the particle capture efficiency of the entire system.
(This article originally appeared in the January 2009 Issue of Indoor Environment Connections and is provided here for the benefit of readers of this site. It should not be reproduced without the permission of Indoor Environment Connections.)