HVAC Filter Selection and MERV Ratings: What Does It All Mean?

By David Klenk, P.E. Mechanical

In 1968, American Society of Heating, Refrigerating and (ASHRAE) created Standard 52.1 to test air filters using established techniques employed by the U.S. National Bureau of Standards and the Institute. During the course of the past 40 years, this standard has been modified several times to reflect changing air filtration testing requirements. The version in use today, Standard 52.1-1992, evaluates filters based upon three key measurements:

ƒ Arrestance. Designated as a percentage, the arrestance of the filter, or ability of a filter to remove synthetic dust, is calculated based upon the known weight of dust fed into a test unit under controlled conditions and the amount of weight added or captured by the filter.

ƒ Atmospheric Dust Spot Efficiency. Designated as a percentage, two procedures can be used to calculate dust spot efficiency, which is the ability of a filter to remove atmospheric dust from the air. Both the intermittent flow method and the constant flow method rely upon white filter paper as a target and the ratio of light transmission. Sampling of atmospheric air is conducted at 25%, 50%, and 75% of dust loading and at final resistance. Efficiency is determined by an equation that incorporates total airflow through the upstream and downstream targets and opacity of dust spot on the upstream and downstream targets. Traditionally, engineers and designers have used the terms 30%, 60%, and 85% filters to indicate which type of filter to install. These terms refer to the dust spot efficiency of the filters.

ƒ Dust Holding Capacity. Expressed in grams, dust holding capacity measures the amount of dust that a filter can hold. Synthetic test dust is fed into a filter and the final capacity is based upon the amount of dust held by the filter.

Standard 52.1 does not evaluate particle size and efficiency of the filter for a particular particle. It is not known what type and size of particles will be captured by the filter. For instance, a 30% filter (based on arrestance) means that 30% of the weight of the dust has been captured. Depending on the size of the dust particles the actual number of particles (and hence the percentage of particles) captured may be substantially less.

MERV Defined

To address the issue of particle size, ASHRAE 52.2 was issued in 1999 and modified in 2007 to complement, not replace, Standard 52.1, by enabling engineers/designers to select a filter based upon the actual known particle size of the contaminant desired to filter. Standard 52.2 indicates the lowest point of filter efficiency (which is normally right at the time of installation1). Filters are evaluated based upon initial efficiency as a function of one of 12 possible particle size ranges. Based on this data, a numerical value is assigned to a filter. This numerical value is designated as MERV: Minimum Efficiency Reporting Value. MERV ratings are based upon specific airflow speeds which must be noted on the manufacturer’s data.

1 Note that synthetic filters are typically electrostatically charged and will have a greater efficiency at time of installation but will lose efficiency quickly as the charge is dissipated. ASHRAE 52.2 currently does not address the dissipation of the electric charge. Refer to the filter manufacturer literature or ask your local representative to determine if a filter is electrostatically charged.

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Testing for MERV

MERV ratings are created using the following testing process:

1. Initial resistance versus airflow is measured. 2. An optical particle counter measures and counts the particles in 12 equally distributed particle size ranges before and after the filter. 3. ASHRAE test dust is introduced to the filter in five predetermined steps (pressure drop across High Efficiency Particulate Air filter increases by 0.04” water gauge (w.g.), 25% (HEPA) Filters of manufacturer’s recommended final pressure drop, 50% of manufacturer’s recommended final Six types of high-efficiency filters are pressure drop, 75% of manufacturer’s available; however only Type A filters are recommended final pressure drop and 100% of typically used in GMP facilities. manufacturer’s recommended final pressure drop). 1. Type A filters. Commonly known as 4. Efficiency performance curves are developed Industrial Grade, Type A filters are based upon the optical particle counter data for used for GMP/industrial processing, each of the six readings (one initial, and five hospitals, and non-critical areas. loading steps). Unlike the other filter types described 5. A composite minimum efficiency curve with the below, which are tested both at the lowest efficiency reading for the six readings for manufacturing facility and after 12 particle size ranges is developed. installation, Type A filters are tested at 6. Twelve particle size ranges are placed in three the manufacturing facility only. larger groups (E1, E2, and E3) and the 2. Type B filters. Commonly known as percentages of each group are averaged. This Nuclear Grade, these filters are used average is deemed the Particle Size Efficiency for Department of Energy nuclear which is the value used to determine the MERV programs (test reactors, R & D, and rating based upon ASHRAE data. (See the weapons) and commercial reactors. ASHRAE Application Guidelines chart listed on 3. Type C filters. Commonly known as page 4 for additional information.) Laminar Flow Grade, Type C filters are used for laminar flow work Filter Selection stations, clean rooms, semiconductor and drug manufacturers. Proper filter selection requires an understanding of 4. Type D filters. Commonly known as several factors: the type and size of particle/contaminant Very Large Scale Integrated (VLSI) to be removed; expected filter service life; and pressure filters, these filters are used for drop. ASHRAE 52.1 and 52.2 should be used in semiconductor and drug conjunction with one another to make a final filter manufacturers. selection. To help in the selection process, the following 5. Type E filters. Commonly known as information will be required: Bio/Hazard Grade, Type E filters are used for hazardous biological ƒ Physical size of particulate. The chart on page 4 containment facilities. should be helpful in determining the MERV rating 6. Type F filters. Commonly known as of the filter once the particulate size has been Ultra Low Penetration Air (ULPA) determined. For example, if bacteria ranges from Grade, these filters are used at semi- 0.3 microns to 4 microns, the chart indicates a conductor facilities. MERV 13 would be the minimum rating to select, and higher MERV-rated filters should be selected HEPA filter efficiencies are based upon with the end users’ level of comfort and 0.30 µm particle sizes. economics in mind.

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659 Van Meter Street Cincinnati OH 45202-1568 T: 513 241 1230 F: 513 241 1287 Email: [email protected] Website: www.hixson-inc.com HVAC Filter Selection and MERV Ratings: What Does It All Mean?

ƒ Service life of filters. Review the dust holding capacity in both ASHRAE 52.1 and the manufacturer’s product data to compare the relative service life of comparable filters. Additional service life can be obtained by simply increasing the depth of the filter (e.g., 2” deep to 4” deep). ƒ Initial and final pressure drop of filters. Review manufacturing product data to determine initial and final pressure resistance. The system should be able to provide design airflow throughout the entire loading process of the filters. Note that increasing the number of filters, and, therefore, the total surface area, will increase the service life of the filters but will not affect the final system pressure drop. ƒ Filter frame material. Determine if the filter will be in a wet or dry environment, and how it will be disposed. (For example, if incineration will be used, a metal frame would not be advisable.) ƒ Design airflow. Pre-filters and final filters should be matched for airflow compatibility and pressure drop. ƒ Physical space allotted for filter bank. Allow space for transition before and after the filter bank.

Remember, the filter rating is based upon the filter only and is tested during ideal lab conditions. Determination of the system rating must include the filter housing. The filter housing must be matched to provide the proper sealing of filters to the filter frame. An inexpensive, poor quality filter frame negates spending funds on a high-rated MERV filter.

ASHRAE Application Guidelines Std 52.2 Std 52.2 Std 52.1 Dust Std 52.1 Minimum Particle size efficiency Spot Efficiency Average Efficiency (%) (%) Arrestance Reporting Value Efficiency (MERV) (%) E1 E2 E3 0.30 to 1.0 µm 1.0 to 3.0 µm 3.0 to 10.0 µm 1 n/a n/a E < 20 < 20 < 65 2 n/a n/a E < 20 < 20 65 ≤ E < 70 3 n/a n/a E < 20 < 20 70 ≤ E < 75 4 n/a n/a E < 20 < 20 75 ≤ E < 80 5 n/a n/a 20 ≤ E < 35 < 20 80 ≤ E < 85 6 n/a n/a 35 ≤ E < 50 < 20 85 ≤ E < 90 7 n/a n/a 50 ≤ E < 70 25 ≤ E < 30 E > 90 8 n/a n/a 70 ≤ E 30 ≤ E < 35 E > 90 9 n/a E < 50 85 ≤ E 40 ≤ E < 45 E > 90 10 n/a 50 ≤ E < 65 85 ≤ E 50 ≤ E < 55 E > 95 11 n/a 65 ≤ E < 80 85 ≤ E 60 ≤ E < 65 E > 95 12 n/a 80 ≤ E 90 ≤ E 70 ≤ E < 75 E > 95 13 E < 75 90 ≤ E 90 ≤ E 80 ≤ E < 90 E > 98 14 75 ≤ E < 85 90 ≤ E 70 ≤ E 90 ≤ E < 95 E > 98 15 85 ≤ E < 95 90 ≤ E 90 ≤ E E > 95 n/a 16 95 ≤ E 90 ≤ E 95 ≤ E n/a n/a 17 99.97 < E n/a n/a n/a n/a 18 99.99 < E n/a n/a n/a n/a 19 99.999 < E n/a n/a n/a n/a 20 99.999 < E n/a n/a n/a n/a "E" indicates efficiency percentage 3

659 Van Meter Street Cincinnati OH 45202-1568 T: 513 241 1230 F: 513 241 1287 Email: [email protected] Website: www.hixson-inc.com HVAC Filter Selection and MERV Ratings: What Does It All Mean?

References

ANSI/ASHRAE. “Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size.” ANSI/ASHRAE Standard 52.2. 2007.

ASHRAE. “Chapter 24: HVAC Systems and Equipment.” ASHRAE Handbook. 2004.

Camfil Farr. “ASHRAE Testing for HVAC Air Filtration: A Review of Standards 52.1-1992 & 52.2-1999.” Technical Services Bulletin January 2001.

Flanders Corporation. “HEPA Filters and Filter Testing: PB-2007-1023.” May 2004.

Thornburg, Don. “Filter Selection: A Standard Procedure.” Engineered Systems June 2000: 74-80.

Advocacy: Hixson is a "No-Bias" Company At Hixson, we seek to serve as extended team members of our clients’ organizations, not as representatives of companies with outside concerns. Therefore, we do not accept income or incentives – or sell/represent products – from equipment suppliers, contractors or any company that could improperly influence our opinion. Because of this, Hixson ensures that the information we present is unbiased and has been developed in the best interests of our clients.

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659 Van Meter Street Cincinnati OH 45202-1568 T: 513 241 1230 F: 513 241 1287 Email: [email protected] Website: www.hixson-inc.com