Filter FAQ

Merv Rating System

MERV or Minimum Efficiency Reporting Value, or MERV for short, is a filter rating system devised by the American Society of Heating, Refrigeration and Air conditioning Engineers (ASHRAE) to standardize and simplify filter efficiency ratings for the public. The higher the MERV rating, the higher the efficiency of the air filter. Simply stated, a MERV 12 filter will remove smaller particles from the air than a MERV 8 filter.

For the consumer this means that you now have the ability to effectively compare one brand to another. Without any value-added additions, any MERV 8 filter will perform about the same as any other MERV 8 filter. The MERV rating only applies to efficiency. 

Merv 1-4 Rated filters will collect most particles of 10 microns or larger. Typical applications of these filters are minimum residential filtration, Light commercials, and minimum equipment production.

Merv 5-8 rated filters are used to trap particles in the 3-10 micron range. Some uses are in industrial and commercial building, high-end residential units, and paint booth/spray and finishing areas. 

Merv 9-12 rated filters are used specifically for particles in the 1-3 micron range. High-end residences, upgraded industrial workplaces and commercial boiling frequently use these.

Merv 13-16 rated Filters remove particles in the 0.3-1 micron range and are used in hospitals, health care and high-end commercial buildings. They are also useful in telecommunication manufacturing facilities.

Understanding MERV
NAFA User’s Guide for
ANSI/ASHRAE
Standard 52.2-2007

Method of Testing General
Ventilation Air-Cleaning
Devices for Removal
Efficiency by Particle Size

Author(s): NAFA Technical Committee

Introduction

This ANSI/ASHRAE Standard 52.2 User Guide was created by the National Air Filtration Association (NAFA), a group of over 600 air filter distributors, manufacturers and engineers. This Guide, and the application of a particle-based contaminant removal standard prescribed by ANSI/ASHRAE Standard 52.2-2007 “Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size,” are intended to assist end-users and specifiers in their selection of appropriate air filtration products and understanding of the MERV values in the 52.2 test reporting.

The ASHRAE Standard 52.2

ANSI/ASHRAE Standard 52.2 features many improvements over the 52.1 standard. In 2009 ANSI/ASHRAE 52.1 "Dust Spot" efficiency testing was removed from the standards while two other parts of the 52.1 Standard were adopted into the 52.2 Standard, allowing for the 52.1 to be retired. The two parts of 52.1 adopted by 52.2 are "Arrestance for determining MERV 1-4 and "Dust Holding Capacity." It should be noted that the ANSI/ASHRAE standard makes it clear that Dust Holding Capacity is reported as the total weight of synthetic loading dust captured by the air cleaning device over all of the incremental dust loading steps. This value should not be used to calculate the expected life of the device in use.

Some of the improvements found in the ANSI/ASHRAE 52.2 standard include:

• The use of mandatory (code) language, which enables the standard to be referenced by other codes that are developed. • Where 52.1 expressed efficiency as an overall percentage, 52.2 expresses efficiency as a function of specific particle size.
• Seventy-two (72) data points are reduced into a single curve that typifies the minimum efficiency of a filter.

Standard 52.2 Test Procedure:
How Data is Obtained

An air filter’s performance is determined by measuring the particle counts upstream and downstream of the air-cleaning device being tested.

Particle counts are taken over the range of particle sizes six times, beginning with a clean filter and then after the addition of standard synthetic ASHRAE dust loadings for five additional measurement cycles.

A laboratory aerosol generator, which operates much like a paint sprayer, is used to create a challenge aerosol of known particle size in the air stream. This will generate particles covering the 12 required particle size ranges for the test (See Table 2).
The challenge aerosol is injected into the test duct and particle counts are taken for each of the size data points.

The filter’s performance, on each of the twelve particle sizes, during the six test cycles (a total of 72 measurements) is determined. For each measurement, the filtration efficiency is stated as a ratio of the downstream-to-upstream particle count. The lowest values over the six test cycles are then used to determine the Composite Minimum Efficiency Curve. Using the lowest measured efficiency avoids the misinterpretation of averaging and provides a “worst case” experience over the entire test.

The twelve size ranges are placed in three larger groups according to the following schedule: ranges 1-4 (or E1, which is 0.3 to 1.0 µm), ranges 5-8 (or E2, which is 1.0 to 3.0 µm), and ranges 9-12 (or E3, which is 3.0 to 10.0 µm). Averaging the Composite Minimum Efficiency for each of these groups will calculate the average Particle Size Efficiency (PSE), and the resulting three percentages (E1, E2, E3) are then used to determine the MERV.

Table 2:  ASHRAE 52.2 Particle Size Ranges

Range	Size	Group
1	0.30 to 0.40	E1
2	0.40 to 0.55
3	0.55 to 0.70
4	0.70 to 1.00
5	1.00 to 1.30	E2
6	1.30 to 1.60
7	1.60 to 2.20
8	2.20 to 3.00
9	3.00 to 4.00	E3
10	4.00 to 5.50
11	5.50 to 7.00
12	7.00 to 10.00

Minimum Efficiency Reporting Value (MERV)

An "overall" reporting value of a 52.2-evaluated air filter is the expression of the Minimum Efficiency Reporting Value (MERV). The MERV is a single number that is used, along with the air velocity at which the test was performed, to simplify the extensive data generated by the method of testing. MERV is expressed on a 16 point scale and is derived from the PSE for each of the three groups. (See Table 3: MERV Parameters.)

The average PSE for each of the three groups (E1, E2 and E3) is referenced against the Minimum Efficiency Reporting Value Parameters (see Table 3: MERV parameters). Move up the appropriate Range Group (E1, E2 and E3) on Table 3 and record the MERV to the left of the first true statement. Do this for all three groups.

Table 3:  MERV Parameters

Standard 52.2 Minimum Efficiency Reproting Value (MERVE)	Composite Average Particle Size Efficiency, % in Size Range, µm			ASHRAE Arrestance
	Range 1 (0.3 - 1.0)	Range 2 (1.0 - 3.0)	Range 3 (3.0 - 10.0)
10	n/a	n/a	E3	Aavg
2	n/a	n/a	E3	65≤Aavg
3	n/a	n/a	E3	70≤Aavg
4	n/a	n/a	E3	75≤Aavg
5	n/a	n/a	20≤E3	n/a
6	n/a	n/a	35≤E3	n/a
7	n/a	n/a	50≤E3	n/a
8	n/a	n/a	70≤E3	n/a
9	n/a	E2	85≤E3	n/a
10	n/a	50≤E2	85≤E3	n/a
11	n/a	65≤E2	85≤E3	n/a
12	n/a	80≤E2	90≤E3	n/a
13	E1	90≤E2	90≤E3	n/a
14	75≤E1	90≤E2	90≤E3	n/a
15	85≤E1	90≤E2	90≤E3	n/a
16	95≤E1	95≤E2	90≤E3	n/a

Standard Test Airflow Rates

The Minimum Efficiency Reporting Value (MERV) must be stated with the air velocity at which the filter was tested. For example, if the filter was tested with an air velocity of 492 FPM and was found to be MERV 8, the filter’s Minimum Efficiency Reporting Value would be MERV 8 @ 492 FPM. ASHRAE Standard 52.2 tests are to be conducted at one of seven airflow rates:

118 FPM (0.60 m/s)
246 FPM (1.25 m/s)
295 FPM (1.50 m/s)
374 FPM (1.90 m/s)
492 FPM (2.50 m/s)
630 FPM (3.20 m/s)
748 FPM (3.80 m/s)

 

Minimum Final Resistance

The minimum final resistance shall be twice the initial resistance, or as specified. Final resistance values have been removed as several products have been designed with efficiencies for markets that will not support the higher pressure drops previously required.

Addendum B average arrestance and dust holding capacity (DHC)

Arrestance and DHC values will be reported on all filters tested per 52.2 testing procedures. While these values will be reported they are not part of the mandatory reporting for MERV unless the values are in MERV's 1 through 4.

Two ASHRAE research projects have revealed a potential loss in efficiency in some filters as they are exposed to superfine particles. Non-ANSI approved Appendix J has been added as an optional conditioning step to the 52.2 Standard to provide a method of identification of the drop in filter efficiency. The reported value per Appendix J would be referred to as MERV 'A.' Thus filters tested per Standard 52.2 with Appendix J option would have a MERV and a MERV 'A' reported value.

Conclusion

Contact your local National Air Filtration Association (NAFA)  member company. Most NAFA members are staffed by NAFA Certified Air Filter Specialists (CAFS) to assist in the proper selection of filters for your application.

Selecting Proper Air Filter Efficiencies for Commercial Buildings.

Introduction

The information provided here is intended to assist those responsible for making technical decisions to improve air filtration in commercial buildings. These would include offices, retail facilities, schools, churches, transportation terminals, and public arenas such as sports coliseums, and malls. The focus here will be on air filter selection concerning particulate contaminants.

Building owners, operators, managers, designers, service contractors and maintenance personnel need reliable and accurate information regarding air filtration and air cleaning options. The decision to enhance and upgrade air filtration in a specific building should be based on the building, occupants, its engineering, and architectural, feasibility and cost. The information learned will allow one to make a more knowledgeable and informed decision about selecting, installing and upgrading air filtration systems. Effective air filtration can also help improve overall Indoor Air Quality, (IAQ) and worker health and productivity. 

Implementation

Cost is always an issue affected by implementing a filtration upgrade to the HVAC system. Total system costs should be evaluated by the decision makers regarding the enhanced filtration upgrade. Life cycle cost analysis should also be conducted. They should include the following:

1. Initial cost of the materials to include shipping, warehousing and "shrinkage"
2. Operating cost, the energy consumption allocated directly to the air filters
3. Replacement cost which is the labor cost to replace filters when they have reached the end of their service life
4. Disposal cost

Higher efficiency filters typically have a higher initial cost than commonly used low to medium efficiency products that are specified in most HVAC systems. Usually, HVAC systems are equipped with filters designed to keep equipment components such as coils, compressors, fans, and ductwork clean. Higher efficiency filters may have a higher resistance to airflow called pressure drop, and fans may have to be changed to handle this increased pressure drop. Although these systems improvements will normally come at a higher initial cost, the benefits achieved by this change can offset many of the operating costs just by delivering cleaner air throughout the building and keeping the system components operating at peak energy efficiency.

Operating Conditions

Building pressure must also be considered for an effective HVAC filter system upgrade. The building envelope should be as airtight as possible but, as with most construction, this is a very difficult parameter to achieve. Some outside building walls leak (infiltration) and significant amounts of unfiltered air can enter the building envelope. Field studies have shown that, unless specific measures have been taken to reduce infiltration, as much air can enter the building through infiltration (unfiltered) as through the HVAC mechanical (filtered) system. Therefore, one cannot expect the HVAC filtration system alone to improve overall IAQ. Instead, one must consider air filtration in combination with other steps, such as building envelope tightness, and building pressurization to, as much as possible, insure that the air entering the building only comes in through the outside air HVAC air intake. The building envelope should be maintained under a slight positive pressure to inhibit infiltration as recommended by the Department of Health and Human Services (NIOSH) in their publication No. 2002-139 "Guidance for Protecting Building Environments from Airborne Chemical, Biological, or Radiological Attacks". 

Particulate Air Filtration

Contaminants of concern should be carefully be evaluated to determine the level of filtration efficiency required for the contaminant size. The size of contaminants is measured in micrometers (microns). Once a comprehensive list of contaminants of concern has been identified one will be able to use the ANSI/ASHRAE Standard 52.2-1999 to select the proper filter with the appropriate Minimum Efficiency Reporting Value, (MERV). A MERV 6 filter for example is the minimum required to comply with ANSI/ASHRAE Ventilation Standard 62.1-2004 located in Section 5.9 Particulate Matter (PM).

Filter selection should be based on ANSI/ASHRAE Standard 52.2-1999 "Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size." This procedure calls for efficiency measurements to be taken on twelve (12) particle size ranges using potassium chloride, (KCI) as the challenge aerosol. Six efficiency measurements for each of the (12) particle size ranges is taken which gives (72) total efficiency measurements. The (12) particle size ranges are grouped into (3) wider ranges. They are as follows: 

• E1 - 0.3 - 1.0 microns
• E2 - 1.0 - 3.0 microns
• E3 - 3.0 - 10 microns

The lowest efficiency value (minimum efficiency reporting value – MERV) of the 6 measurements taken is recorded. The 52.2 test prescribes that the procedure is to be conducted at one of 7 airflow rates. The tested filters run from 118 feet per minute, (fpm) up to 748 fpm. The MERV allows you to be able to select the proper number to capture and remove the contaminant of concern. Standard 52.2 provides the industry accepted procedure for measuring filter efficiency by particle size. The need for a more precise measurement of a filter’s ability to remove specific particle sizes has become a concern over Indoor Air Quality, (IAQ) as well as the protection, products, processes and most importantly people. It is very important that the filters selected for the specific application are provided with an ASHRAE 52.2 Test Report documenting the filter efficiency. It is also critical that the filters selected have the test data showing the airflow rate of the filter being tested should be of the same velocity rating of the HVAC system using one of the 7 flow rates used in Standard 52.2. 

Emerging Technologies

Increasing the HVAC air filtration efficiencies typically results in higher pressure drops. Today there are several air filtration products that are manufactured that provide higher efficiencies with little or negligible increase in pressure drop. The mini-pleat V-cell filters incorporate up to 4 times the media in the same 24x24x12 filter pack. This is accomplished by manufacturing 1 inch min-pleat panels in a V-style filter pack. The principle here is the very same as a V-bank filter housing. It allows more filter surface area, thus reducing air flow resistance. In cases where there is only one filter track the highest MERV number should be considered, providing the airflow pressure drop is not increased beyond the point of system design capabilities. 

This V-cell mini-pleat filter incorporates about four times the area of a regular filter, greatly reducing static pressure and lasting about twice as long.

Installation

In addition to proper air filter selection, several issues must be considered before installing or upgrading filtration systems. Air filter bypass is a common problem found in many HVAC filtration systems. Filter bypass occurs when air moves around the filter rather than moving through the filter. This will result in a decrease of collection efficiency and defeating the intended purpose of the filtration system. By simply improving filter efficiency without properly addressing filter bypass, the system will provide very little if any added benefit. If the system hardware/frames or housing leaks or if the filters are poorly fitted then subsequently filtration efficiency and performance will drop off significantly. The filters must be installed with the proper filter holding clips. Gasket material should be used on the vertical side between filters, on frames, tracks, and definitely on the doors of the unit to insure an airtight seal. Simply put, in order to have the filtration system perform effectively they must be forced to pass through the filters. Air filter gages must also be installed in order to measure pressure drop across the filter bank. If the system cannot be measured with an air filter gauge then it cannot be monitored, and if not properly monitored, the filtration system performance cannot be effectively managed. 

One More Thing

Everyone assumes that technicians understand the proper way to install and maintain air filters. Experience shows a different story. The author has personally observed incorrect filters, improperly installed and/or missing, with gaps and worn or missing filter holding clips and gasketing. A 10 millimeter gap (less than ¼ inch) between filters can lower a filter’s MERV rating by at least two levels, thereby taking a high efficiency filter and moving it to a medium efficiency filter. Only adequately trained personnel should perform filter maintenance. The National Air Filtration Association (NAFA) has designed an accredited program for HVAC field technicians called a NAFA Certified Technician (NCT). This program is comprehensive in its approach with a complete text and tutorial study program followed by a national exam. This certification program has been designed for North American Technician Excellence (NATE) CEU’s, and more information can be found at a local NAFA-member air filtration company or on the NAFA web site at www.nafahq.org. 

Conclusion

Consider using periodic quantitative evaluation to determine the total system efficiency. Building operators should perform various field inspections to insure filter seals and gaskets are installed properly and gauges are reading pressure drops accurately. This will allow you to properly apply the 3 M’s Measure, Monitor and Manage their HVAC air filtration systems.

HVAC systems should be (locked out/tagged out) while conducting maintenance to avoid and prevent contaminants from being entrained into the moving air stream. Follow OSHA Standards 29 Code of Federal Regulations, (CFR) 1910.132 and 1910.134 regarding appropriate personal protective equipment, i.e. (gloves, respirators, glasses) etc. when performing filter change-outs.

Maintenance plans and schedule of operations should also be put in place to make sure that the filtration system works as intended. Life cycle cost analysis will also insure that the filtration system will satisfy the building needs while providing adequate protection to the building occupants in the office workplace today. NAFA certified field technicians will assure personnel are trained in the proper installation, application and maintenance of the system.

(all articles are excerpts from NAFA Library)