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MERV / MERV-A - While both ratings provide the performance rating of the HVAC Filter, there is a difference in how an HVAC Filter is assigned either a MERV or MERV-A rating.

Difference between MERV and MERV-A

Why this difference matters to you

Most filter manufacturers use the MERV rating to indicate the particle efficiency of their HVAC filters. However, using only the MERV rating can have implications of which facility managers and building owners are not aware.

According to ASHRAE Standard 52.2, an HVAC filter with a MERV 14 rating for example, is required to perform at MERV 14 at the time of installation. The same filter however, might drop in performance (sometimes from MERV 14 to as low as MERV 12 or 11) during actual use.

ASHRAE has recognized this shortcoming and therefore added a test protocol (= Appendix J) to determine the
MERV-A rating of an HVAC filter.

To ensure that you buy the HVAC Filter that meets your minimum efficiency requirements, you should therefore look for the MERV-A rating when choosing the HVAC filter.

In the following, we will provide you with more detailed information about the difference of MERV and MERV-A, and expand on why it's important to look for the MERV-A rating when buying HVAC filters.

Understanding the basics

While both ratings provide the performance rating of the HVAC Filter, there is a difference in how an HVAC Filter is assigned either a MERV or MERV-A rating. This is of particular importance when using electrostatically charged (electret) HVAC Filters, as opposed to conventional / mechanical (non-electret) HVAC filters.

First, let's take a closer look at the main characteristics typically seen with mechanical (non-electret) and electret filters:

 
Mechanical filter
Electret filter
Filter Media Material
Microglass fiber
Synthetic charged fiber
Electret charge
No
Yes
Pressure drop
(at equal filtration efficiency
of same filter construction)


Higher

Lower
Filtration efficiency
(at equal pressure drop of
same filter construction)


Lower

Higher
Filtration mechanism
Mechanical filtration only
Mechanical filtration and
charged based filtration
Durability in use / Tear resistance
Low
High
Moisture absorption
Yes, unless treated
No

Our 3M™ Commercial HVAC Filters use electret media with an innovative, open structure media for low pressure drop. Using 3M technology, we are able to provide our customers with filtration media that supports low air flow resistance which can lead to less electrical energy consumption. It also allows for depth-loading of the filter, which leads to longer filter life.

 
Innovative, open structure media used in Filtrete™ Commercial HVAC Filter Products Innovative, open structure media used in 3M™ Commercial HVAC Filter Products

Questions?

Learn More or call us at
1-800-648-3550.

 

Now, let's look at the difference between MERV and MERV-A and the test protocols that are being used to test the filters accordingly.

MERV Rating

MERV (Minimum Efficiency Reporting Value) is a particle capture efficiency metric for air cleaning devices (e.g., air filters) defined by the American Society of Heating, Refrigeration and Air-Conditioning Engineers (ASHRAE) Standard 52.2-2007. MERV reporting values range from 1 to 16.

The reporting value is determined by testing the filter for its capture efficiency, using salt particles (potassium chloride) as a function of particle size over the size range from 0.3 to 10 µm. The MERV designation increases with the rise of the particle capture efficiency of a filter, especially for small particles.

However, the test protocol used to determine the MERV reporting value is not appropriate for filters that contain an electret charge.

The following table lists criteria for each MERV rating per Standard ASHRAE 52.2-2007.

 
Average Particle Reduction Efficiency (in %)
MERV Rating
(ASHRAE 52.2: 2007)
E1
0.3 - 1.0 microns
E2
1.0 - 3.0 microns
E3
3.0 - 10.0 microns
1
--
--
--
2
--
--
--
3
--
--
--
4
--
--
--
5
--
--
≥ 20
6
--
--
≥ 35
7
--
--
≥ 50
8
--
--
≥ 70
9
--
--
≥ 85
10
--
≥ 50
≥ 85
11
--
≥ 65
≥ 85
12
--
≥ 80
≥ 90
13
--
≥ 90
≥ 90
14
≥ 75
≥ 90
≥ 90
15
≥ 85
≥ 90
≥ 90
16
≥ 95
≥ 95
≥ 95

According to the table, a MERV 14 filter would reduce:
According to the table, a MERV 14 filter would reduce greater than 75% of 0.3 – 1.0 micron sized particles (E1)> 75% of 0.3 – 1.0 micron sized particles (E1)
According to the table, a MERV 14 filter would reduce greater than 90% of 1.0 – 3.0 micron sized particles (E2)> 90% of 1.0 – 3.0 micron sized particles (E2)
According to the table, a MERV 14 filter would reduce greater than 90% of 3.0 – 10.0 micron sized particles (E3)> 90% of 3.0 – 10.0 micron sized particles (E3)

Building owners, facility managers and all personnel involved in buying HVAC Filters should know that the MERV rating is a laboratory metric, indicative of how a brand new HVAC filter performs at time of installation. It does not indicate the minimum particle capture efficiency over a filter's life. This is where the MERV-A rating is relevant when using electret media filters. It is a more accurate reflection of the minimum filter performance in actual use / over a filter's life.

MERV-A Rating

MERV-A (Minimum Efficiency Reporting Value-A) is a particle capture efficiency metric for air cleaning devices (e.g., air filters) with electret charged media. The applicable standard to determine the MERV-A rating is defined by ASHRAE Standard 52.2. – 2007 Appendix J.

In Appendix J, the test protocol includes a conditioning step that is designed to determine the magnitude of the efficiency loss that an electret based filter may realize in actual use. This conditioning step is representative of the best available knowledge to simulate the minimum efficiency expected in actual use, in a controlled laboratory setting.

Once the conditioning protocol is applied to a filter, the standard test protocol to measure efficiency is carried out and the MERV-A rating determined, based on the table used for the MERV rating.

Looking at the MERV and MERV-A ratings, can also help you categorize a filter:

If MERV Rating = MERV-A rating If MERV Rating = MERV-A rating then it's categorized as a mechanical filter categorized as a "mechanical filter"

If MERV Rating > MERV-A rating If MERV Rating greater than MERV-A rating then it's categorized as an electret filter categorized as an "electret filter"

Why 3M uses the MERV-A rating for 3M™ Commercial HVAC Filters

All 3M™ Commercial HVAC Filter Products use an innovative, open filtration media design with a robust electrostatic charge. In addition to testing our filters to the industry conditioning standard – ASHRAE 52.2: 2007 Appendix J, our filters are also tested to several rigorous internal test protocols. Ultimately, all of the testing and the use of the MERV-A rating provide our customers with the maximum amount of information and enable them to feel confident that the filters they select will meet their requirements.

Here are some of the key benefits of 3M™ Commercial HVAC Filters when compared to mechanical (non-electret) filters.

 
Mechanical filter
(Microglass fiber)
Electret filter
(Synthetic charged fiber)
Benefits of 3M™ Commercial HVAC Filters



Electret charge



No



Yes
Electret media allows for lower flow resistance resulting in potential lower electrical energy consumption while maintaining filter efficiency
Pressure drop
(at equal filtration efficiency
of same filter construction)


Higher

Lower
Lower pressure drop leads to potential lower electrical energy consumption
Filtration efficiency
(at equal pressure drop of
same filter construction)


Lower

Higher

Better IAQ










Filtration mechanism










Mechanical filtration only










Mechanical filtration and charged based filtration
Relying solely on mechanical filtration has the downside of a higher pressure drop, resulting in higher electrical energy consumption.

Relying solely on electret charged filtration is not feasible, as filtration efficiency from the charge may be lost over time.

Use of both, mechanical and electret mechanisms provide customers with the optimal solution, balancing lower electrical energy consumption and high filtration efficiency over the filter life.

Durability in use
/
Tear resistance

Low

High
Long filter life; lower risk of damage to filters during transport and installation

Moisture absorbption

Yes, unless treated

No
Does not support microbial growth

Why looking at the MERV / MERV-A rating is important when buying HVAC Filters

We believe that the MERV-A rating offers our customers a better guidance regarding the in-use minimum efficiency performance for HVAC filters.

With our 3M™ Commercial HVAC Filter Products, we have demonstrated that the in-use efficiency of the filter and the filter performance over its life cycle, are accurately represented with the MERV-A rating. This is the reason why 3M is using the MERV-A rating for all 3M™ Commercial HVAC Filter Products.

When buying HVAC filters, careful consideration should be paid to the efficiency ratings that filter manufacturers provide for their electret products. To ensure they buy HVAC filters that meet their minimum in-use efficiency requirements, building owners, facility managers and personnel should use the MERV-A rating rather than the MERV rating when selecting HVAC filters.

Bottom Line

ASHRAE has recognized that the MERV-A rating is providing additional information, to help customers make informed decisions. If you do not see a MERV-A rating, we recommend that you ask your supplier or filter manufacturer for that information. 3M provides more information related to the MERV-A rating in our product information sheets, than any other manufacturer. Understanding the difference between MERV and MERV-A will help you compare the performance capabilities of 3M™ products, to HVAC filters from other manufacturers and recognize the benefits the 3M products offer you.

 
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