HVAC air filter

The Minimum Efficiency Reporting Value, or MERV for short, is an industry rating to determine the effectiveness of air filters. Commonly used to the specify the filtration efficiency  of commercial grade filters, the industry standard rating system allows facility managers to compare different HVAC filters as per their MERV values and choose the one that meets their filtration requirements. To help, here is a summary of MERV ratings according to particle size.

The MERV Rating Chart

MERV Minimum Particle Size Typical Controlled Contaminant
1-4 > 10.0 μm Pollen, dust mites, cockroach debris, sanding dust, spray paint dust, textile fibers, carpet fibers
5–8 10.0–3.0 μm Mold, spores, dust mite debris, cat and dog dander, hair spray, fabric protector, dusting aids, pudding mix
9–12 3.0–1.0 μm Legionella, Humidifier dust, Lead dust, Milled flour, Auto emission particulates, Nebulizer droplets
13–16 1.0–0.3 μm Bacteria, droplet nuclei (sneeze), cooking oil, most smoke and insecticide dust, most face powder, most paint pigments
17–20 < 0.3 μm Virus, carbon dust, sea salt, smoke

MERV ratings usually range between 1-16, but the upper limit has been extended to 20. The MERV rating scale is designed to represent worst-case performance of an air filter that is used for filter particles ranging from 0.3 to 10 micrometers. While the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) recommends using air filters with MERV value 6 or higher, the US Department of Energy recommends products with MERV 13 and LEED recommends filters with at least MERV value 8.

How is the MERV Rating of Air Filter Determined?

Determining the MERV rating of an air filter involves a series of tests. The process includes:

  • Test particles are added into the air in the testing area. These particles are of different sizes and fall into 1 of the 12 categories.
  • The air is passed through the filter that is tested. The density of the particles in the air is measured both before and after the air passed through the filters to determine its efficiency in order to remove the pollutants.
  • The entire process is carried out six times to get six ratings for the twelve categories each.
  • The MERV rating is given on the basis of the worst result.

The Significance of MERV Rating

Maintaining the Indoor air quality is very critical especially in healthcare facilities, pharmaceutical units and several other manufacturing units. The MERV rating assists these facilities to select an ideal filter that removes the impurities from air efficiently. Installing a filter with an MERV rating lower than required may lead to health problems in the facility as the indoor air quality is not clean. Here are the details of which MERV ratings that are ideal for specific sectors:

MERV 1-4

Air filters with MERV 1-4 ratings are recommended for facilities that require minimum filtration. These are used exclusively in residential buildings.

MERV 5-8

Air filters with MERV 5-8 ratings are recommended for residential, commercial and industrial workspaces.

MERV 9-12

Air filters with MERV 9-12 ratings are recommended for residential, commercial facilities and hospital laboratories.

MERV 13-16

Air filters with MERV 13-16 ratings are ideal to be used in hospitals and general surgery units.

MERV 17-20

Air filters with MERV 17-20 ratings are ideal for electronics and pharmaceutical manufacturing units.

Conclusion

MERV rating of a filter is determined through rigorous testing and it gives the performance of a filter in worst-case scenario, which makes it a reliable criteria for air filter selection. It is however essential for commercial establishments and industrial units to understand that highest MERV unit is just one selection criteria. There is a need to pay attention to other factors such as the size of the unit, price factor and the nature of filtration, when choosing an air filter.

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