Understanding the MERV Rating

Changing your furnace filters regularly and using the most efficient furnace filters on your heating and air conditioning units are the first two steps toward improving your indoor air quality. In fact, the only way to actively reduce dust particles and airborne bacteria is through the use of efficient air filters.

When choosing air filters for your heating and air conditioning units, it is important to buy the most efficient filter that your system will allow. Finding the most efficient filter possible is easy, thanks to the Minimum Efficiency Reporting Values, or MERV rating.

The MERV rating is an industry-standard system for rating the minimum efficiency of air filters. The MERV rating is a scale from 1 to 20, with one being the least efficient designation and 20 the highest. For example, a simple fiberglass panel would have a MERV rating of 5, and only traps the largest of airborne particles. A more efficient filter with a MERV rating of 14 will trap even the most miniscule particles.

Shopping for an air filter is not as easy as buying the model with the highest MERV rating, however. Filters with a higher MERV rating also create more resistance to airflow through the filter. The high resistance that comes with the increased MERV may tax your furnace fan power too extensively. Be sure to select a filter with the highest MERV rating that your unit's fan can maintain.

Also, some filters may use an electrostatic charge to improve their MERV rating. These filters offer a high MERV rating, but their efficiency dwindles as the filter absorbs contaminants. Look for a "discharged MERV" rating to find the filter's actual efficiency.

Taking a little extra time to learn about and inquire as to MERV ratings will pay off in the long run, as you will be able to choose the most efficient filter for your home.

Typical applications for the major MERV value ranges

MERV 1 – 4

Minimum filtration, used almost exclusively in residential buildings

MERV 5 – 8

Most commercial applications and better residential buildings

MERV 9 – 12

Superior residential buildings and better commercial buildings

MERV 13 – 16

Hospital inpatient and general surgery; found in superior commercial buildings

MERV 17 – 20

Clean rooms and pharmaceutical manufacturing

MERV ratings are used to rate the ability of an air conditioning filter to remove dust from the air as it passes through the filter. MERV is a standard used to measure the overall efficiency of a filter. Higher MERV ratings mean fewer dust particles and other airborne contaminants pass through the filter.

MERV ratings are determined by adding particles of varying sizes (1-12) into a controlled testing environment. The particles are added upstream of the test filter and a laser particle counter samples the air before it enters the filter and after it leaves the filter. The two particle counts are compared to calculate the Particle Size Efficiency of the tested filter. Once this is determined, a MERV Parameters chart is used to determine the MERV rating.

MERV ratings range from 1 - 16 and measurements are in microns. Some of the common particles related to MERV ratings are pet dander, insecticide dust, smog, dust, viruses, wood, tobacco smoke, spores, bacteria and pollen.

MERV Ratings Explained

MERV rating 1 to 4

Some of the most common filters found in residential use only have a merv rating of 1 to 4. These are typically disposable panel type filters and do not do a good job of filtering the air because they will not stop particles smaller than 10 microns.

MERV rating 5 to 8

Merv 5 to 8 rated filters are a better choice and are more commonly found in commercial applications. These filters will collect particles as small as 3 microns and are a good choice for home owners also.

MERV rating 9 to 12

Filters with a MERV rating of 9 to 12 are used in commercial and industrial applications and will stop particles in the 1 to 3 micron range. These filters are a great choice for home owners who want the best dust control possible. When using filters with Merv 9 ratings and above it is important to clean or replace them when recommended by the manufacturer because they will have a negative effect on air flow when they become dirty. This can lead to performance problems and decreased operating efficiency.

MERV rating 13 to 16

The most efficient filters have MERV ratings of 13 to 16 and will stop particles as small as .3 microns. These filters are used in hospitals and other super clean environments.

MERV Filter Efficiency Guide

Efficiency MERV Rating Contaminant Filtered
to 20% 1 - 4 Pollen, Spanish Moss, Dust Mites, Sanding & Spray Paint Dust, Textile & Carpet Fibers
to 20% 5 Pudding Mix, Snuff, Powdered Milk
20-30% 6 Dusting Aids, Cement Dust
25-30% 6-7 Hair Spray, Fabric Protector
40-50% 8 Mold Spores
50-60% 9-10 Welding Fumes, Nebulizer Drops, Coal Dust, Auto Emissions
60-70% 10-11 Lead Dust, Milled Flour
80-90% 12-14 Legionella, Humidifier Dust, Smoke, Copier Toner, Rigid Cell Filters, Face Powder, Paint Pigments, Insecticide Dust
90-95% 14-15 Sneeze, Cooking Oil
95% 16 Bacteria, Tobacco Smoke

Your filters MERV Rating is a good way to help judge the effectiveness of the filter. MERV stands for Minimum Efficiency Reporting Value which was developed by the American Society of Heating, Refrigeration and Air Conditioner Engineers - ASHRAE. MERV values vary from 1 to 16. The higher the MERV value is the value the more efficient the filter will be in trapping airborne particles. Another consideration is air flow through the your HVAC system. Leaving a dirty air filter in place or using a filter that is too restrictive may result in low air flow and possibly cause the system to malfunction.

ASHRAE recommends MERV 6 or higher
US Department of Energy recommends MERV 13
LEED recommends MERV 8 at a minimum.

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