What is the MERV rating system?
We usually associate air purifiers with a standard rating level of efficiency. For smaller portable air purifiers, this number is the CADR (Clean Air Delivery Rate). That, and the HEPA certification, of course. For larger purifiers or HVAC systems meant to clean an entire home or building, however, the MERV rating system applies.
MERV stands for Minimum Efficiency Reporting Value. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) created the system in 1987. The rating system starts at 1 and climbs to 16 as filters grow more efficient. Occasionally, the ASHRAE will extend the rating to 20 to increase flexibility.
Related: You Need a HEPA Filter – Here’s Why
How is a MERV rating determined?
MERV focuses on particles from 0.3 to 10 micrometers. Companies use a series of tests to conclude what a purifier’s final rating will be. Testers introduce particles of different sizes into a test area. Each particle ranges in size within 12 different categories. The smallest particles are 0.3 to 0.4 micrometers and the largest 7 to 10 micrometers. A fan pushes air through the filter and testers measure the density of the particles both before and after the test. This process is repeated six times and the worst result is taken for the MERV rating.
How does the MERV rating help you?
When looking for an air purifying system, the MERV rating will help you choose what is best for your home or building. This number is not the only thing to consider when selecting an air purifier. However, it provides a good guideline to determine efficiency based on the size of your space and quality of your air. Common particles include things like pollen, dust mites, carpet fibers, mold spores, dust, pet dander, and tobacco smoke. The MERV rating gives an indication of how effectively an air purifying system can rid your environment of these pollutants.
Which MERV rating is best for you?
Many people believe that the higher the MERV rating, the better it is. The United States Environmental Protection Agency suggests a rating between 5 and 13 for your home. However, it primarily depends on the type of HVAC system you have. The EPA also suggests that filters with ratings between 7 and 13 will perform similarly to HEPA (High-Efficiency Particulate Air) filters. Which we highly recommend in plug in purifiers and hospital applications. The National Air Filtration Association recommends that residential systems have MERV ratings between 9 and 12. Higher ratings are better for larger buildings such as hospitals and offices.
The reason why higher ratings are not necessarily better is that HVAC systems have smaller pores when they have higher ratings. Smaller pores can cause resistance and restricted airflow which ultimately results in diminished air quality. Ideally, you want to research and discover what the highest rating you can have in your space while still maintaining substantial airflow.
Limitations to the MERV
It is important to reiterate that the MERV rating system is only applicable to large purifiers or HVAC systems. Because MERV testing is done under controlled conditions, it is not an exact simulation of what it will be like in your home or building. Installing such a large system can actually have a negative impact on the filter’s results. The “installation effect”, as it is called, may result in worse performance than expected.
In addition, the MERV rating is specific to particles but not all indoor air pollutants are particle-based. Gases, for example, are not included in the system so the filter you purchase may or may not be effective for gasses based on this rating.
Finally, you will need to swap out filters with higher MERV ratings more often. You will need to keep an eye on this to maintain a good degree of airflow. MERV filters are also a custom size, making it necessary to stick to one rather expensive standard.
The MERV rating system is a useful tool to assist in choosing a large air purifying or HVAC system. It creates a standard by which you can compare filters and discover what the best system might be for you. Once you understand the rating, it is less complex than meets the eye. However, there are some important limitations to consider when deciding on a MERV filter system. Installation, the cost of the filter, and other air pollutants all play their part.