Commercially available HVAC filters are rated based on something called MERV which stands for the Minimum Efficiency Reporting Value. The MERV rating criteria often confounds homeowners. However, it’s actually a very simple formula, and a very useful one too.

The MERV rating simply answers the question: how efficient is your air filter? It describes the size of the holes that allow air to pass through the filter material. A higher rating means the filter holes are smaller which is great for efficiency. Why? Because smaller holes in the filter allow fewer dust particles and other contaminants to pass through. In addition to improving your indoor air quality, filters with higher MERV ratings can prolong the life of your system and reduce the need for frequent heating and AC repairs.

MERV ratings range from 1 to 16 with a rating of 16 being the best possible and a rating of 1 being the worst possible. A simple test is used to determine the rating of each air filter:

  1. Test particles are introduced into the air of the test area. The particles generally fall into 12 different categories, based on size. The largest particles range from 7 to 10 micrometers while the smallest particles range from .3 to .4 micrometers.
  2. The pollutant-filled is then passed through the filter being tested and the efficiency of the filter determined based on how effective it is at removing pollutants in each of the 12 size categories.
  3. This process is repeated five times for each pollutant size category and the filter assigned a MERV rating based on the worst result.

When choosing a filter for your HVAC system, it is important to choose one that can keep out as much pollutants as possible.

Keep in mind, though, that very high MERV ratings are not necessarily better when shopping for residential filters. Since these filters have extremely tiny holes, it may create resistance to air flow in the home. Reduced airflow can worsen the quality of air in the home and puts a lot of pressure on the furnace fan or the AC system.

The United States Environmental Protection Agency (EPA) has provided some useful guidelines on MERV ratings that you should consider depending on intended use. According to the agency, the best MERV ratings for residential properties are 7-13. The EPA states that filters rated 5-13 are all okay, but those rated 7-13 are more effective at controlling airborne indoor particles. Filters in this range are also generally less expensive compared to HEPA filters (currently considered the best air filters on the market) and allow higher airflow and quieter HVAC fan operation than HEPA fans. The National Air Filtration Association (NAFA) holds the same position.