MERV's The Word: Explaining Air Filter Performance

Most people don't give much thought to their air conditioner's filter as they replace it. After all, a filter is just a filter, right? As it turns out, there's more than meets the eye when it comes to your air filter and how it relates to your A/C system's overall performance and energy efficiency.

What MERV Has to Do with Your A/C

When it comes to rating and comparing air filters, industry experts rely on the Minimum Efficiency Reporting Value system or "MERV" for short. MERV provides a relatively effective way for consumers and HVAC professionals to quickly and safely choose the best air filter for their A/C system.

MERV ratings are largely based on an air filter's ability to effectively trap a broad range of airborne pollutants, including pollen, household dust, pet dander and stray carpet fibers. This ability usually relies on the size of the airborne particles trapped in a typical air filter. These particles range from 0.3 microns to 10 microns in size.

The MERV rating scale goes from 1 to 16 for normal air filters and 17 to 20 for HEPA filters. At higher MERV ratings, air filters become capable of collecting airborne pollutants in ever-decreasing sizes. Some filters can even block the spread of airborne viruses, certain bacteria and other incredibly microscopic pollutants.

By the Numbers

  • Ordinary fiberglass air filters normally achieve MERV ratings between 1 and 4. That's good enough for keeping most forms of dust and debris out of the A/C system, but it doesn't do much for improving your home's indoor air quality.
  • At MERV 5 to 8, you start seeing cartridge-style and pleated air filters that effectively trap mold spores, pet dander and other airborne particles, some as small as 3 microns in size.
  • Air filters rated at MERV 9 to 12 offer superior filtration performance by blocking airborne particles as small as a single micron in size. In addition to the usual household dust and debris, these filters can block out Legionella droplets as well as particles from vehicle exhaust, hair spray and other volatile organic compounds (VOCs).
  • Filters rated at MERV 13 to 16 are normally used in medical, commercial and industrial applications and for good reason. These filters are capable of capturing up to 99.99 percent of pollutants, some as small as 0.3 microns in size.

At MERV 17 and above, you'll start running into HEPA filters – specialized air filters that often require a modified fan blower system and air filter enclosure. For most household applications, a HEPA filter is likely to be overkill – according to the U.S. Environmental Protection Agency, air filters rated between MERV 7 and 13 are just as effective at filtration as a HEPA filter under most circumstances and at a lower cost.  

Why Higher Isn't Better

Contrary to popular belief, there comes a point when high MERV ratings offer diminishing returns. This is largely due to the fact that as MERV ratings increase, air filters restrict greater amounts of air flow. For instance, a filter rated at MERV 13 offers better filtration than a MERV 8 filter, but at the expense of reduced air flow.

This is one reason why it's not a good idea to use HEPA filters in place of standard air filters. Not only must the air conditioner be properly upgraded to provide adequate airflow through the filtration media, but such efforts offer less than favorable returns under ordinary household conditions.

Checking and replacing your air conditioner's filter on a monthly basis is one of the most important steps you can take to ensure your system operates at peak efficiency and performance. Just as equally important is making sure your air filter is capable of removing dust and other airborne pollutants effectively and efficiently. For more information on air conditioning maintenacne and repairs, you can pop over to this site.