Furnace Filter Basics: Understanding the Various Ratings

Published on: January 3, 2014

Your furnace’s air filter protects the equipment from dust and dirt infiltration. Air filters also offer your home’s occupants cleaner air. Selecting the right furnace filter and changing it on schedule can make a huge difference in your home comfort and health, as well as energy use.

The MERV Rating System for Air Filters

In 1987, the American Society of Heating, Refrigerating and Air-Conditioning Engineers developed the MERV rating system for air filters. MERV stands for Minimum Efficiency Reporting Value. It measures a filter’s ability to capture particulates ranging from 0.3 to 10 micrometers in size. The MERV scale runs from 1 to 20, though air filters rated above MERV 13 are rare in standard residential HVAC systems. This rating system offers a way to measure the effectiveness of air filters across various manufacturers.

Here is a summary of the MERV rating scale and the kinds of particulates each level captures. You can assume that with each higher rating, the filter will capture everything that the lower-rated air filters can remove.

  • MERV 1 to 4 filters handle particulates over 10 micrometers, including pollen, dust mites, carpet fibers and cockroach debris. These cheap, flat-panel filters are designed mainly to protect system components from dirt and debris, rather than improve indoor air quality.
  • MERV 5 to 8 filters handle particulates over 3.0 micrometers, including mold spores, pet dander, dust mite debris and home dusting liquids. These filters are good for residential and some commercial applications. At this MERV level, you’re often talking about pleated filters.
  • MERV 8 to 12 filters handle particulates over 1.0 micrometers, including legionella, lead dust, auto emissions, and humidifier dust. Filters at this level are good for superior residential and some commercial applications.
  • MERV 13 to 16 filters handle particulates above 0.3 micrometers, including bacteria, cooking oil, sneeze droplets, and most smoke and insecticides. These filters are approved for hospital use. For residential use, since they likely will impede system airflow, serious system modifications may be required to install them in your HVAC system.
  • MERV 17 to 20 filters handle some particulates below 0.3 micrometers in size. This includes viruses, smoke, carbon dust, and salt. These filters – also called HEPA – (high efficiency particulate air) – are used in manufacturer clean rooms for electronics and pharmaceuticals. They can be used in your home, but as with the other high-end MERV filters, they’ll require system modifications to prevent airflow restrictions. They also come in stand-alone air purification systems or air cleaners.

Are There Other Filter Rating Systems?

Other filtration ratings systems pop up occasionally. Many filter manufacturers and retailers have tried to replace the MERV ratings with their own versions. They change all their filters over to the new rating system and let their customers try to make sense of it all. All it does is make it more difficult for property owners to figure out what furnace filters to use. If possible, stick with filter manufacturers that use the MERV rating system.

Which MERV rating Is Best for Your Home’s Filters?

As a rule of thumb, most household furnaces and air conditioners can handle filters with a MERV rating of 8-12. This captures a good deal of household pollutants, but does not strain the average domestic HVAC system.

For those who have allergies, selecting a filter with a MERV rating of 9 or 10 may help a bit more. But, going too high will be counterproductive since it puts too much strain on the air handler.

What Else Do You Need to Know About Furnace Filters?

You can save money on filters by buying them in bulk. A single filter can be three of four times more expensive when bought individually in the store. Store the filters near the ductwork where you need to make the change.

Set a time each month to check the furnace filter. You can choose a specific date (like the 1st or 15th) or a certain day (like the first or last Saturday of the month). Many filters have a rating for 90 days or more. However, when the system is working full-time to cool the house in summer and heat it in winter, it may need changing more often. If the filter is clogged, change it.

For further information on furnace filters, contact us to find a contractor in your area. We work with contractors throughout the Southeast U.S.