Decoding the MERV ratings of Heating and Air Conditioning Filters
Monday, September 23, 2013
Changing filters is the simplest maintenance task you have for your home heating and air conditioning system to keep it running efficiently and to assure high air quality within your home.
When you go to replace a filter, however, you may still have questions. It’s not just an issue of getting the right size filter to fit your system. You’ll face a range of filters with MERV ratings. Just what is a MERV, anyway, and how high a rating is good enough?
Let’s start with the acronym first. MERV stands for Minimum Efficiency Reporting Value, developed as a measurement scale by the American Society of Heating, Refrigerating and Air-condition Engineers in 1987 to rate effectiveness of air filters.
MERV ratings range from 1-16, with the higher numbers representing increased effectiveness. Filters for homes rarely have MERV ratings above 12. The U.S. Environmental Protection Agency notes that filters with ratings 13 to 16 have a higher resistance to air flow and are typically used in hospitals were the heating and cooling systems can handle it. Filters with MERV ratings higher than 12 can be installed in homes but may require modification to the home’s heating and air conditioning system to handle the filter’s air-flow resistance.
Filters with a MERV rating of 1-4 meet the minimum standards of filtering pollen, dust mites, dust and other large particulates. These filters are made of fiberglass or synthetic media panels one-inch thick, or washable aluminum mesh or foam rubber panels. They’re generally used in residential window air conditioning units.
Filters with a MERV rating of 5-8 will filter mold spores, cat and dog dander, hair spray and dusting aids. They can be pleated filters with extended surface of cotton, polyester or both, and one to six inches thick. These also come as cartridge filters in viscous cubes or pockets and as synthetic media panel filters.
The high-end residential filters come with MERV ratings from 9 to 12. Besides what lower-rated filters work on, the high-end filters will also filter legionella, humidifier dust, lead dust, milled flour and auto emission particles. These come as pleated filters of one to six inches thick or box filters with a rigid style cartridge six to 12 inches thick.
Determining how often to change a filter depends both on the filter and the activity in the house. A home with pets, for instance, may require a more frequent filter changing. Generally, a flat filter with a low MERV rating may need to be changed monthly. Pleated filters with higher ratings should be changed every three months.