Minimum Efficiency Report Value (MERV) is a rating system used to measure the ability of air filters to capture particles larger than 0.3 to 10 microns (µm). This value is useful for comparing the performance of different filters and determining the size of air particles they can capture. In short, a MERV rating lets you know what size of air particles an air filter will catch. A higher rating means that the filter can trap smaller air particles.
A filter with a MERV rating of around 10 to 12 is sufficient to remove most of the particles that cause allergies from the air in your home. The filters are rectangular in shape, so they will have a length, width, and thickness (usually 1 to 2 inches for residential HVAC systems). The MERV classification system is designed to determine the filter's ability to remove particles from the air. The ASHRAE 52.2 standard describes methods and procedures for evaluating the performance of air filters based on particle size.
In simple terms, it sets the minimum amount of particles that a filter must capture to obtain a specific MERV rating. For example, if an air filter can capture at least 20% of E1 particles, 65% of E2 and 85% of E3 particles, you'll get a MERV 11 rating. It should also be noted that the filters at the lower end of the MERV scale are not even tested for their efficiency in capturing E1 and E2 particles. That doesn't mean they don't capture any E1 or E2 particles, it just means that they're not proven to capture them at a specific level.
Particles E1, E2 and E3 are particle range sizes used to test air filters to determine a MERV rating. E2 particles range in size from 1.0 to 3.0 microns. Typical E2 particles include pet dander and PM2.5 are particles that are 2.5 microns or less in diameter, so larger PM2.5 particles are considered E2 particles. Only filters with a MERV rating of 8 or higher are tested on particles in this size range. It is recommended that you check your air filters once a month to see if they are dirty enough to change them.
Most people will need to change their filters at least every 2 months. If you have pets in your house, you'll probably need to change your air filter more often. Basically, the higher the MERV rating, the greater the air filtration capabilities of a particular filter. Unlike other scales, which emphasized the need to increase the amount of particles that pass through an air filter, the MERV was designed to offer a balance between particle filtering and energy efficiency. ISO-Aire developer Kevin Albers describes this difference between a MERV filter and a HEPA filter in this short video. However, you should check the air filter every month to see its condition and if it needs to be changed.
Once you understand how the MERV classification chart works, you can choose the air filters with the MERV ratings that are right for you. Getting the right oven filter is about matching indoor air quality to the capabilities of your HVAC system. Changing the air filter on your HVAC system is almost the easiest maintenance you can do as a homeowner. There is no difference between an oven and an air conditioning filter, and the terms are often used interchangeably.