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Everyone knows that air filters are an important part of maintaining fresh, clean indoor air, but many people are unclear about how they actually work. All the different rating systems can feel a little confusing and overwhelming. However, it is actually easy to figure out air filter ratings once you know a bit about them. This collection of frequently asked questions can help you learn a lot about how air filter ratings work.

What Are MERV Ratings?

This is the most common type of rating scale for air filters. MERV stands for “minimum efficiency report value.” It starts at 1 and can go as high as 20. The majority of filters used today tend to be somewhere between a MERV 4 and a MERV 16.

How Are MERV Filter Ratings Determined?

Filter ratings are determined with a test method that the American Society of Heating, Refrigerating, and Air Conditioning Engineers developed. This test involves sending a variety of particles sized from 0.3 to 10 microns through the filter. Then, the filter is rated based on the largest particles that manage to squeeze through. A lower rating means bigger particles get through. Meanwhile, a higher rating means that the filter catches more small particles.

Are MERV and HEPA Rated Filters the Same Thing?

Another type of rating that often gets brought up alongside MERV is HEPA. HEPA filters are an especially high-efficiency filter that can catch things as small as 0.3 microns in diameter. Most HEPA filters have a MERV rating of about 17 to 20. Some are actually so efficient that they can even exceed MERV ratings altogether.

HEPA filters are typically only needed in labs, hospitals, and other areas where air filtration is the highest priority. Typically, they filter so effectively that they will not work with the average home system. They usually need a special fan to keep air moving through the extremely tight filter.

What Are FPR and MPR Ratings?

When shopping for filters, you may also see some with an MPR and FPR rating. MPR stands for “microparticle performance rating.” It is a rating developed by 3M, a major air filter manufacturer. MPR goes from 100 to 2800, with higher numbers indicating more filtration.

FPR stands for “filter performance ratio,” and it is also a rating developed by an air filter manufacturer. FPR goes on a scale from 4 to 10, and just like other air filter ratings, a higher number means it filters out more. These ratings are useful for comparing filters within the same product line. However, if you are comparing filters made by different brands, you will need to convert to MERV.

Is the Highest Filter Rating Always the Best?

A higher filter rating means a filter catches more particles, so many people assume that higher is always better. This is not necessarily true though. A higher filter rating does more than just catch more debris. It also causes a big drop in air pressure because your system cannot pull air through the filter as easily. Different HVAC systems are designed to work with a different level of interior pressure. An excessively large pressure drop can make your system wear out quicker and use more energy.

Therefore, you need to always check your HVAC manual before picking a filter rating. A rating compatible with your system will filter air while still helping your system run as efficiently as possible.

What Particles Will Your Filter Rating Catch?

Typically, a MERV of around 8 to 12 works for the average homeowner while those with allergies or respiratory problems need one of at least 16. Here are some of the types of things that can be caught with the various MERV ratings.

  • MERV 4: Pollen, dust mites, sanding dust, and carpet fibers
  • MERV 7: Mold spores, hairspray, and cement dust
  • MERV 10: Some chemical fumes and auto emissions
  • MERV 12: Lead dust and legionella bacteria
  • MERV 14: Tobacco smoke and respiratory droplets
  • MERV 16: Most bacteria
  • MERV 20: Virus particles and smoke

Does a Filter’s Material or Shape Affect Filter Rating?

Yes, the types of materials your filter is made from will affect its rating quite a bit. Filters made with fiberglass and cardboard tend to have lower MERV ratings while filters with cotton and polyester tend to have higher MERV ratings. Many modern filters are pleated instead of flat because the pleating provides a higher surface area for catching particles. The pleats themselves do not change the MERV rating. However, since pleated filters tend to be sturdier and made out of higher quality materials, they end up with higher MERV ratings.

Do Reusable Filters Have a MERV Rating?

Also called aluminum filters, washable filters, or electrostatic filters, these filters use a charged metal surface to trap debris in the air. They might not come in a box with a big MERV rating printed on them, but they still have a rating. Depending on the filter you get, a reusable filter will usually have a rating of around 4 to 6. This is enough to catch most larger particles of dust and debris, but it may let some harmful particles slip through.

Does an Air Filter’s Rating Drop With Use?

Air filters are not actually damaged or worn out from long periods of use, so the age of a filter does not lower its MERV rating. Instead, filters’ MERV ratings actually tend to increase with age. As the filter fills up, there is less space for particles to squeeze through. This increase in the MERV rating puts more strain on your system. The material buildup on the filter also traps moisture, so it is easier for mold to grow inside your HVAC. Therefore, you still need to change your filter regularly even if age improves your air filter rating.

Does Filter Rating Affect How Often You Change the Filter?

Yes, the rating on your filter does affect how often you change it. Air filters with a higher MERV rating become loaded with debris more quickly, so they should be changed more often. If you do not change highly rated filters often, your system can become clogged.

However, many other factors, like filter size and material, alter how often a filter should be changed. Therefore, there is no hard and fast rule about how often to change the filter for various MERV ratings. Instead, you will need to check the recommended change rate for your specific filter. If your home has a lot of dust, pets, or other potential pollutants, you may need to change it more often.

Interested in learning more about air filter ratings? JW Plumbing, Heating & Air has plenty of experience helping Los Angeles homeowners improve indoor air quality. Our highly skilled, NATE-certified technicians can assist you with installing, repairing, and maintaining HVAC units. We also work on plumbing systems and water heaters. To learn more about our services, go ahead and give us a call now.