What's a MERV Rating?

It's time for a new air filter for your HVAC system — it should be an easy purchase, right?

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You may actually find that it isn't so easy when you head to the air filter aisle and see all the options. What size do you need? What is a MERV rating? Which one is best? It can be overwhelming, but starting with the MERV rating can help you master your selection.

Your house naturally has dust, allergens and other small particles floating around in the air. The air filter that goes into your HVAC system helps trap those particles. But not all air filters are the same, which is where the MERV rating comes into play.

But what does MERV mean? MERV stands for “minimum efficiency reporting value,” and it's a description of the size of air particles the air filter can trap effectively. If you see a higher MERV rating, it means the filter can trap smaller particles. In other words, filters with higher ratings pull more particles out of the air.

MERV ratings run from 1 to 16, with HEPA (high-efficiency particulate air) filters falling beyond the MERV rating maximum. Particle sizes measured in MERV ratings run from 0.3 to 10 microns. Each level can handle different-sized particles at different efficiency rates.

For example, at a MERV rating of 5, a filter can trap particles from 3 to 10 microns with 20% or higher efficiency. If you jump up to a MERV rating of 10, the filter traps particles from 1 to 10 microns, so it pulls out smaller particles than the MERV 5 filter. The efficiency rate for particles of 1 to 3 microns is 50% or higher, while the efficiency for particles of 3 to 10 microns is 80% or higher.

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Is a Higher MERV Rating Always Better?

Since higher MERV ratings trap more particles, it would seem logical to always go with the filter that has the highest rating. However, that isn't always the best option for your system. Those higher MERV numbers can have some side effects you might not think about.

Since those filters trap more air particles, it can reduce airflow through the system. The filter material is more tightly woven as the MERV rating increases so it can trap smaller particles. The increased resistance can make your HVAC system work harder, which can cause strain and wear that damages the system. It can also make your HVAC system less efficient and actually hurt your indoor air quality, instead of improving it. Plus, you might have to change or clean the filter more often since more particles get stuck in it.

Your home comfort system should have a recommendation for the MERV rating on filters. Check the manufacturer's recommendations to help you choose the correct air filter. Avoid using filters that are higher than the recommended rating.

A filter with a MERV rating between 5 and 8 can cut down on come allergens and remove dust from the air. This can improve your indoor air quality a bit without lowering airflow through the system too much. If someone in your family has severe allergies, or you want greater protection against smaller particles that could make your family sick, going with a MERV rating of 9 or higher can help. Verify that the higher rating works with your system.