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What's a MERV Rating and How Does It Affect My HVAC System?

merv ratingThe Minimum Efficiency Reporting Value, or MERV, is a third party rating system for air filters. It provides a standard way for consumers to compare the effectiveness of various filters. The higher the MERV number, the more effective a filter is at removing contaminants.

MERV Ratings for Home Use

Most filters for home use have a MERV rating from 3 to 8. There are higher ratings than that, but just because a filter has a higher MERV rating doesn’t mean that it’s the right choice for you. There are two things to consider for your needs:

  1. If your goal is to remove pollen because of allergies, then a MERV rating of 6 or 8 will do just fine. Buying a higher-rated filter would be like paying for premium gas when all your car needs is regular—it’s just a waste of money.

  2. Higher MERV filters have more resistance to airflow. To filter out the smaller particles, the filter media has smaller openings which may slow down the airflow and create other problems. It’s a little like using a coffee filter to strain noodles. The noodles will separate just fine, but the water flow will be painfully slow.

Keith Hill, technical support manager for Minnesota Air, reminds homeowners that higher MERV filters require more horsepower to push or pull air through them. 

“If your system does not have enough oomph, you may have trouble with your heating and cooling equipment, possibly even do damage,” he says. “Your system needs a minimum amount of air to reject the heat or your furnace will overheat, and the same for cooling, or your A-coil (part of the AC that helps cool the air) will get too cold and freeze.”

Most residential furnaces are equipped with, and work well with, a 1” thick filter that is MERV-rated 3 to 5, or a 4” or 5” thick media type filter that is MERV-rated 6 to 8. If you want to try out a filter with a MERV value above either of those, we strongly recommend you contact your HVAC professional to make sure that the filter will work in your system without causing any airflow related problems.

Also keep in mind that higher-rated MERV filters will need to be replaced more often as they “plug up” more quickly.

How Filter Size and Thickness Affect Airflow

Filter length, width, and depth all make a big difference in the amount of airflow. As Keith points out, “Length and width are obvious, but thickness is a huge factor. If you took a 1” thick filter and a 4” thick filter out of their frames and stretched them out, you would see that the 4” is roughly four times as long. That means the filter media in the 4” filter has essentially 400% more available area for the air to pass through. That makes it easy for the air to go through – very low restriction to flow." And you’ll want good airflow, which is a major factor in the effectiveness and efficiency of your HVAC system.

Table of MERV Ratings

Use this table to help guide you in selecting the filter that works best for your needs.

MERV Rating & Description

Contaminants Removed

3 – 5 Basic filtration

Most dust and lint; some pollen

6 – 7 Very good filtration

Dust, lint, and pollen

8 – 9 Better filtration

Dust, lint, and pollen; some mold spores and mites

10 – 11 Best filtration

Dust, lint, and pollen; some mold spores, mites, lead dust, coal dust, and mineral dust (from a humidifier)

12 – 13 Near hospital grade filtration

Dust, lint, and pollen; most mold spores, mites, lead dust, coal dust, and mineral dust (from a humidifier); any smoke particles, insecticide dust, legionella, and some other bacteria

14 – 18 Hospital and commercial grade filtration

For Clean Rooms and industrial processes. Removes all bacteria and some viruses.

19 – 20 HEPA filtration

For Clean Rooms and industrial processes. Removes all bacteria, viruses, and smoke particles.

Higher MERV Ratings Aren't Necessarily The Best Choice

The bottom line is: you don’t want to spend money on a better filter just because the MERV rating is higher. Keith breaks it down like this:

For residential use, a filter with a MERV of 6 or 8 is more than enough for most people. 

For those with allergies to dust and smaller pollen particles, choose a MERV of 8 or 10. 

Nevertheless, if your system is already working to capacity regarding airflow, adding a more restrictive filter can create trouble, Keith says. That’s where choosing a thicker filter can help. If you read our Filters 101 Guide, you’ll see that a thicker filter allows for better airflow and efficiency, but may require some duct modifications to use it. In some cases with undersized ducts, even a MERV 6 can be problematic, so when in doubt, ask a pro before installing a high-efficiency filter.

A General Recommendation

Given all these factors, Keith’s general advice is “to stick with MERV 7 or less in a 1” filter and 12 or less in a 4” or 5” media filter."

If you’re in any doubt as to the best filters to use for you system, give your local HVAC pro a call – they’ll be able to help out. Need to contact an HVAC pro in your area? Just use our convenient dealer locator.

Additional Resources

Check out these articles for more information about filters and air quality for your home.

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