Whether you live in a house or an apartment, you're likely aware of the filter you need to replace once or twice a year. But did you know that not all filters are created equal? Besides getting the right physical size to fit in your HVAC system, you have a choice of different filter ratings.
Two standard ratings are MERV and HEPA, and while they both remove impurities from the air in your home, they are pretty different. To learn the difference between these two filter ratings, how they work, and how to choose the best one for your home, continue reading this detailed guide!
What is a MERV 13 Filter?
MERV stands for Minimum Efficiency Reporting Value and is a test method ASHRAE (the American Society of Heating, Refrigeration, and Air Conditioner Engineers) created.
Higher MERV ratings indicate finer filtration and the capability of trapping more and smaller particulates, ranging from 0.3 to 1.0 microns (µm). The MERV ratings for air filters range from 1 to 16.
Info: A MERV rating of 13 means the filter will trap no more than 75% of air particles in the 0.3 to 1.0-micron size. For reference, the average virus is 0.1 microns in diameter.
Since MERV 13 filters are so fine, they can only let so much air through. In many cases, MERV 13 filters put too much stress and load on the fan of the HVAC system. This will lead to low airflow and cause faster wear and tear on your system, leading to equipment failure.
As such, many HVAC units are limited to MERV 8 and MERV 9 filters, predominantly residential units.
What is a HEPA Filter?
A HEPA filter is a pleated mechanical air filter that removes at least 99.97% of dust, pollen, bacteria, mold, viruses, etc., with 0.3 microns or larger.
Note: HEPA filters perform even better with larger and smaller particle sizes- 99.97% of 0.3-micron particles is just a "worst-case" scenario. You might be wondering why particles smaller than 0.3 microns are easy to capture; we'll explain why later.
HEPA stands for "High-Efficiency Particulate Air" filter as defined by the United States Department of Energy (U.S. DoE).
A HEPA filter is the best air filtration solution and greatly exceeds what a MERV 13 filter can do. However, HEPA filters have a significant drawback, the same limitation as MERV filters - the amount of air that can pass through it.
Since HEPA filters are too fine, it is almost impossible for most standard HVAC systems to circulate adequate airflow through them.
As such, the best way to add HEPA systems to an existing building or home is with portable HEPA air filtration units. Check out our detailed reviews of the h0meLabs HEPA air filter and the highly efficient Enviroklenz mobile UV air purifier for our top recommendations.
What MERV rating is a HEPA filter? By default, all HEPA filters meet a MERV 17 rating or higher.
How Filters Work
Filters work precisely how you might expect them to - they physically block particles from passing through them. In broad strokes, this is correct, but in actuality, the mechanisms by which filters trap particles are much more complex.
All HVAC filters are made from multiple layers of crisscrossed fibers that go in many directions. There are thousands to millions of these tiny fibers in a single air filter.
Air is small enough to pass between these fibers, but most particles cannot. The filter fibers block, snatch, and attract particles through four main filter mechanisms:
1. Diffusion
Particles that follow the air stream through the filter hit the filter fibers randomly. These special particles primarily include exhaust particles and other VOCs (volatile organic compounds) and follow the Brownian motion of small particles.
2. Interception
Like an interception in football, interceptions of particles occur when a nearby filter fiber grabs a particle as it is floating on by. An interception is more of a side-swipe than a direct inertial impact.
3. Inertial Impaction
As particles traveling with the air pass through the filter, they smash face-first into a filter fiber. After this head-on collision, the particle gets stuck to the fiber and doesn't make it out of the other end.
4. Electrostatic Attraction
If you've ever enjoyed the wonders of static electricity, electrostatic attraction should be easy for you to understand. Just like your hair will stick to a balloon if you rub it on your head, particles will stick to filter fibers because of their differences in charges.
How MERV 13 and HEPA Filters Differ
The difference between HEPA and MERV filters is the size of particles they can capture. HEPA filters capture 99.7% of particles 0.3 microns in size. On the other hand, MERV 13 filters capture no more than 75% of 0.3-micron particles.
However, most residential HVAC systems can only handle a MERV 13 filter, MERV 14 through 20, and HEPA filters have too much resistance. As such, you should only use filters that your HVAC is rated for- check your manual or a tag on your air handler for recommended filter type.
Why You Shouldn't Use the Wrong Filter
If you use filters that have too much resistance, they restrict airflow and will prevent your HVAC system from heating and cooling your home.
You'll end up with higher energy bills because your HVAC unit will have to run longer, and you'll be uncomfortable. Plus, it will eventually lead to mechanical problems, like a burnt-out blower motor, frozen coils, or a blown compressor- all of which are expensive to repair.
Particle Sizes
Most HVAC filters have ratings of how well they capture 0.3 micron-sized particles. Why this particular size and not smaller sizes? After all, HEPA filters can capture particles as small as 0.01 microns.
0.3 microns is used in filter ratings because they are actually the most difficult to capture! Yes, it is true - even compared to smaller particles. Particulates with the 0.3-micron size are the "Most Penetrating Particle Size" or MPP. They're essentially the MVP when it comes to filtering.
Experts, scientists, the EPA, and the entire HVAC industry have concluded (through extensive testing) that 0.3-micron particles are the hardest to trap in a filter.
One reason smaller particles are easier to capture has to do with electrostatic attraction.
Do HEPA and MERV 13 Capture Viruses?
Yes, HEPA and MERV 13 filters can capture airborne viruses. However, HEPA is more effective.
With the modern dangers of the pandemic, the filtering of viruses is a cause for concern for many homeowners. Virus particles are about 0.1 microns in size, so how effective are HEPA and MERV-13 filters against them?
Well, now that we know 0.3-micron particles are the hardest to capture, a 0.1-micron virus should be a breeze to filter.
Note: Remember, HEPA filters can catch 99.7% of 0.3 microns particles, which means they can catch an even higher percentage of 0.1-micron particles like many common viruses!
On the other hand, MERV 13 filters can nab more than 75% of airborne viruses since they can trap no more than 75% of 0.3-micron particles.
Adding HVAC UV lights will eradicate most pathogens that might slip past your filter.
How to Choose One for Your Home
Choosing the right filter for your home isn't complicated. You can only use a HEPA filter in your HVAC unit if it is designed to handle it. Almost all residential HVAC systems cannot handle HEPA filters.
If you'd like to add HEPA filtration to your home, you can use a portable HEPA air purifier to cleanse your air further.
Otherwise, you can use the highest MERV filter rating your central HVAC is rated for, which is typically MERV 9. While MERV 9 is lower, it still vastly improves indoor air quality (IAQ).
With that said, some residential HVAC systems can handle MERV 10-13 without a problem. It all depends on your specific models. You can determine which filter your HVAC system can handle from the manufacturer's instructional manual or a label on the HVAC unit.
