MERV vs HEPA – Which Filter Is Best For Your HVAC System?
Do you find the MERV vs HEPA question a little bit confusing? If you’ve ever gone looking for a filter called MERV, probably so.
Or maybe you know nothing about either, and you’re just starting your research because you’re now a homeowner and realize you need to start buying filters for your furnace. Either way, you’re in the right place.
This article will explain exactly what MERV and HEPA are and help you decide which one might be better for your HVAC system.
MERV—Minimum Efficiency Reporting Value—is a rating system that addresses the efficiency of air filters by measuring the size of the particles that can pass through them. HEPA—High-Efficiency Particulate Air—are filters that aren’t rated since they’re manufactured to meet an efficiency of 99.97%.
Now that you understand the difference, read on so you can find out which is better for you.
MERV vs HEPA: Which One Should You Choose?
Okay, I know I said that MERV is merely a rating system, not an actual filter. So why am I comparing the two, you ask? Because since you do want a filter that has a good MERV rating listed on it or a HEPA filter, the comparison really isn’t that far out in left field.
Since explaining MERV is pretty complicated, let’s get through understanding that one first.
What is MERV?
I mentioned this above, but for the sake of having things in sensible order here, I’ll mention it again.
MERV, which is an acronym for Minimum Efficiency Reporting Value, is a system that is used to evaluate and rate the efficiency of air filters. The job of your furnace filter is to stop and catch particles that are floating around in the air of your home—some of them harmful. MERV rates filters on how well they do at catching particles of different sizes. Filters that have a higher MERV will catch the smallest particles.
Now, you might be tempted to stop reading right there to run out and buy yourself the filter with the highest possible MERV value you can find. Because that would be a good thing, right?
Depending on your HVAC system a high value MERV filter has the potential to kill your furnace. Nobody wants to destroy the heat exchanger on their furnace—which at the very least will cost you a fortune, and at the worst will kill you due to carbon monoxide poisoning—because of a $20 filter.
Before you buy a high MERV filter, make sure your system was built to handle the reduced airflow.
How Does the MERV Rating System Work?
Okay, this is all a little mind-bending and there are lots of numbers and data, so take a deep breath.
MERV rates and reports the ability of a filter to capture particles sized between 0.3 and 10 microns (µm). Essentially, it will let you know how effective an air filter is at its job.
The rating system is between 1 and 20. Filters rated at 1 are only capable of the lowest levels of filtration, while at the other end of the scale, you guessed it, filters rated at 20 will catch the tiniest particles.
You won’t find filters with a MERV of over 16 in your local Home Depot, though. They are typically only found in hospitals, schools, and even nuclear power plants.
For homeowners, the recommendation is a MERV between 5 and 13. And as I pointed out above, don’t buy a 13 without knowing the amount of airflow your duct work needs. And in my opinion, you’re pretty much wasting your money on anything under a MERV of 5.
And unless you live alone and don’t have any pets—or visitors—I’d strongly advise against buying cheap filters that have no kind of rating at all.
Particles measuring 0.3 to 10 microns (µm) in diameter are created in a lab to use in testing. And because no one I know knows the size of a micron, I’ll provide you with a visual reference.
Reach up and pull out one of your hairs—just kidding—and have a look at the diameter. The average strand of hair is about 50 microns in diameter, meaning your eye won’t even be able to detect 0.3. In fact, the smallest you can see is about 40 microns in diameter, so you can’t see the larger 10 micron particles either.
The lab divides these particles into three size categories—E1, E2, and E3—and then divides them again into four sub-categories. So that it looks like this:
|4 particles in the 0.3 to 1.0 µm range||4 particles in the 1.0 to 3.0 µm range||4 particles in the 3.0 to 10.0 µm range|
From there, each filter goes through a total of 72 tests per particle to determine the MERV rating.
Once that is done, they have the data to create a chart something like this.
Before getting into which MERV filters might be the best for you, I’m going to go off on a bit of a lateral tangent.
I’ve mentioned a few times that filters with higher MERV ratings can be bad for your HVAC system, but I haven’t really explained why. I’m going to explain why now.
The Dangers of Pressure Drop
Let’s start with an analogy that many of us now unfortunately have firsthand knowledge of. Have you worn a mask at all since the beginning of COVID-19?
What happens when you inhale and exhale? That mask, which is the same principle as your HVAC filter, restricts your air. And the more layers the mask has—the better it does at stopping tiny particles—the more your air is restricted.
That’s not fun, and it makes us work harder to pull in the air we need to live.
Pressure drop is the same idea.
It’s basically resistance to airflow. Too much restriction is bad for humans and bad for HVAC systems. In more mathematical terms, a pressure drop is equal to the amount that your filter slows down airflow between your vents and your furnace.
Just like the type of mask you wear will impact airflow and pressure drop, so will the type of filter you use. If it’s tightly woven and will withstand those 0.3 microns, you could be facing a dangerous pressure drop. Assuming your ductwork wasn’t designed to handle it.
And I’m sorry, but I don’t know if anyone but a professional can assess your system and let you know what airflow it’s been designed to take.
Okay. I told you MERV was complicated! Now let’s move on to HEPA filters.
What are HEPA Filters?
HEPA stands for High-Efficiency Particulate Air.
HEPA, or High Efficiency Particulate Air filters, are designed for applications where contaminants must be trapped on the first go-round.
These types are filters are found in places like hospitals or labs where airborne contaminants need to be captured in their first cycle through the facility’s ventilation system.
If I were writing this more than a year ago, I would be mentioning something scary like MRSA that can be spread via droplets. We have something a lot scarier to deal with now, and all of us are more familiar than we want to be with airborne bacteria or viruses.
HEPA filters are created using a collection of dense fibers in a specific configuration. Particles either hit the filter straight on and get caught or they are trapped in the curved contours of the fibers.
Do HEPA filters have MERV ratings?
HEPA filters are not MERV rated. Not because they don’t qualify for MERV testing and rating, but because the manufacturing process of HEPA filters already guarantees a rating higher than MERV.
The lab testing done to rate filters for MERV mentioned above is the ASHRAE (The American Society of Heating, Refrigerating, and Air-Conditioning Engineers) test protocol 52.2.
HEPA air filters are the ONLY mechanical air filters that are tested and certified to meet a specific efficiency at a specific particle size. All HEPA air filters must meet a minimum efficiency of 99.97% at 0.3 microns. ASHRAE or MERV air filters are tested using the Dust Spot tests that incorporate some fine dust, powdered carbon, and some cotton linters. The Dust Spot test particle size range is from 0.3 microns to 50 microns in size with an average size of approximately 20 microns.
So what does that mean?
Are HEPA Filters Better Than MERV Filters?
Let’s do a MERV 16 vs HEPA comparison here.
If you were to use the Dust Spot test that is used on HEPA filters on a MERV 16 filter that uses the ASHRAE test protocol, the MERV 16 filter would only sit at about 50% efficiency.
Meaning that HEPA filters are at least 50% better at doing their jobs when it comes to removing respirable sized airborne particles. And that’s compared with a MERV 16 rated filter which is 95% or better and trapping particles sized 0.0 to 1.0 microns.
- A MERV 16 filter is rated at 95% or better at 0.3 microns
- A HEPA filter is guaranteed to have a minimum efficiency of 99.97% at 0.3 microns
But again, a word of caution. This doesn’t mean you should run out and buy a HEPA filter for your furnace because they are better.
In an apples to apples testing ground where all variables and environments are equal, yes. There is no question that HEPA is better than MERV.
But I’m guessing your home isn’t in a sealed lab the likes of a MERV or HEPA testing site.
Remember the section above on the dangers of pressure drop? The same applies to HEPA filters.
Which filter is best against COVID-19?
I mentioned above that a year ago we were still pretty innocent and naïve about our fears of airborne bacteria and viruses. Not to say what we had before wasn’t bad, but we’ve reached a whole new level of scary.
In my city, HVAC companies are getting worked off their feet trying to keep up with ventilation upgrades in commercial buildings, schools, and so forth. The goal is to get more fresh air in while improving central air filtration. The CDC has an entire page of upgrade recommendations.
But what about your home?
Remember, the smallest airborne particle the best MERV 16 filter catches is 0.3 microns. If you don’t remember, take a look back at that nice colorful chart above.
And the EPA says that a HEPA filter is guaranteed to capture particles at least that size.
Yet since the COVID virus is only about 0.1 um in diameter, there is no filter guaranteed to catch that.
Could a HEPA filter theoretically catch the COVID-19? Yes. Because it could, maybe, and probably has caught 0.1 micron particles in testing. But the flip side would be it’s also let some through. Therefore, and this is a biggie, there is no guarantee it will offer you 100% protection from spreading the virus in your home.
I’d say a filter may be false hope, but certainly, something is still better than nothing.
So, at the end of the day, which is better for your home? A MERV of HEPA filter.
If you’re read through this entire article, you know by now there is no one size fits all answer.
The first, and most important deciding factor has to be your own HVAC system, and the amount of pressure drop your airflow can handle. If you use a very tight MERV 16 or HEPA filter, you could destroy your furnace.
You’ll either need a professional to come in and test your system, or you’ll need to get your hands on some specialized testing equipment and learn how to use it. And do lots of math.
Let’s assume your system was built to handle the pressure of a top of the line filter. Meaning MERV 16 vs HEPA.
Buy a HEPA filter if:
- You want the highest possible guarantee of airborne pollutants to be caught
- The initial cost is not a factor
- Longer replacement schedule of 6 months to a year vs every 90 days for MERV filters
Buy a MERV 16 filter if:
- You’re looking for a cheaper, more accessible option
- You’re not too concerned about the few extra percentage points of protection that a HEPA could provide
- You’re looking for more options
Thanks for reading. Hopefully, I’ve answered any questions you might have had in your search for information around MERV vs HEPA.
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