As much as I’ve written about filters and filtration in this blog, I have to confess there’s one aspect of filter efficiency I’ve resisted learning about. Every article I’ve written that mentions filter efficiency talks about only one rating scale: Minimum Efficiency Reporting Value (MERV). A couple of other scales are in common use, too, but I just didn’t want to know anything about them because I buy and recommend MERV rated filters. But I took the plunge recently, and here’s what you ought to know about the three main filter rating systems.
Minimum Efficiency Reporting Value – MERV
The standard rating system for filters is called Minimum Efficiency Reporting Value, or MERV. When I say it’s the “standard” rating system, I mean it. This rating system is defined in an ASHRAE standard (52.2), which specifies the performance required to meet each MERV level and the testing required. Higher-MERV filters remove more particles from your air. They also remove smaller particles. A standard fiberglass one-inch filter is typically a MERV 2. The high end of the scale is MERV-16. (It used to be 20, but ASHRAE dropped it to 16 years ago.)
Before we go further, we need to define the three general size ranges of particulate matter (PM).
- Range 1: 3.0 to 10.0 microns (includes PM10)
- Range 2: 1.0 to 3.0 microns (includes PM2.5)
- Range 3: 0.3 to 1.0 micron
The words making up the MERV acronym are a bit unwieldy – What did you expect from engineers? – but the “minimum efficiency” part is the most important. For a filter to get the MERV-13 rating, for example, it must filter out a minimum of 90% of the Range 1 particles (PM10), a minimum of 85% of the Range 2 particles (PM2.5), and a minimum of 50% of the Range 3 particles. Below is a chart from John Semmelhack showing the percentage of particles removed by size range of particle and the MERV rating of the filter.
As you can see, you’ve got to step up your filter game to at least MERV-10 to get even half of the PM2.5. But you really should think of MERV-13 as the minimum because it removes more than 85% of those particular little bitty invisible pieces of stuff. For the COVID-19 pandemic of 2020, ASHRAE has started recommending at least MERV-14.
Filter Performance Rating – FPR
The Home Depot sells a lot of filters. Rather than use MERV ratings, though, they decided to create their own rating system. The details of their testing are a bit murky, as their website explains it in broad strokes only. They look at how well a filter catches both large and small particles and rank the filters on a 1 to 10 scale. From that, they reduce the scale to four numbers (4, 7, 9, or 10), based on a weighted average calculated according to these percentages:
Sixty percent of the rating is determined by the filter’s ability to capture large particles, thirty percent is determined by its ability to capture small particles and weight gain over the filter’s lifetime determines the final ten percent.
Here are the four FPR ratings you’ll find on filters at Home Depot:
- 4 Good
- 7 Better
- 9 Best
- 10 Premium
They must be using a different definition for the word “best” than the one I’m familiar with because their premium is better than best. If you’re wondering how FPR compares to MERV, the Home Depot website has a section on that. They start by saying, “FPR 10 is similar in strength to MERV 20, since they are both the highest rating value.” They’re a bit out of date, however, because the MERV rating goes up to a maximum of 16 now, not 20. Some filters on their website have both an FPR and a MERV rating, and at least one with an FPR of 10 is shown as equivalent to either MERV-8 or MERV-13. (See screenshot below.) Clearly, this isn’t clear. Is an FPR-10 the same as MERV-16 or MERV-13 or MERV-8?
They don’t provide easily available information on the sizes of what they call large and small particles, so it’s hard to know what you’re getting with a particular FPR rating. When you combine that with the difficulty of comparing FPR to MERV, it seems the best you can do with this scale is to use it as a rough guide. And if you want to filter out as much of the PM2.5 as you can, stick with the FPR-10 or find one with that also has a MERV rating and go with MERV-16.
Microparticle Performance Rating – MPR
As with Home Depot’s Filtration Performance Rating, the Microparticle Performance Rating was developed by a company with a financial interest in the sale of filters. In this case, it’s 3M, the maker of the Filtrete line of filters. Unlike the FPR, though, the MPR measures the effectiveness of a filter’s capturing only the smallest particles on the MERV scale: 0.3 to 1.0 microns. Their website says, “Your filter’s MPR (Microparticle Performance Rating) indicates its ability to capture tiny particles between 0.3 and 1 micron in size.”
Chances are good, though, that if a filter captures a lot of the small particles, it will capture even more of the larger particles. The scale just won’t tell you anything about how well it does with particles larger than 1.0 micron. And we should note that 2.5 microns, the upper limit of PM2.5, is in the group of larger particles not covered by MPR. Apparently they do measure the larger particles, however, because they have a page comparing MPR to MERV, and there you can download a table of capture efficiency data showing the three ranges of particle sizes used in the MERV scale.
The MPR system, like the other two, uses a numerical scale on which higher numbers indicate better filtration. The MPR scale, however, uses much larger numbers. Their basic filter is rated at 100 and their most efficient filter is rated at 2800. The MPR-2800 would qualify as a MERV-14, so even though 3M emphasizes the smallest size range of particles, their best filter doesn’t qualify for the highest MERV rating.
My recommendation is to buy filters with a MERV rating whenever possible. All three ratings have their problems but even Home Depot and 3M admit on their websites that MERV is the industry standard for rating filters. Also, if you want the filter to improve your indoor air quality and not just protect the heating and air conditioning system, go with MERV-13, FPR-10, MPR-1900, or higher. But make sure you can do that without reducing air flow and possibly damaging your system.
Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.
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