The Furnace and Air Conditioner Filters We Would Buy

Most HVAC systems use a standard 1-inch-thick filter, so we focused exclusively on 1-inch medium-efficiency MERV filters. These are installed either in wall-mounted air-return vents (most common in the South), or at the air handler (more common in the Mid-Atlantic and northern states, where the air handler is often placed in the basement, near the furnace). A correctly sized 1-inch MERV filter simply rests behind a bracket at the return register; if you’re not sure what size you need, check the narrow side of an existing filter for the dimensions (like “16 x 25 x 1”).

If your HVAC system uses a thicker filter (usually in the 4- to 5-inch range, and usually mounted at the air handler), it was likely designed specifically for medium-efficiency MERV filtration. You don’t need our advice; simply replace the existing filter with one that has the same specs.

MERV stands for Minimum Efficiency Reporting Value, and is a measure of how well a filter removes particulates from the air. In that way it’s akin to the more familiar HEPA rating used on air purifiers. And like the test for HEPA rating, MERV testing measure a filter’s efficiency—the percentage of particles it captures—in a single pass. Air is loaded with particulate matter, sent through the filter a single time, and the results measured. In real-world use, with the air in your home constantly recirculating through the ducts and passing through the filters each time, the cumulative effect of the filters rises.

But MERV is different in some key ways. First, instead of being a simple pass-fail measure like HEPA (a filter is either HEPA-rated or it’s not), MERV ratings fall on a scale from 1 to 16, with higher numbers indicating better performance.

Also unlike HEPA, MERV tests measure particulates of different sizes, from 0.3 micron (very small and difficult to filter—the size HEPA tests measure) up to 10 microns (relatively large and easy to filter). MERV also doesn’t have a target percentage for removal, the way HEPA does. Instead, the actual percent removal is recorded for each of 12 particle sizes, and these numbers are referenced against a performance table to determine the appropriate rating.

So when you look at the full spectrum of what’s floating around in the air in your house, MERV ratings reflect a filter’s performance on all of it. Higher numbers indicate a better filtration of the smaller particles, like pollen and smoke, that cause many respiratory issues. And medium-efficiency MERV filters in the 8 to 13 range—the kind we would get—can remove 90 percent or more of these. For a really thorough explanation of how MERV ratings are calculated, visit the website of the National Air Filtration Association (PDF).

One more thing: Some companies, including major filter manufacturer 3M/Filtrete and major filter retailer Home Depot, use their own rating conventions (MPR by Filtrete (PDF) and FPR by Home Depot). Both claim to be based on the American Society of Heating, Refrigeration, and Air-Conditioning Engineers’s ASHRAE 52.2 standard that’s used to set MERV ratings, but neither company provides an easily accessible explanation of how its own ratings differ from or compare with MERV. (Filtrete does print MPR and MERV equivalents on the frames of its filters, but you can’t see that until you’ve actually got one in your hands.)

After we inquired, Filtrete replied that its MPR filters capture more of the very small particles (0.3 to 1 micron) than equivalent MERV-rated filters when tested to ASHRAE 52.2 protocol, and shared a test summary that backs this up; on the larger particles (1 to 10 or more microns), MERV and MPR performance was nearly identical. Filtrete said MPR 1900 is equivalent to MERV 13, MPR 1500 to MERV 12, and MPR 1000 to MERV 11.

Home Depot shared a basic comparative chart but no test data or protocols; it equates FPR 9 and 10 to MERV 12, FPR 7 and 8 to MERV 11, and FPR 6 to MERV 10.

To decide what filter we’d buy, we needed a more pragmatic explanation of MERV, so we turned to Kathleen Owen, a research engineer specializing in air filtration. Owen almost literally wrote the book on MERV: In the 1990s she helped define the American Society of Heating, Refrigeration, and Air-Conditioning Engineers’s ASHRAE 52.2 standard that’s used to set the ratings. She later chaired the ASHRAE 52.2 committee, which regularly updates the standard to address changes in filter technology.

In a conversation about the most common filter levels for sale at retail, MERV 1 through 16, Owen said this range “gets you from filters that might catch a golf ball—I’m exaggerating a little bit—up to filters that will catch almost everything.” Owen added, “I like to tell people that each of those filters has a purpose.”

• MERV 1 to 4 (and unrated): These are the cheapest (some under $1 apiece) and most basic MERV-rated filters. Their purpose is to keep particles larger than 10 microns—including pet hair (but not dander), carpet fibers, and heavy dust—from clogging the heating/cooling coils and ducts in your HVAC system. They have little to no impact on fine particulates like pollen and smoke, so they will not do much to ease respiratory issues. But even with their limitations, these filters aren’t pointless. They prolong the life of your equipment and help keep it running efficiently in the heating and cooling seasons. “Think of how important your screen door is in summer when there’s flies out,” Owen said. If you are reading this guide, you probably care too much about air quality to consider one of these filters, but if you’re living (or vacationing) in a rental and don’t want to invest much—especially if the existing filter is problematically old or dirty—replacing it with a MERV 1 to 4 filter would be a fine quick fix.

• MERV 5 to 7: Owen said filters in this range (like this MERV 7 Nordic Pure) were created as an improvement on the basic MERV 1 to 4 filters and designed to also filter out midsize particles (3 to 10 microns, which includes most pollen and mold spores) that could encourage mold growth on heating/cooling coils. These filters are not as widely sold as the lower and higher MERV-rated filters in hardware stores, but they can be found online easily. There’s nothing wrong with these types of filters; we just didn’t focus on them as much as we did those in the 8 to 13 range mostly because this class, by comparison, usually isn’t any cheaper, doesn’t perform quite as well, and often isn’t as easy to find.

• MERV 8 to 13: This is the category we’d recommend for people who care about indoor air quality, and there are many equally good examples, such as the Nordic Pure MERV 10 and MERV 12, the Honeywell FPR 9, and the Filtrete 1500. (For more detail on these models, see the A perfectly good HVAC filter and Two other filters that are also fine sections.) We focused on filters in the MERV 8 to 13 range because they are good to excellent at removing fine particulates (as small as 0.3 to 3.0 microns in diameter, including smoke and exhaust particles and bacteria), plus anything larger. This MERV range, Owen said, “covers most of the stuff that people are likely to breathe in unless they’re really close to the source.” The EPA says filters rated MERV 7 to 13 “are likely to be nearly as effective as true HEPA filters at controlling most airborne indoor particles.” The higher end of the range, MERV 11 to 13, removes 20-plus to 50-plus percent of the very smallest, 0.3-to-1.0-micron particulates, and filters in this range are especially popular. An NIH literature review indicates that filters in this range can significantly reduce indoor fine-particulate pollution and alleviate some respiratory ailments.

• MERV 14 through 16: These are specialized filters that require a purpose-built HVAC system; they’re widely used in hospitals and some commercial and industrial environments, but generally not in homes.

So why doesn’t everyone get a MERV 8 to 13 filter? Beyond their higher cost, there is also a concern your equipment might not be able to handle them, according to engineers we interviewed from two of the major HVAC manufacturers: Jay Ayers of Trane and Doug Powell of Carrier. They (and Owen) all told us MERV filters in this range restrict airflow more than low-MERV filters, as measured by the drop in air pressure the filters create when installed. That can potentially strain your equipment.

Powell noted that, “You have a fair amount invested in your HVAC system, and unless you know how much pressure drop you have in it and how much more it can handle, you could end up shortening the life of your blower. And a blower’s not cheap.” Ayers said, “That’s the danger of looking at 1-inch filters and trying to go higher-efficiency and do the job that’s needed for allergies: You run the risk of really damaging your blower and your HVAC system.” (If you are concerned about this, to be absolutely punctilious you can hire an HVAC firm to measure pressure drop in your system.¹)

Against these warnings, however, is a mountain of owner testimony that MERV 11, 12, and even 13 filters have worked just fine in their systems for years.

We feel the manufacturers’ concerns are a bit overcautious, in part because a recent innovation in air filters permits high MERV ratings (11 to 13) with low pressure drop.² So-called electret filters use a mat of fibers that are electrostatically charged. The charge helps attract airborne particles (rather than just passively impeding them). As a result, the mat can be more porous, causing better airflow and a smaller pressure drop. However, Owen emphasized, electrets become less efficient over time, often significantly so. “Electret media will drop in efficiency upon use, sometimes by as much as 50 percent,” she said. “Basically what happens is the tiny particles—we’re talking 40, 50 nanometers, which there are gazillions of in the air—will coat the charged fibers and keep them from attracting larger particles.” That means regularly replacing electrets is absolutely key to maintaining good filtration, and most come with 60- or 90-day replacement schedules. If you run your system year-round, that’s potentially six filters a year.

Given all this, we decided that if we were buying new filters today, we’d look for an electret filter around the MERV 8 to 13 level with a low pressure drop that was both affordable and easy to find.

Nordic Pure’s MERV 12 filters are not the only products that meet all our criteria, but they are perfectly good examples, and they’re usually competitively priced and easy to find online. We like that they’re genuinely MERV-certified, because unlike the proprietary ratings used by some bigger-name brands, the MERV criteria are publicly available. Being rated MERV 12, the Nordic Pures will remove at least 35 percent of particles in the 0.3-to-1.0-micron range, which includes bacteria and smoke; at least 80 percent of 1.0-to-3.0-micron particles, which include fine dust and soot; and at least 90 percent of larger pollen, dust, pet hair, and lint particles. Again, MERV stands for Minimum Efficiency Reporting Value, so this is the worst performance measured during third-party testing to the ASHRAE 52.2 standard. And again: the MERV test measures a single pass of polluted air through the filter. In your home, the air will continuously recirculate when your HVAC system is, leading to cumulatively greater filtration with each trip through the return registers.

Nordic Pure filters are mostly sold online, and the MERV 12s typically retail for a much lower price than competitors—about $40 for a six-pack of 16-by-25-by-1-inch filters, or about $7 each (with some competitors you’ll pay about double that price for MERV 11/12 or the equivalent). At the recommended three-month replacement cycle, a Nordic Pure can save you about $30 to $40 a year.

Nordic Pure reports the filter’s pressure drop as 0.24 inch, which puts it a shade higher than the similar, MERV 12–equivalent Filtrete 1500 at 0.22 inch. But most people seem to have had no issues with the Nordic Pures straining their HVAC system—the six-pack has an overall score of 4.3 stars (out of five) across more than 2,800 reviews at Amazon at this writing (we should note that the reviews once received an A rating from Fakespot; as of May 2019 that rating was downgraded to a C). The Nordic Pure MERV 12 filters are available in many sizes, so you should be able to find one that fits your return vent opening.

If you can’t find Nordic Pure in your size, or if you prefer to buy and pick up a new filter in a store immediately, Honeywell’s FPR 9 filters and Filtrete’s MPR 1500 filters are similar, and they’re widely available at retail. These filters perform on a similar level as the Nordic Pure MERV 12, they are well-reviewed by many owners, they come in a wide range of sizes, and the high likelihood that they’re in stock at a nearby hardware store is a big advantage if you need one ASAP.

We don’t love that neither HDX or Filtrete is MERV-certified, but their proprietary ratings are roughly equivalent to MERV 11 or 12.  The Filtrete’s pressure drop is 0.22 inch, virtually identical to the Nordic Pure MERV 12’s at 0.24 inch. We couldn’t find pressure-drop information on the Honeywell, but it is likely similar. Data Filtrete shared with us indicates the the 1500 removed 54 percent of 0.3-to-1-micron particles in its own tests, versus a minimum of 35 percent for the MERV 12-certified Nordic Pure. On larger particles, the two filters performed very similarly.

A major reason we’d look for Nordic Pure as an option first comes down to price: Both the Honeywell and Filtrete options usually cost more than twice the price of the Nordic Pure filters. As electret filters, all these models will lose effectiveness as they capture particles, and should be replaced at least every three months, per the manufacturers’ recommendation. But homes with certain challenging conditions—if you smoke, frequently use candles or fireplaces, or have pets—will significantly reduce that lifespan.³

Owen also offered a simple way to judge if it’s time for a swap: “If you can get to your filter easily, one simple thing you can do is just go take a look. If you can’t see the filter media anymore, it’s been in there too long. Change it!”

During a wildfire, you’ll need to change your filters more often than you would under normal air conditions, and there are a few indicators that can help you know when it’s time to replace one.

We asked Kathleen Owen, the filtration expert we spoke with for this guide, for her advice. She replied in an email: “If I lived near the wildfires and didn’t find the cost prohibitive, I’d probably change a 3-month filter every month during the season. Or possibly after each major event— say if a fire gets close enough to be a big deal (rather than simply you know about it from the news).”

Owen also noted that wildfire smoke is a complex mixture, incorporating both physical smoke particles (which filters trap permanently) and water vapor and volatile organic compounds (gases), which are not trapped—and the latter of which create the smoky scent. “The wet particles or ones with gases adsorbed on them can be collected, then give off the smelly stuff the next time the fan comes on,” she wrote. “So, one recommendation is that if folks feel like the air breathes easier or smells OK with the HVAC on but are gagging in the first minute or two the HVAC is on again, changing the filter could help.”

There are also visual cues you can look for when judging whether to replace your furnace filter, Owen wrote. Clogging increases the pressure differential between the intake and outflow sides, and this can cause the filter’s pleats to flatten out or the frame of the filter to buckle. Both are evidence that the smoky air is finding a path around the clogged filter rather than through it, and that it’s time to install a new one.

In addition to obvious steps like keeping windows and doors closed, Owen also advised shutting off bathroom fans and range hoods, both of which vent directly to the outside and, as a result, draw an equal volume of smoky outdoor air in through window frames and other leaky spots.

Owen added that she’d also consider adding a portable air purifier with a VOC filter. All our purifier picks have them, and though in our tests against an extreme load of ethanol vapors we found most of them ineffective, anecdotally we can say that they seem to perform well under fairly heavy smoke conditions. Both in guide author Tim Heffernan’s apartment and in our New York test space, after running our top picks for a few hours, there was little olfactory evidence that hundreds of matches had been burned nearby.

For more on this topic, please see our reporting on how to clean wildfire smoke from your home.