Which Furnace Filter Should I Buy

The research

Who this is for
What is a MERV rating?
How to choose a filter by its MERV rating
A perfectly good HVAC filter: Nordic Pure MERV 12
2 other filters that should do a fine job
How often should I replace my filters during wildfire smoke events?
Footnotes

Who this is for

Most HVAC systems use a standard ane-inch-thick filter, so we focused exclusively on i-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 mutual in the Mid-Atlantic and northern states, where the air handler is frequently 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 y'all 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 (unremarkably in the 4- to 5-inch range, and usually mounted at the air handler), information technology was likely designed specifically for medium-efficiency MERV filtration. Yous don't need our advice; but replace the existing filter with one that has the same specs.

What is a MERV rating?

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

Just MERV is different in some key ways. First, instead of existence a simple pass-fail measure like HEPA (a filter is either HEPA-rated or information technology's not), MERV ratings fall on a scale from 1 to 16, with college numbers indicating ameliorate operation.

Too different HEPA, MERV tests measure particulates of different sizes, from 0.three micron (very minor and difficult to filter—the size HEPA tests measure) up to 10 microns (relatively large and like shooting fish in a barrel to filter). MERV also doesn't have a target percentage for removal, the way HEPA does. Instead, the bodily percent removal is recorded for each of 12 particle sizes, and these numbers are referenced against a performance table to determine the appropriate rating.

And then when you wait at the full spectrum of what's floating around in the air in your firm, MERV ratings reverberate a filter'due south operation on all of it. College numbers indicate a better filtration of the smaller particles, like pollen and smoke, that crusade many respiratory issues. And medium-efficiency MERV filters in the 8 to 13 range—the kind we would get—tin can remove ninety percent or more of these. (For a actually thorough explanation of how MERV ratings are calculated, visit the website of the National Air Filtration Association.)

One more matter: Some companies, including major filter manufacturer 3M/Filtrete and major filter retailer Habitation Depot, use their own rating conventions (MPR by Filtrete and FPR by Home Depot). Both merits to be based on the American Society of Heating, Refrigeration, and Air-conditioning Engineers'south ASHRAE 52.2 standard that's used to set MERV ratings, merely neither visitor 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, just you tin can't run across that until you've actually got one in your hands.)

Afterwards we inquired, Filtrete replied that its MPR filters capture more of the very small particles (0.three 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 x or more microns), MERV and MPR functioning was nearly identical. Filtrete said MPR 1900 is equivalent to MERV 13, MPR 1500 to MERV 12, and MPR 1000 to MERV eleven.

Home Depot shared a basic comparative nautical chart simply no exam information or protocols; it equates FPR 9 and x to MERV 12, FPR 7 and 8 to MERV eleven, and FPR half-dozen to MERV 10.

How to choose a filter past its MERV rating

A stack of the filters we tested — Photo: Michael Hession

To decide what filter we'd buy, we needed a more businesslike explanation of MERV, so we turned to Kathleen Owen, a enquiry 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 Ac Engineers'due south ASHRAE 52.2 standard that'south used to set the ratings. She after chaired the ASHRAE 52.two commission, which regularly updates the standard to address changes in filter technology.

In a conversation about the most mutual filter levels for sale at retail, MERV one through 16, Owen said this range "gets you from filters that might catch a golf ball—I'yard exaggerating a little bit—up to filters that volition catch about everything." Owen added, "I similar to tell people that each of those filters has a purpose."

• MERV i to four (and unrated): Filters like the MERV iv Eastward-Z Flow, MERV 2 True Blue, MERV 1 Filtrete Basic Cut-To-Fit, and the unrated Flemish region, are the cheapest (some under $one apiece) and nearly bones MERV-rated filters. Their purpose is to keep particles larger than 10 microns—including pet hair (but non dander), carpet fibers, and heavy dust—from bottleneck the heating/cooling coils and ducts in your HVAC system. They have piffling to no affect 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 aid keep it running efficiently in the heating and cooling seasons. "Remember of how important your screen door is in summer when there's flies out," Owen said. If y'all are reading this guide, you lot 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 erstwhile or dirty—replacing it with a MERV 1 to 4 filter would be a fine quick fix.

• MERV five to 7: Owen said filters in this range (like this MERV vii Nordic Pure) were created as an comeback on the bones MERV 1 to 4 filters and designed to also filter out midsize particles (3 to 10 microns, which includes near pollen and mold spores) that could encourage mold growth on heating/cooling coils. These filters are not as widely sold as the lower and college MERV-rated filters in hardware stores, simply they can exist plant online easily. At that place's naught wrong with these types of filters; we just didn't focus on them every bit much as nosotros did those in the 8 to 13 range more often than not because this class, by comparison, unremarkably isn't any cheaper, doesn't perform quite too, and often isn't as easy to find.

• MERV eight to xiii: This is the category we'd recommend for people who care most indoor air quality, and in that location are many equally adept examples, such equally the Nordic Pure MERV 10 and MERV 12, the Honeywell FPR 9, and the Filtrete 1500. (For more detail on these models, run into the A perfectly adept HVAC filter and Two other filters that are besides fine sections.) We focused on filters in the MERV 8 to 13 range because they are skilful to splendid at removing fine particulates (as small as 0.3 to 3.0 microns in bore, including smoke and exhaust particles and bacteria), plus anything larger. This MERV range, Owen said, "covers most of the stuff that people are probable to breathe in unless they're really close to the source." The EPA says filters rated MERV 7 to xiii "are likely to be nearly equally effective as truthful HEPA filters at controlling nearly airborne indoor particles." The college end of the range, MERV 11 to 13, removes xx-plus to 50-plus percent of the very smallest, 0.3-to-1.0-micron particulates, and filters in this range are specially popular. An NIH literature review indicates that filters in this range tin significantly reduce indoor fine-particulate pollution and convalesce some respiratory ailments.

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

So why doesn't everyone get a MERV eight to xiii filter? Across their higher cost, there is also a business organization 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 force per unit area the filters create when installed. That tin can potentially strain your equipment.

Powell noted that, "You accept a off-white amount invested in your HVAC system, and unless you know how much pressure drop you lot have in it and how much more it can handle, you could cease upwards shortening the life of your blower. And a blower'south not cheap." Ayers said, "That'south the danger of looking at one-inch filters and trying to become college-efficiency and do the job that'due south needed for allergies: You run the chance of actually damaging your blower and your HVAC system." (If you are concerned about this, to exist admittedly punctilious you can hire an HVAC firm to measure pressure level drop in your system.¹)

Against these warnings, however, is a mountain of possessor testimony that MERV 11, 12, and even 13 filters have worked just fine in their systems for years. Besides, an often-cited comprehensive independent test of MERV 8 to xiii filters' effects on HVAC airflow and free energy consumption (a proxy for how hard the equipment is working) concluded that even "if no accommodations are made for the greater pressure drop of high-MERV filters, air flow and energy penalties are not probable to be severe—at least, not until the filter is loaded with dirt."

We experience the manufacturers' concerns are a flake overcautious, in role because a contempo innovation in air filters permits high MERV ratings (xi to 13) with low pressure drib.² So-called electret filters use a mat of fibers that are electrostatically charged. The charge helps attract airborne particles (rather than but passively impeding them). As a issue, the mat can be more porous, causing better airflow and a smaller pressure drop. Still, Owen emphasized, electrets go less efficient over fourth dimension, oftentimes significantly so. "Electret media volition driblet in efficiency upon employ, sometimes by as much as 50 percentage," she said. "Basically what happens is the tiny particles—we're talking 40, fifty nanometers, which there are gazillions of in the air—will coat the charged fibers and go along them from attracting larger particles." That means regularly replacing electrets is absolutely key to maintaining adept filtration, and most come with threescore- or 90-24-hour interval replacement schedules. If you run your system year-round, that'due south potentially 6 filters a year.

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

A perfectly proficient HVAC filter: Nordic Pure MERV 12

Nordic Pure's MERV 12 filter, a white corrugated fabric-looking material. — Photo: Michael Hession

Nordic Pure MERV 12

Nordic Pure's MERV 12 filters are not the just products that see all our criteria, just they are perfectly good examples, and they're ordinarily competitively priced and easy to find online. We like that they're genuinely MERV-certified, because different the proprietary ratings used past 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 lxxx percent of i.0-to-three.0-micron particles, which include fine grit and soot; and at to the lowest degree 90 percent of larger pollen, grit, pet hair, and lint particles. Again, MERV stands for Minimum Efficiency Reporting Value, then this is the worst functioning measured during third-party testing to the ASHRAE 52.2 standard. And once again: the MERV examination 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 toll than competitors—most $40 for a six-pack of 16-past-25-by-1-inch filters, or about $7 each (with some competitors you'll pay nigh double that cost for MERV xi/12 or the equivalent). At the recommended 3-month replacement cycle, a Nordic Pure can save y'all nigh $30 to $xl a year.

Nordic Pure reports the filter'south 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 about people seem to have had no issues with the Nordic Pures straining their HVAC organisation—the six-pack has an overall score of 4.iii stars (out of five) across more than 2,800 reviews at Amazon at this writing (we should notation that the reviews in one case 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, and then you should be able to find one that fits your return vent opening.

Two other filters that should do a fine job

A tester holding two 16 by 25 inch filters — Photograph: Michael Hession

If you can't discover Nordic Pure in your size, or if you prefer to purchase and choice 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 bachelor at retail. (Habitation Depot stocks Honeywell; Lowe's and Ace stock Filtrete). These filters perform on a similar level as the Nordic Pure MERV 12, they are well-reviewed by many owners, they come up in a broad range of sizes, and the high likelihood that they're in stock at a nearby hardware store is a large advantage if y'all demand one ASAP.

We don't love that neither Honeywell or Filtrete is MERV-certified, but their proprietary ratings are reportedly equivalent to MERV 11 or 12. We couldn't find pressure-drop information on the Honeywell, so that's a possible concern. The Filtrete's pressure drib is 0.22 inch, fractionally lower than the Nordic Pure MERV 12's at 0.24 inch. Data Filtrete shared with us indicates the the 1500 removed 54 percent of 0.iii-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 2 filters performed very similarly.

A major reason we'd await for Nordic Pure as an option commencement comes downwards to price: Both the Honeywell and Filtrete options usually toll more than twice the price of the Nordic Pure filters. Equally electret filters, all these models will lose effectiveness equally they capture particles, and should be replaced at least every three months, per the manufacturers' recommendation. Just 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 bandy: "If yous tin get to your filter easily, one simple thing you tin practise is just go accept a look. If yous tin can't meet the filter media anymore, it'southward been in in that location too long. Change it!"

How often should I replace my filters during wildfire smoke events?

During a wildfire, you'll need to change your filters more often than you would under normal air conditions, and at that place are a few indicators that tin can assist you know when it's time to supervene upon i.

We asked Kathleen Owen, the filtration expert nosotros spoke with for this guide, for her advice. She replied in an email: "If I lived near the wildfires and didn't detect the cost prohibitive, I'd probably change a 3-month filter every month during the flavor. Or possibly later each major issue— say if a burn gets shut enough to be a large deal (rather than only you lot know about information technology from the news)."

Owen also noted that wildfire smoke is a complex mixture, incorporating both physical fume 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 moisture particles or ones with gases adsorbed on them tin exist nerveless, then give off the smelly stuff the adjacent time the fan comes on," she wrote. "So, one recommendation is that if folks experience 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 assistance."

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

In improver to obvious steps like keeping windows and doors closed, Owen too brash shutting off bath fans and range hoods, both of which vent straight to the exterior and, equally 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 confronting an farthermost load of ethanol vapors we found near of them ineffective, anecdotally nosotros tin say that they seem to perform well nether adequately heavy smoke conditions. Both in guide writer Tim Heffernan's apartment and in our New York exam space, after running our top picks for a few hours, there was piffling olfactory evidence that hundreds of matches had been burned nearby.

Finally, Owen noted that the American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) has begun working on building-filtration guidelines specific to wildfire weather. Nosotros'll exist following upwards on this every bit it develops. For more than on this topic, delight see our reporting on how to clean wildfire smoke from your home.