Air Filter Buying Guide

    Air Filter Buying Guide

    Running your furnace, air conditioner, or heat pump probably constitutes a big part of your utility bill. But it’s a small, often inexpensive part—the air filter—that keeps these heating and cooling systems humming along.

    Every forced-air heating and cooling system uses at least one filter, so homeowners (and some renters) with these systems need to know how and when to replace those filters. 

    Some background: Forced-air heating, ventilating, and cooling (HVAC) systems work on a loop. They “inhale” through return vents in your living space, pulling air through ductwork and across your heating or cooling source (either a furnace’s burners, or an AC or heat pump’s coil). Then they then “exhale” through supply ducts and vents, delivering warm or cool air to your living space. Inevitably, some of the hair, dust, and other debris in your home gets sucked into that loop as air circulates around your house.

    It’s an HVAC air filter’s job to catch that debris. In doing so, it first and foremost protects your heating and cooling equipment from damage. As a bonus, it can improve your indoor air quality. In fact, many of today’s filters are claimed to capture allergens, microbes, and other particles while they’re doing their primary job of keeping your system gunk-free. 

    There are a range of HVAC filter designs: The basic models are made from strands of fiberglass (for disposable filters) or metal mesh (for reusable filters), and can stop big debris. Higher-end filters are made from material more like a surgical mask’s, arranged into pleats, and at their best, they can capture particles as tiny as smoke, bacteria, and sometimes even viruses before they can recirculate. 

    You should expect to replace your filter every three to 12 months of use, depending on the size of the filter. For 1-inch-thick filters, three months is the usual recommendation. For 4-inch and thicker filters, 12 months is common.

    You might need to replace the filter more frequently if you have a few very furry pets, for example, or you live in an area with a lot of air pollution (like from wildfires). When HVAC filters get dirty and clogged with debris, air can’t flow freely through your ducts, which makes your home less comfortable and can eventually break your HVAC equipment. 

    The good news is that fresh HVAC filters can be budget-friendly and are usually simple to replace. 

    In this guide, we’ll help you figure out which filter size your HVAC system needs, how to pick the right amount of filtration, and how to swap in a filter on your own—usually with no tools required.

    CR members can also see our comprehensive air filter ratings for HVAC filters, based on extensive lab testing. We have ratings for 45 different models across a range of thicknesses, prices, and performance ratings. Many filters that we review are available in multiple width and height configurations, so our recommendations should cover almost any kind of forced-air system.

    How We Test Air Filters

    We test HVAC filters in an isolated room with its own air circulation duct, cut off from the rest of our building’s heating, cooling, and ventilation systems. The temperature and humidity are controlled, and we clean the air in the room prior to each test. Then we inject two substances into the room: a mixture of dust in various particle sizes, and smoke from standardized research cigarettes.

    Once the air quality gets as bad as we need it to be for our test, we turn on the air circulation in the room and use a particle analyzer to measure how quickly the filter reduces the particle count. We also monitor how much each filter restricts airflow, using a differential pressure transducer installed across the air filter.

    Our top-rated models are fantastic at capturing pollutants while allowing enough air to flow through to keep your heating and cooling system running optimally. The worst models capture very little pollution or significantly restrict airflow—or sometimes both. 

    We also calculate the annual costs of replacement filters, assuming that your system needs only a single filter, and based on the manufacturer’s recommendation for when to change them. Keep in mind that prices may vary by retailer and that it may be cheaper if you buy filters in bulk.

    How HVAC Filtration Is Measured: The MERV Scale

    The industry standard for an HVAC filter’s performance is measured according to the MERV scale (Minimum Efficiency Reporting Value—rolls off the tongue!), from MERV 1 through MERV 16. Some sources have claimed that the scale reaches MERV 20, but the testing standard has been updated to clarify that MERV 16 is the maximum. 

    At the low end of the scale, the filters are made from fiberglass or mesh, and can capture large particles like hair, fibers from clothes and carpets, and some pollen. Disposable fiberglass filters can cost just $2 each.

    As the rating increases, the filters can progressively capture smaller particles—and they tend to cost more. Around MERV 8, filter designs typically switch to pleated media (nonwoven material made of things like fiberglass paper, polyester, and plastic), and can reliably capture pollen, mold spores, and many types of household dust. 

    At MERV 13—the highest rating you’ll find for most of the popular residential HVAC filter sizes—you can count on the filter to stop bacteria, smoke, and other microscopic particles. These filters can cost around $40 or more.

    Most home improvement stores sell HVAC filters ranging from MERV 1 to MERV 13, though CR has tested models as high as MERV 16. But some filters with midrange and higher MERV ratings might cause problems in residential HVAC systems. (More on that later.)

    Some filter brands and retailers use alternative scales, like Home Depot’s Air Filter Performance Rating (FPR) system or MPR (on 3M Filtrete air filters). But the MERV rating will also be printed on the packaging or in the online product descriptions. 

    Many HVAC filters also call themselves “allergen” filters (or something similar). But these are usually just filters with a rating of at least MERV 11, the rating where filters begin to catch the majority of particles that are the size of common allergens. There’s nothing uniquely anti-allergenic about these filters, and other filters with the same MERV rating should capture allergens just as well.

    MERV vs. HEPA

    The MERV standard is used mainly to measure the performance of filters meant for forced-air HVAC systems. 

    HEPA (high-efficiency particulate air) is a much stricter standard than MERV. Because HEPA filters have such tight filtration, they tend to restrict airflow so much that they’re impractical for most residential forced-air systems. Instead, HEPA filters are typically used in air purifiers and vacuum cleaners

    How to Shop for Replacement Air Filters

    One easy way to pick a replacement filter is to simply buy the same filter that you’re already using in your system. But if that’s unavailable, or you’d prefer something that costs less or performs better, here’s how you can go about the process. Air filters generally come in a range of standard sizes, with a few that can adapt to fit different-sized filter-box or return-air openings.

    1. Figure Out Which Size You Need

    This typically means first finding your existing filter (or filters). HVAC filters are often located in a slot next to your furnace or air handler. But some HVAC systems have filters inside the return air ducts scattered in multiple rooms throughout your home. (Those are the grates that suck air back in, rather than blowing air out.) 

    Once you’ve found each filter, make sure it’s the proper size, measured in inches of height, width, and depth (or thickness). If there are gaps around the sides or it doesn’t line up with any gaskets, it might be the wrong size. If your existing filters are a perfect fit, write down (or take a picture of) the size printed on the filter frame. But when in doubt, check for the proper filter dimensions in your equipment’s owner’s manual, or get in touch with the manufacturer. 

    You’ll often find an exact fit in stock at a hardware store, but sometimes you’ll need to order the correct filter size online. (And if you’re reading this from the filter aisle at the hardware store, don’t bother trying to guess the size—there are at least a dozen common dimensions, and you’re likely to choose incorrectly.) 

    Take particular note of the thickness because it will affect the next decision you have to make (in step 2). Most residential HVAC filters are 1 inch thick, but 4-inch filters are becoming common, too.

    2. Choose the Right Filtration Level

    Any pleated HVAC filter can improve your home’s indoor air quality by snagging dust, pollen, and other small particles—sort of like a quieter, more passive vacuum cleaner. 

    But a filter that’s too dense for your setup can make it harder to heat and cool your home effectively and efficiently. It might also damage your furnace, AC, or heat pump over time as parts overheat or freeze solid.

    And yet a filter that’s unnecessarily loose isn’t great, either. It’s a missed opportunity to breathe cleaner air—and it also could damage your HVAC equipment over time as gunk accumulates on the blower and coils.

    So the right amount of filtration is about finding the sweet spot: a filter that allows ample airflow through your ducts while capturing as much debris as possible. 

    Remember how we said to take particular note of your filter’s thickness, which will usually be 1 inch or 4 inches? This is where that detail becomes important, because it might affect how much filtration your system can handle.

    1 inch thick: These can be tricky to shop for, according to many HVAC professionals, because 1-inch filters with high MERV ratings are especially likely to be too restrictive for some HVAC systems. 

    Nevertheless, CR has tested several 1-inch, high-MERV filters (MERV 11 and up) that earn a Very Good rating on our airflow test (that’s like scoring a 4 on a scale of 1 to 5). They tend to be pretty expensive, though. Models that earn the highest rating (Excellent) on our airflow test also tend to have lower MERV ratings, and cost less, though they don’t perform as well on our particle-capture tests. CR members can see the top performers in our full air filter ratings

    If you want to be certain about how much airflow your HVAC system requires to function properly, you’ll need to measure the static pressure. (Basically, that’s the resistance to airflow in your ducts, based on a handful of factors.)

    A qualified technician with basic tools of the trade can take the measurement in a couple of minutes—for example, when they’re already at your home performing a system tuneup—and advise you on how to pick filters that will work well with your equipment.

    If you can’t take a measurement, experts sometimes recommend sticking with a MERV 8 pleated filter as a safe-harbor option. Filters with this rating allow plenty of airflow, so they’ll be safe for most systems. They also don’t cost much, and they catch a lot more debris than basic fiberglass filters can.

    4 inches thick (or greater): If you already have a filter cabinet that can handle a filter of this thickness, you can feel confident that even a high-MERV filter will work well with your HVAC system.

    The secret is that they can capture tons of debris while using relatively porous filter material, thanks to the huge surface area that dirty air has to pass over—about four times as much filter media as on a 1-inch filter. A MERV 13 is a great choice at this size, delivering excellent air quality for a reasonable price. 

    If you currently have 1-inch filters, you could consider upgrading your system to accommodate 4-inch filters. Your HVAC pro would install a small cabinet next to your furnace or air handler (on the air-intake side) to hold the thicker filter. The work should cost only hundreds (not thousands) of dollars.

    3. Installing Your New HVAC Filter

    You might need a screwdriver or pliers to open a hatch or cover to get at the filter, but often you won’t. Installation is usually as simple as sliding or popping out the old, dirty filter, then sliding the clean one into the same slot.

    Make sure to install the new filter facing the right way—look for arrows on the filter frame indicating the airflow direction.