Best Furnace Filter: MERV 8, 11 or 13 for Your Blower

A furnace filter has two jobs: keep dust off the blower and the cooling coil, and clean the air you breathe. The two pull in opposite directions. The finer the media, the more particles it catches and, usually, the harder the blower has to work to pull air through it. Get the balance wrong and you can trip a furnace's high-limit switch or freeze an AC coil, with a filter that looked like an upgrade.

Our call for most homes: in a standard 1-inch slot, buy a MERV 11 filter with a published low resistance and change it every one to three months. Drop to MERV 8 if the system already struggles for air. Save MERV 13 for a 4-inch media cabinet, or for a 1-inch slot only after a static pressure check says the blower can take it. The slogans you'll hear (buy the cheapest and change it often, never go above MERV 8, MERV 13 is fine everywhere) are each right for some systems; the numbers below show which one is right for yours.

What MERV measures

MERV comes from ASHRAE Standard 52.2. The test sorts particles into three size bands and rates the filter by the minimum share it captures in each. MPR (3M's Filtrete scale) and FPR (Home Depot's) are proprietary; the EPA points out that only MERV comes from a national consensus standard. 3M maps its own grades: MPR 300 is MERV 5, 700 is MERV 8, 1000 is MERV 11, 1500 is MERV 12, 1900 is MERV 13 and 2800 is MERV 14.

The jump that matters for health is in the first column. A MERV 8 doesn't have to catch any of the smallest particles; MERV 11 must catch a fifth of them and MERV 13 half. That first column is why the EPA points people toward MERV 13 when fine particles are the worry. For keeping a coil clean, MERV 8's 70% capture in the 3-10 micron band does most of the work.

Why a better filter can hurt the system

Your blower pushes air against the resistance of everything in the loop: return ducts, filter, coil, supply ducts and registers. That total is the external static pressure, and residential equipment has a ceiling. Goodman lists its furnaces' cooling capacity at 0.5 in. w.c. of external static and says static shouldn't exceed the rating plate; one Goodman sheet says not to run above 0.5 in. w.c. in heating at all. Goodman's airflow charts are also measured without a filter, so whatever your filter costs comes out of that budget.

Carrier's furnace manual puts a number on the filter's share: best practice keeps the filter under 0.2 in. w.c., and the blower does its best work under 0.1. Past that, Carrier says, the system loses airflow, the rooms at the end of the ducts go short, and the fan gets louder and uses more electricity. ASHRAE puts the general rule flatly: more efficient filters generally mean more pressure drop, so check that the system can handle an upgrade first. The EPA adds the important qualifier, "generally, but not necessarily," because a filter with more pleats can be denser and still flow well.

When a filter is too much for the system, it shows up the same way again and again:

• Furnace shutting off early: the high-limit switch trips because too little air carries heat off the heat exchanger. Owners describe a furnace that runs a few minutes and quits, or throws a limit fault.
• AC coil freezing: low airflow lets the coil drop below freezing; a frozen coil and water from the thaw follow.
• Filter bowing or pulled into the blower: the pressure difference across the filter flexes it out of its rack. A blower set too fast can do this with any filter, so it is worth a tech checking the speed.
• Weak airflow at far registers: the back bedroom runs warm in summer and cool in winter.

A dirty filter does all of this too. The EPA says pressure drop across fibrous media "will slowly increase over the filter's useful life," and ENERGY STAR warns a dirty filter "can even cause the air handler to shut down." The MERV 13 that was fine in week one can be the problem in month four.

Do the math for your filter slot

The number that decides how hard a filter is to breathe through is face velocity: airflow divided by filter area. Carrier's manual gives the formula, and it's simple enough to do on the back of the filter box. A 16x25x1 is about 15.5 by 24.5 inches in reality, or 2.64 square feet. Using the common rule of thumb of 400 CFM per ton of cooling:

Now hold that against the published resistance. Carrier's table of representative clean filters puts a generic 1-inch pleated filter at 0.30 in. w.c. at 300 FPM and doesn't list it at 400 FPM or above; for a 16x25 1-inch pleated filter it shows 0.29 at 800 CFM and nothing at 1,000 CFM. In other words, a typical pleated filter in a single 16x25x1 slot on a 2-ton system is already past Carrier's 0.2 best-practice line, and a 3-ton system pulling through one slot is off the chart. That's not a reason to run fiberglass; it's a reason to want more filter area.

Published clean resistance for the filters in our picks, plus the media filters that fit 4-inch cabinets:

The test conditions differ, so treat the table as a ranking within each brand, not a contest between brands. Two things still jump out. The 1-inch MERV 13s sit above Carrier's 0.2 line even when clean, while a 4-inch MERV 13 in a 20x25 cabinet starts at 0.12 at 1,000 CFM because the same air spreads across far more media. Goodman's own minimum filter sizes point the same way: its GME80603B can use one 16x25 on the side, while larger models in the same table call for two 16x25s on the side or one 20x25 on the bottom.

Filtrete, Nordic Pure and the numbers on the box

3M's product pages give no pressure-drop figure for any Filtrete grade (owners say the frames print the numbers needed to work it out). What 3M publishes is capture data, and on its own numbers the MPR 1900 clears the MERV 13 minimums comfortably (62%, 87% and 95% across the three bands), plus a claim that the 1900 has better airflow than any other brand's 1-inch retail MERV 13, based on California Energy Commission data. Owners who own a manometer tend to back that up: the 1900 is the 1-inch MERV 13 they keep coming back to. One reads about 0.25 on a new one and changes it at 0.30, when their supply air temperature starts to show the lost airflow. That's above Carrier's best-practice line, which is the point: it works in that system, measured. Yours may differ.

Nordic Pure is the opposite case: full resistance tables on its site, made in the USA, and a Tru Mini Pleat line built to cut resistance at the same MERV. One owner who measures particles found a 2-inch Nordic Pure MERV 14 performing like a MERV 8 in their house, which is one report, but it's why we keep the Nordic Pure picks at MERV 8 and 11, where you're buying airflow first. We left Filterbuy out after owner reports of a MERV 8 box relabeled as MERV 13 and of filters shipped at nominal rather than actual size.

Size matters too. Filters are sold by nominal size and run smaller: Nordic Pure says a 16x25x1 may measure 15.5 x 24.5 x 0.75 inches. Measure the old one, not the slot. The EPA says the filter should fit snugly so air doesn't leak around it, and Nordic Pure says never to stack two filters in one place.

The upgrade that solves it: a 4-inch media cabinet

If you want MERV 13 without arguing with your blower, give the air more media to pass through. A 4- or 5-inch media cabinet holds a deep pleated filter, and the EPA notes these "will increase the area of the filtration medium and limit the airflow resistance." AprilAire's 213 runs 0.12 in. w.c. at 1,000 CFM in its 20x25 cabinet and is rated for a 12-month change, against one to three months for a 1-inch filter. Owners running 4- and 5-inch MERV 13 to MERV 16 filters report vent airflow on par with their old MERV 8 and low pressure drop many months in.

Two catches. A thick filter isn't magic on a system that's already starved; one owner's 5-inch MERV 12 still tripped a limit fault three days past a monthly change, which points at ducts or blower settings, not the filter alone. And adding a cabinet means cutting into the return duct or plenum. That's sheet-metal work, and on most installs it's a job for an HVAC contractor; if you're having a new system put in, ask for the bigger cabinet then, when it's easiest to add. AprilAire is phasing the 213 out in favor of the UK213, a 3-inch filter with upgrade rails for existing cabinets; the 213 still fits, and AprilAire publishes resistance data for it that it hasn't yet published for the new one.

Measure instead of guessing

Every argument about MERV ends the same way: it depends on your system. So find out. An HVAC tech can measure total static pressure and the pressure drop across your filter in a few minutes during a tune-up; ask for both numbers, with the filter you want to use installed. Some owners go further and mount an inexpensive manometer across the filter, then change filters by reading instead of by calendar.

Without a gauge, the routine is still simple. ENERGY STAR says to check the filter monthly and change it at least every three months, and the EPA says makers typically recommend every 60 to 90 days. Change sooner with pets, construction dust or a system that runs nonstop. Don't vacuum a pleated filter to stretch it another month; replace it.

Which filter we'd buy

• A 1-inch slot on a typical system: MERV 11 with published low resistance, the Nordic Pure Tru Mini Pleat, changed every one to three months.
• A system that already trips limits, freezes coils or whistles: MERV 8, the Nordic Pure pleated at 0.14 in. w.g., while a tech looks at the ducts and blower.
• You want MERV 13 and have only a 1-inch slot: the Filtrete MPR 1900, after a static pressure check with it installed, and change it on time.
• You shop at a big-box store: the Filtrete MPR 1000 is the MERV 11 you can find almost anywhere.
• You have an AprilAire media cabinet: the AprilAire 213 at MERV 13, or its replacement when your cabinet gets the new rails.
• Allergies, smoke or a new system on order: skip the 1-inch fight and have a 4- or 5-inch media cabinet installed.

If moisture is your air-quality problem rather than dust, a filter does nothing for it; our AprilAire E070 review covers a whole-house dehumidifier that ties into your ductwork.