Furnace Filter MERV Ratings Explained

Your furnace filter has two jobs. It protects the blower and other parts of your heating and cooling system from dust, and it removes some particles from the air you breathe. The MERV rating printed on the frame tells you how well it does the second job. Picking the right number means balancing better filtration against what your system's fan and filter slot can handle.

What MERV means

MERV stands for Minimum Efficiency Reporting Value. According to the EPA's explanation of MERV ratings, the rating reports a filter's ability to capture particles between 0.3 and 10 microns, based on a test method developed by ASHRAE. The scale runs from 1 to 16. A higher number means the filter traps a larger share of smaller particles.

For reference, a micron is one-millionth of a meter. Particles in the 0.3 to 1 micron range include some smoke and other fine particles. Larger particles in the 3 to 10 micron range include much of the dust and pollen you can see settle on surfaces.

What each range captures

The EPA's MERV table shows these minimum capture levels:

• MERV 1–4: captures less than 20 percent of large particles (3–10 microns). These are basic fiberglass filters that mainly protect equipment.
• MERV 8: captures at least 70 percent of large particles and at least 20 percent of mid-size particles (1–3 microns). It does not have a rating for the smallest range.
• MERV 11: captures at least 85 percent of large particles, 65 percent of mid-size particles and 20 percent of small particles (0.3–1 micron).
• MERV 13: captures at least 90 percent of large particles, 85 percent of mid-size particles and 50 percent of small particles.
• MERV 16: captures at least 95 percent of particles in all three size ranges.

HEPA filters are rated on a different system and are not part of the MERV scale. They are common in portable air cleaners.

What the EPA recommends

The EPA advises that if you upgrade your filter, you choose one rated at least MERV 13, or as high a rating as your system fan and filter slot can accommodate. The EPA also says you may need an HVAC technician to determine the most efficient filter your system can handle.

That second part matters. A higher-MERV filter has denser media, which can make it harder for air to pass through. If the fan cannot move enough air through the filter, the system can lose airflow. The EPA guide to air cleaners in the home also points out that a furnace filter only cleans air while the system is running, which is usually less than 25 percent of the time during heating and cooling seasons.

Filter size and depth

The filter slot on your furnace decides much of what you can use:

• 1-inch filters fit most standard furnace slots. They have less media surface area, so high-MERV versions can restrict airflow more and fill up faster.
• Deeper media filters sit in a dedicated cabinet next to the furnace. The extra depth gives the filter more surface area, which lets it use higher-efficiency media with less restriction.

If you want MERV 13 or higher and your furnace has a 1-inch slot, ask whether a media filter cabinet can be added. This is a common upgrade during furnace replacement or as part of whole-home air purification.

What you can check yourself

1. Read the current filter. Note the size (printed on the frame as length x width x depth) and the MERV rating.
2. Check the arrow. The airflow arrow on the frame should point toward the furnace, in the direction air moves.
3. Inspect on a schedule. ENERGY STAR advises inspecting, cleaning or changing filters once a month and notes that a dirty filter can raise energy costs and damage equipment. See the ENERGY STAR maintenance checklist. Replace the filter on the schedule its manufacturer recommends, or sooner if it looks gray and loaded.
4. Watch for warning signs after an upgrade. Weak airflow at the registers, rooms that heat or cool unevenly, or a louder blower can mean the new filter restricts air too much.
5. Turn the system off before changing the filter. Do not remove panels beyond the filter access door.

Choosing the right number for your home

• If you only need to protect the equipment and no one in the home has allergies or asthma, a mid-range filter that your system handles well may be enough.
• If you deal with wildfire smoke, pet dander or allergies, the EPA's MERV 13 recommendation is a good target, as long as your system can accommodate it.
• If your system cannot handle a higher-MERV filter, a deeper media cabinet or a portable HEPA air cleaner in key rooms are other options.

Minnesota homes keep windows closed for much of the long heating season, and smoke from distant wildfires can drift into the state in summer. A furnace filter that matches your system is one of the simplest ways to keep particles under control during both seasons.

When to call a professional

Call for service if you want to move to MERV 13 or higher and are not sure your furnace can handle it, if you notice weak airflow or uneven heating after changing filters, or if your filter slot is damaged or lets air bypass the filter. A technician can measure system airflow, check the blower, and recommend a filter or media cabinet that fits. Filter checks are also part of routine furnace maintenance.

Common questions

Sources

• U.S. EPA — What is a MERV rating?
• U.S. EPA — Guide to Air Cleaners in the Home
• ENERGY STAR — Maintenance Checklist