How Often to Change Your Furnace Filter (and What MERV Means)

A dirty furnace filter makes the blower work harder and lets dust pass through the house, and swapping it takes only a few minutes. The two questions are which rating to buy and how often to swap it. The short answer from the EPA is to change the filter about every 60 to 90 days, check it more often if it looks heavily soiled, and consider a MERV 13 filter if your system can handle it.

Summary

A furnace filter sits in the ductwork and traps particles as air passes through, and the MERV rating tells you how well it does that job. MERV stands for Minimum Efficiency Reporting Value, and it reports a filter's ability to capture particles between 0.3 and 10 microns, based on a test method from ASHRAE. The higher the rating, the better the filter traps specific particle sizes.

If you upgrade to a higher efficiency filter, the EPA advises choosing at least MERV 13, or as high a rating as your system fan and filter slot can accommodate. All filters need periodic replacement to work properly, and the EPA notes that manufacturers typically recommend replacement every 60 to 90 days. Change it sooner if the filter looks heavily soiled when you take it out.

Furnace Filters at a Glance

The table shows that the 60 to 90 day guidance applies to filters in general, and what changes with the rating is what gets caught. A higher rating traps smaller particles, but it can also load up faster, which is why the EPA pairs the MERV 13 advice with a reminder to replace the filter frequently.

The 4- to 5-inch media filter is the exception. It has more surface area, so follow the maker schedule for that type and check it monthly until you see how fast it loads in your home.

Which Filter to Buy

Fiberglass, MERV 1-4

A fiberglass filter is a thin, flat panel that mainly protects the furnace itself. The EPA table shows MERV 1-4 filters capture under 20% of particles in the 3.0 to 10.0 micron range, which covers large dust and lint. They let air pass easily, but they do little for the air you breathe. Check the maker label for the change interval, and look at the filter monthly, because even a thin filter stops working well once it is loaded with dust.

Good fit if: your only goal is protecting the furnace, and no one in the home has dust or pollen sensitivity.

Pleated, MERV 8

A pleated MERV 8 filter has folded media, so it holds more dust than a flat fiberglass panel before it clogs. The EPA table shows it captures at least 20% of particles from 1.0 to 3.0 microns and at least 70% of particles from 3.0 to 10.0 microns, a range that includes dust, pollen, and mold spores. It is a common everyday choice for many homes.

Good fit if: you want a basic step up from fiberglass without checking whether your system can take a denser filter.

Pleated, MERV 11

A pleated MERV 11 filter steps up to finer particles, including pet dander. The EPA table shows it captures at least 20% of particles from 0.30 to 1.0 microns, at least 65% from 1.0 to 3.0 microns, and at least 85% from 3.0 to 10.0 microns. Homes with pets may find it a useful middle option. Check the filter monthly at first, because a denser filter can load faster than a MERV 8 in the same house.

Good fit if: you have pets or dust sensitivity and your blower handles a denser filter.

Pleated, MERV 13

MERV 13 is the EPA's suggested minimum when you upgrade to a higher efficiency filter. The EPA table shows it captures at least 50% of particles from 0.30 to 1.0 microns, at least 85% from 1.0 to 3.0 microns, and at least 90% from 3.0 to 10.0 microns, which covers fine dust and smoke.

The EPA notes that most furnaces and HVAC systems can accommodate a MERV 13 filter without equipment problems, provided the filter is replaced frequently. Consult a professional HVAC technician to be certain. If the filter looks gray across most of the media before 60 days, shorten the interval rather than stretching it.

Good fit if: you want finer filtration and your system can take it, and you will change the filter on schedule.

Media Filter, 4-5 Inch

A media filter is a thick pleated cartridge, usually 4 to 5 inches deep, that fits a wider filter cabinet. The EPA notes that filters commonly come in thicknesses of 1 to 5 inches, and a thicker cartridge has more surface area, so it can run longer between changes. The exact interval depends on the maker and the cabinet, so check the maker schedule and write the install date on the frame.

Do not force a thick filter into a 1-inch slot, and do not bend or crush a filter to make it fit, because air will leak around it.

Good fit if: your furnace has a media cabinet and you want fewer changes per year.

Choosing Order

1. Read the old filter first: the size is printed on the frame, such as 16x25x1, so buy the same size and check the thickness before you pick a higher rating.
2. Check what your system can take: if you want MERV 13, ask an HVAC technician whether your fan and filter slot can handle it, because a filter that is too dense can restrict airflow.
3. Set the schedule: write the install date on the frame, check the filter every month, and change it about every 60 to 90 days, or sooner if it looks heavily soiled.

Before You Buy

• Read the size printed on the old filter frame, and buy the same length, width, and thickness.
• Check the airflow arrow on the frame and install the new filter with the arrow pointing toward the furnace blower, unless the maker says otherwise.
• If you move up to MERV 13, confirm with an HVAC technician that your system can handle it.
• Keep one spare filter on hand so a dirty filter never waits a week for a store trip.
• If someone in the home has asthma or a dust allergy, consider asking a clinician what fits your household, and do not treat a filter as a substitute for cleaning or ventilation.
• Note that the EPA does not certify or recommend specific brands or models, so compare ratings and maker guidance rather than brand names.

Sources

• U.S. Environmental Protection Agency, What is a MERV rating?
• U.S. Environmental Protection Agency, Guide to Air Cleaners in the Home