AC Filter Replacement: Which Filter to Buy and How Often to Change It

What a filter does for air quality, and what it cannot do

The filter sits in the path of the air returning to your indoor unit, which is the air handler or furnace. Inside that unit are a blower, the fan that moves the air, and a cooling coil, the cold metal that chills it. The filter catches dust before the air reaches them. ENERGY STAR says a clean filter prevents dust and dirt from building up in the system. It also says a dirty filter slows airflow and makes the system work harder.

A filter cleans the air only while the system runs. EPA says furnace and HVAC filters work to filter the air only when the system is operating. It also says no filter removes every pollutant, and that most filters are designed for either particles or gases.

Finding the right filter size

Start with the old filter. Its size is printed on the cardboard frame as length, width and thickness in inches. Buy the same size. If the print has worn off, measure the old filter or the slot, and match your numbers to the package.

EPA says filters commonly come in thicknesses of 1 to 5 inches, with lengths and widths to match common filter frame sizes. It adds that the filter should fit snugly, so air does not leak around it and you do not have to bend or crush it to make it fit.

A deeper filter needs a deeper housing. The Florida Solar Energy Center describes filters in boxes 4 inches deep or more that attach to the return duct, the duct that carries air back to the unit. If your slot holds a thin filter, do not force a thicker one into it. Ask a technician first.

MERV ratings explained

MERV stands for Minimum Efficiency Reporting Value. EPA says it reports a filter's ability to capture particles between 0.3 and 10 microns. A micron is one-millionth of a meter, so these particles are very small. The rating comes from a test method developed by the American Society of Heating, Refrigerating and Air Conditioning Engineers, known as ASHRAE.

EPA's table runs from MERV 1 to MERV 16 and lists average capture efficiency by particle size. A MERV 1 to 4 filter captures less than 20 percent of particles 3 to 10 microns across. A MERV 8 filter captures at least 70 percent of those, and at least 20 percent of particles 1 to 3 microns across. A MERV 13 filter captures at least 50 percent of particles 0.3 to 1 micron across.

Two other rating systems are in common use: the Filter Performance Rating (FPR) and the Microparticle Performance Rating (MPR). EPA says MERV rests on a national consensus standard, while FPR and MPR are proprietary, meaning company-owned. For all of these methods, a higher rating means higher removal efficiency for most particle sizes.

Why the highest MERV is not always the right choice

EPA's advice has two parts. First, if you decide to upgrade, choose at least MERV 13, or as high a rating as your system's fan and filter slot can accommodate. Second, EPA says most furnaces and HVAC systems can accommodate a MERV 13 filter without equipment problems, provided the filter is replaced frequently.

The hard part is knowing your own system's limit. EPA says it can be difficult to tell, because HVAC systems are not commonly labeled with any filter recommendations. To be certain, EPA says to consult a professional HVAC technician. Trane's homeowner page adds that you should always double-check your owner's manual for specific recommendations.

Why can a filter be too dense? A tighter filter resists the air more. A Florida Solar Energy Center research proposal says consumer demand for higher-efficiency filter media can result in filters with more air restriction, which may lower airflow rates and increase static pressure. Static pressure is the resistance the blower pushes against. That document is a proposal, not a test result, so it names the concern without measuring it.

Our reading: EPA's two lines fit together. A higher rating is a good choice when the system can take it and the filter is changed often.

How often to change an AC or furnace filter

The published schedules differ, so it helps to read them side by side. A fiberglass filter is a thin flat panel, a pleated filter has folded material that gives it more surface area, and a media filter is a deep pleated filter.

• ENERGY STAR: check the filter every month, especially in heavy-use months. If it looks dirty after a month, change it. At a minimum, change it every 3 months.
• EPA: manufacturers typically recommend replacement every 60 to 90 days. If filters look heavily soiled when you change them, consider changing them more often.
• Carrier: replace a 1-inch fiberglass filter every 30 days, a 1-inch pleated filter every 30 to 90 days, and a 4-inch media filter every 6 to 12 months.
• Trane: generally, clean or change your filters once every 3 to 9 months, and check the owner's manual for specific recommendations.

Our reading: the numbers differ because the filters and homes differ. Carrier says its schedule varies with filter thickness, household factors and system usage, and EPA says many factors affect how quickly filters get dirty.

A practical rule that fits all four: look at the filter every month and change it when it looks dirty. For a 1-inch filter, treat ENERGY STAR's three months as the longest gap. For a deeper media filter, follow the maker's stated interval and keep checking it. That rule is our reading, not a quotation from any one source.

What changes in the Tampa Bay cooling season

The National Weather Service Tampa Bay office describes a humid rainy season. It dates the season May 25 to October 10 for West Central Florida outside Southwest Florida, and says it typically comes with persistent dew point values above 70°F. Dew point is the temperature at which water vapor in the air starts to condense, so a high dew point means wet air.

ENERGY STAR says to check the filter every month, especially during heavy use months, which it names as winter and summer. Carrier's homeowner page says a system that runs continuously, for example during a hot Florida summer, needs its filter changed more often than the standard recommendation. Our reading: a Tampa Bay system running through the humid season is the kind of heavy use those two sources describe. This page cites no measured data on how many hours Tampa Bay systems run.

Where the filter sits matters too. In 2017 the Florida Solar Energy Center proposed a study of deep filter boxes. Its proposal says their large access covers have been observed to have air leakage, based on visual inspection. The proposal reports no test result. Our reading: after each change, close the cover or grille fully, so the air passes through the filter and not around it.

The rest of the Florida upkeep list, such as the condensate drain and the coils, is in the Florida AC maintenance checklist.

What a filter will not fix: mold and humidity

A filter will not fix a moisture problem. EPA says furnace and HVAC filters do not address the cause of mold and musty odors, which it describes as a water or moisture problem in the building. It adds that filters may remove some of the particles mold produces.

Mold and humidity have their own pages: mold in HVAC systems and why a house feels humid with the AC running.

Filter maintenance: how to change it

Changing a filter is homeowner work. Trane's page says to check your owner's manual before performing any maintenance. Only the filter grille or slot cover is yours to open. Leave every other panel to a technician.

1. Set the thermostat to OFF so the blower stops, and follow any extra power-off step in your owner's manual.
2. Open the filter grille or slot cover. Note the size and the airflow arrow printed on the old filter's frame.
3. Slide the old filter out, holding it level so the dust stays on it.
4. Check that the new filter is the same size as the old one.
5. Slide the new filter in with the arrow pointing the way the air flows. If you are not sure, see which way a furnace filter goes.
6. Close the cover fully. The filter should fit snugly, with no bending or crushing.
7. Write the date on the frame, turn the thermostat back on, and set a reminder to look at the filter in a month.

Do not run the system with the slot empty. ENERGY STAR says a clean filter prevents dust and dirt from building up in the system. If the cover will not close or the filter will not fit, stop and ask a technician.

Questions to ask a technician

EPA says a technician can determine the most efficient filter your system can use. These questions work with any contractor.

1. What size and thickness of filter does each filter slot take, and where is each one?
2. What does the manufacturer's documentation say about the filter for this equipment?
3. Can my system take a MERV 13 filter? If not, what is the highest rating it can take?
4. If I want a higher rating or a deeper filter, can the housing be modified? How will you check airflow afterward?
5. How often should I change the filter in my home, given how much the system runs?
6. Does the filter cover seal fully when it is closed?