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Updated November 2022
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Buying guide for Best furnace filters

Choosing a replacement furnace filter might not sound like a difficult task, but when you look at the numerous options available, your decision may suddenly become surprisingly complex and confusing.

The easiest choice is to get an exact replacement for what you already have, but is that costing you more than it should? Are you getting the most efficiency from your system?

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Most manufacturers suggest replacing filters every three months. If any members of the household have conditions that affect breathing, however, you should change them more often.

Why changing your furnace filter is important

A filter might seem like a fairly insignificant part of a furnace or A/C system, yet good airflow is vital. If your filter is clean, your system works efficiently. If it's clogged and dirty, your system works harder than it should. That means higher bills and lower air quality.

Some suggest that a little dust is no bad thing, that it can actually improve filtration. That's not really an accurate reflection of what's going on. Less-efficient filters get more efficient as they load since the pores begin to fill and spaces between the media fibers get smaller, and they start trapping finer dust. However, this “advantage” is short-lived. The filters continue to clog quickly, and the dust soon dramatically restricts airflow.

The best solution is a proper maintenance schedule. It's a good idea to keep a log, check your filter regularly, and replace it as recommended by the manufacturer (or if you notice its performance starting to deteriorate).

It's false economy to try to extend the life of filters beyond their specifications. Any money you might save on the filters themselves is lost on higher energy bills. If anyone in the household has respiratory difficulties, you're also making things tougher on them.

How a furnace filter is made

Filters usually have an aluminum, a steel, or a plastic framework containing a filtering element made of fiberglass, polyester, cotton, or paper. They're designed to trap particles of dust and dirt that adversely affect the components and performance of your system and have a negative impact on household air quality.

The majority of filters are pleated, offering a large surface area without increasing the exterior dimensions.

Filters can be disposable or washable. Disposable filters are convenient, easy to change, and usually last several months. Washable filters can last years if looked after properly. But they must be cleaned regularly (usually every 30 days) and thoroughly. If not done correctly, there is a danger of spreading bacteria.

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For your safety
Most furnace filters have a front and back. It's important you fit yours the right way, or airflow will be compromised. Instructions will be provided, so read carefully. Often, there's also an arrow on the side indicating the correct direction.

Choosing the right furnace filter

There are two things to check when choosing a replacement furnace filter: size and filtration level.


It's important to get the right size, as there's no room for maneuvering. Furnace filters either fit or they don't.

Sizing should be clear from your existing filter and will be given as the width x height x thickness –16 x 25 x 1, for example. These measurements are in inches, but the inch marking is not normally shown.

Most filters are one inch thick, but some are two or even four inches thick. In theory, a thicker filter gives greater filtration – but they aren't interchangeable. If your existing filter is one-inch thick, a four-inch filter isn't going to fit without serious modifications to your system. Even if that were practical (which is unlikely), your system likely only has sufficient airflow for a one-inch filter anyway. Putting in a thicker version would actually reduce effectiveness.

Filtration level

The vast majority of disposable furnace filters are rated according to the MERV scale created by the American Society of Heating, Refrigerating, and Air-Conditioning Engineers. The higher the number, the finer the filter, so the more particles it traps.

  • Disposable filters offered for household use vary between MERV 6 and MERV 12. However, experts recommend MERV 8 as a minimum and MERV 10 or 11 for better odor control.

  • Commercial filters are available with much higher MERV ratings, but most household systems don't generate sufficient airflow, so they should not be used.

  • Washable filters are not covered by MERV ratings, and their specifications can seem confusing. Some claim to trap a certain percentage of airborne particles, but they don't tell you what size those particles are.

However, manufacturers' claims such as “removes 95 percent of dust” are meaningless without knowing the size and type of particles. A very poor (airy) filter can trap 95% of larger particles, but only 3% of very small particles. Some filters that claim to remove 99% of pollen or mold are basing that on removing whole pollen or mold grains and do not factor in particle fragments, since pollen and mold can break up into finer particles.

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Did you know?
Pleated filters pack more surface area into the same frame size as standard filters and thus are more efficient.

Electrostatic furnace filters

Many furnace filter manufacturers now use the expression "electrostatic" in their product descriptions. An electrostatically charged filter is better than a standard pleated filter with many contaminants, and they’re recommended for homes with allergy sufferers, smokers, or pets. The electrostatic charge is added during manufacturing. There is no requirement for a separate electrical supply. It's not something that needs to be re-charged.

Furnace filter prices

A good furnace filter isn't expensive. However, it is difficult to give accurate prices because of the enormous variety of materials and sizes and the competitive nature of the available furnace filters.

  • As a rough guide, disposable furnace filters start at around $7 or $8 each for 20 x 20 x 1. The price rises to between $15 and $20 for 20 x 30 x 4. MERV rating has little impact.

  • Washable filters, not surprisingly, are considerably more expensive. Expect to spend anywhere from $30 to $50 each,  depending on construction.

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Never run your furnace or HVAC system without the proper filter. Not only will it adversely affect your air quality, it will also allow dirt and dust into parts of the system that should remain uncontaminated. The result will be poor performance, higher bills, and breakdowns.


  • Filter quality and worth depend on the application, including the size and airflow of the HVAC system. For example, the higher the MERV filter number, the better the dust removal – if your system can push air through it.

  • Running the HVAC system on a continuous fan is the best way to clean the air regardless of the type of filter you use.

  • Less-efficient filters get more efficient as they load since the pores begin to fill, spaces between the media fibers get smaller, and they start trapping finer dust. Of course, this also means the filters are clogging and will soon obstruct airflow.

  • How often you need to change the filter will depend on how efficient it is, how much you run the fan on your HVAC unit, and how many dust sources you have inside.

  • The best filters tell you the percentage of particles removed by size at a given airflow.

  • HVAC systems and filters are like vacuum cleaners. They should move enough air to clean the environment without allowing the dust to blow back into your home.

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Replacing filters regularly helps your system perform at its best. We also recommend regular maintenance visits from a properly qualified HVAC specialist.


Q. What is a micron?

A. A micron (or micrometer) is one-millionth of a meter or 0.000039 inch. To give you an idea of just how small that is, a human hair is about 70 microns thick.

Q. What is the purpose of a HEPA filter, and are they the best?

A. HEPA (high-efficiency particulate air) filters are extraordinarily efficient, removing nearly 100% of particles as small as 0.3 microns. However, they have a very dense structure, and most furnaces don't produce sufficient pressure to drive air through them. If that's the case, all you're doing is blocking up your system and actually getting no filtration at all.

A great filter is the most efficient one your HVAC system can handle while still moving enough air to make a difference.

Q. How often do I need to replace my furnace filter?

A. It depends on a variety of factors. Standard, one-inch-thick filters should be changed after around 90 days, but thicker filters can last much longer.

If you have pets or are a smoker, your filters will become clogged more quickly. If you have breathing difficulties, you'll probably want to change them more often because you'll be more susceptible to airborne particles.

Your furnace or HVAC manual should give you the information you need. You can also get information from filter suppliers. Follow the instructions carefully.

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