How to Choose the Right Furnace Filter

Every time I purchase furnace filters, I stare at the wide variety available and wonder which to purchase. Should I just get the most affordable? Is there really any benefit to the pricy ones that filter micro-allergens and have odor reduction?

If you’ve ever wondered the same things, then you’ll appreciate this post. Here is what I’ve learned after taking a close look at what to look for in a furnace filter.

What’s the purpose of furnace filters?

Before selecting a furnace filter, it’s useful to understand why furnace filters are important. There seems to be a common misconception that furnace filters are made to clean the ambient air in homes. However, their primary purpose is actually to protect furnaces. They filter out dust, hair, and other particles in order to protect the blower fan. They can help improve indoor air quality because these particles are removed before the air is circulated, but their function is not to clean the ambient air.

Types of filters

There are several different types of furnace filters.

• Fiberglass or cellulose pads. These pads, which are surrounded by cardboard frames, are able to protect furnaces from large dust particles and debris, but they are not effective at removing smaller particles. These are the most affordable filters on the market.
• Washable/reusable filters. Flat plastic or metal foil pads comprise these filters. They can be rinsed with a hose, reinstalled, and reused. Some of these can be sprayed with a sticky coating material to increase their ability to trap smaller particles. Reusable filters should be washed monthly and they may last 3 to 5 years.
• Pleated polyester filters. Like fiberglass or cellulose pads, these filters are held in cardboard frames. Unlike those filters, though, these provide much greater filtering capacity. Many are made from electrostatically charged fibers that attract smaller particles. These generally need replaced every 3 months.
• Deep-pleated, high-efficiency air filters. These are similar in many ways to pleated polyester filters, but they are typically 4 to 6 inches thick. They require a special setup in the duct system because they do not fit in standard filter holders.
• Electronic filters. Electronic filters are comprised of high-voltage grids that apply a positive charge on particles in the air stream. These particles are attracted to negatively charged elements in the filter where they become trapped. Electronic filters are roughly the same size as the holders for deep-pleated filters. They require electricity to operate and must be washed monthly. These are the most expensive filters on the market.

Not all filter types can be used with all furnaces, so be sure to check the user manual for your furnace to see which types are suitable for your unit.

Which type of filter is best?

As you can tell from these descriptions, some filters are able to trap small particles and others aren’t. It’s easy to tell how well a given filter works by looking at its MERV rating. MERV stands for “minimum efficiency reporting value,” which is a measure of how well a filter traps particles. MERV ratings range from 1-16, with higher MERV values indicating that a filter can remove more dust particles and other contaminants.

At first glance, you may assume that the best choice is buying a filter with a high MERV rating. However, there are a couple of other things you may want to consider.

• While higher rated filters trap more particles, they also allow less air to flow through them. This means your furnace has to work harder (which will increase utility costs and decrease its longevity, especially if these filters aren’t changed frequently).
• The Canada Mortgage and Housing Corporation, a Canadian government agency that has researched indoor air quality, discovered that air filters with higher MERV ratings don’t really improve indoor air quality. These filters reduce the number of particles in the air that just passed through them (i.e., air in the furnace and ducts), but not ambient air in the home. Why? Because the air in the home contains particles that are stirred up by our activities (e.g., dust becomes airborne when we sit on furniture, soot is released when burnt toast pops up from the toaster).

Does this mean we shouldn’t purchase high quality furnace filters? Not exactly. Here are a few general guidelines to use when selecting and using a filter.

• Check the user manual for your furnace to see which types of filters are suitable for it.
• A MERV rating between 8 and 11 is sufficient for most homes. Unless your user manual recommends otherwise, choose a recommended filter that falls in this range. Purchase one that fits in your budget (filters with higher ratings typically cost more). You will probably also want to consider the following factors.
  • If your furnace fan is old or has had a lot of strain on it and you are concerned about longevity, then you may want to choose a filter with a MERV rating at the lower end of this range.
  • If your home needs superior filtration because a family member has a health issue (e.g., asthma, severe allergies) or the air in your home is especially dirty (e.g., you have numerous pets, a family member smokes cigarettes indoors), then you may want to use a MERV 11 or 12.
• Replace or clean the filter at the interval recommended by the manufacturer.
• Use trusted approaches unrelated to furnace filters to improve the air quality in your home.
  • Encourage family members and visitors to remove their shoes at the door.
  • Don’t allow family members or visitors to smoke in the house.
  • Dust and vacuum often. Use a vacuum equipped with a HEPA filter.
  • Consider keeping pets outdoors or restricting them to specific rooms.

This information has helped me in selecting the right furnace filters for my home. I hope it helps you, too! For additional information, visit my sources: here, here, and here.

Do you have any questions or additional tips? Please share these with us in the comments section.

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Monday’s Musings and The Art of Homemaking.

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