How Did My Furnace Filter Get So Dirty?

How Did My Furnace Filter Get So Dirty?

If your home is heated by a furnace, air circulates from room to room through ductwork. On its way to the furnace to be heated, air passes through a filter that removes large particles that could settle on and damage the furnace if they don’t get trapped.
As you can imagine, furnace filters get congested the longer they are used. Learn why your furnace filter gets dirty and when it’s time to replace it.

Pollutants that Clog Furnace Filters

  • Dust, dirt and fibers: Dead skin cells, dirt tracked in from outside, and fibers from upholstered furniture, carpet and bedding contribute to household dust. As these pollutants are whipped up, they enter the ductwork and clog up the furnace filter.
  • Smoking indoors: Cigarette smoke wafts through the air when you smoke indoors, making its way to the filter and clogging it up faster than if you choose to smoke outside.
  • Pet dander: Dogs and cats shed, which gets hair everywhere. Much of this hair is swept up into the ductwork, where the furnace filter catches it so it can’t re-circulate back into your home.
  • Pollen: Even though flowers grow outside, it’s possible for pollen to hitch a ride on clothing, shoes, hair and skin. Furnace filters for allergy season are most effective at trapping pollen particles for cleaner indoor air.
  • Mold spores: High efficiency furnace filters trap mold spores as they travel through the ductwork. If excess humidity dampens the filter, mold could start to grow here if you don’t change the filter frequently enough.

Other Factors that Clog Up Your Furnace Filter Faster

  • Cold weather: When the temperature drops, your furnace works harder to keep your home warm. This sends more air flowing through the filter and increases the amount of particles that become trapped there, dirtying your filter faster than normal.
  • Thermostat set to “on”: This setting means the furnace blower runs all the time. “Auto” is a better setting for energy efficiency as it runs only when needed and helps your filter last longer as well.
  • A home full of contaminants: More pets, people and dust in your home cause more contaminants to fly through the air, dirtying the filter faster.

How Frequently to Change the Furnace Filter

  • The general guideline is to check your filter once a month and change it if it looks clogged. No filter should be installed longer than three months.
  • Try to change your filter with the changing seasons to keep up with this recommendation.
  • Now that you know what causes your furnace filter to become dirty, it’s time to stock up on replacements. Remember, choosing high-efficiency furnace filters for allergy season can be of great help to keep the air clean.


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