Signs of a Dirty Furnace Filter

Learn the factors that cause dirty furnace filters, dirty air filter symptoms to look out for, and what to do about them

While the average lifespan of an HVAC system is approximately 20 years, one dirty furnace filter can cut that time in half – or worse. It is crucial to regularly replace your air filters to avoid a catastrophically expensive, potentially dangerous situation.

Whether your home has a furnace or an air conditioner, the air in it runs through an HVAC air filter. The filter is meant to capture particles like dust, pollen, pet dander, and – depending on the efficiency of the filter – even bacteria and viruses. A clogged air filter can create big problems, so you'll want to recognize the signs and know what to do about it.

Dirty Air Filter Symptoms

Stay vigilant in keeping an eye out for dirty air filter symptoms so you don’t end up with a clogged AC filter becoming a burden on your HVAC system.

  • More dust has been piling up in your home.
  • Your air filter appears gray in color and seems to be covered in pollutants.
  • You see unusual increases in your energy bill.
  • The cycles in your HVAC system have increased in length.

Pollutants that Cause Dirty HVAC Filters

Most homes in the US have a central air conditioning unit that cools the house. In these kinds of systems, air circulates from room to room through ductwork. On its way to the unit, air passes through a filter that removes large particles that could settle on and damage the unit if they don't get trapped. The question on most people's mind is – Why does your air filter get so dirty?

  • Dust, dirt, and fibers – Dead skin cells, dirt tracked in from outside, and fibers from upholstered furniture, carpet, and bedding all 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 pets, clothing, shoes, hair, and skin. Furnace filters made specifically for allergy season are the most effective at trapping pollen particles for cleaner indoor air.
  • Mold spores – Efficient furnace filters for the home 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 May Create a Clogged AC Filter and Cause Dirty Air Filter Symptoms

  • 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. Summertime heat can cause similar problems for air conditioners. Both systems utilize air filters so it’s important to keep an eye out for dirty air filter symptoms year round.
  • Thermostat set to on – This setting means the furnace blower or air conditioner runs all the time. Auto is a better setting for energy efficiency 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. If you have multiple pets and people living at home, you should expect your furnace filter to become dirty faster.
  • Infrequent air filter replacement – The general guideline is to check your filter once a month and change it if it looks clogged. No filter should be installed for longer than three months. Try to change your filter with the changing seasons to keep up with this recommendation or follow our guide on how often to replace your furnace filter.

What Does a Dirty Furnace Filter Look Like?

A dirty air filter is gross – often coated in gray or tan filth that may flake or fall off when disturbed. If it looks like this, it's time to replace your filter.

Potential Problems Caused by a Dirty Furnace Filter

Simply ignoring or accepting a clogged air filter is never a good idea. Not only can the dust cause health issues due to increased dust and allergens in the air, but your HVAC system and furnace filter can malfunction, decreasing the flow of heat or air in your home or freezing condensation on the cooling coils (also preventing proper airflow). Dirty air filter symptoms can lead to serious problems:

  • Recycling of air filled with pollen, dust, mold, and other allergens trapped in the filter.
  • Rise in allergic reactions like clogged sinuses, headaches, itchy eyes and throat, and more.
  • Inflation of energy bills due to an upsurge of up to 15% energy usage.
  • Growth of bacteria, as a result of frozen coils.
  • Ultimate destruction HVAC system, potentially costing you thousands of dollars.

Now that you know what causes your air filter to become dirty, it's time to stock up on replacements. Remember, choosing high-efficiency air filters for extreme seasons can be of great help in keeping the air in your home clean.

For more useful information about caring for your HVAC unit – like how to safely dispose of a dirty air filter, check out our Helpful Tips page.

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