Dirty furnace filter symptoms and how to fix it

Dirty air filters can cause havoc on your health and HVAC system. Luckily, dirty furnace filter symptoms can give you clues that it’s time to replace a clogged filter.

Will you see the signs before you pollute your air or cause furnace damage?

Luckily for you, today’s article is all about identifying the symptoms of a dirty air filter. We’ll show you the exact steps you need to take in order to protect your air quality and avoid costly HVAC repairs.

Dirty Furnace Filter Symptoms and How to Fix it

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What are the symptoms of a dirty air filter?

Surprisingly, there are many signs your ac filter is dirty and needs to be replaced. Have you experienced any of these problems?

Dirty furnace filter symptoms

  1. Increased energy bills.  A dirty furnace filter means your AC’s fan or blower needs to work harder to maintain your home’s temperature. With the increased work, your energy bills will increase as the system is running inefficiently.
  2. Longer heat time. A dirty air filter restricts the amount of air that gets heated or cooled. A lack of airflow means your system can’t heat air and pump it into your home. So yes, your centeral heat will be affected by a dirty filter!
  3. Dirty home. Dirty air vents are the first clue that your furnace filter needs replacing. If you are seeing streaks of dust near the vents, check your filter. Your home will become dirty because your furnace can filter out dust so it gets pumped into your home.
  4. Increase in allergies. Again, your filter cannot remove any dust, dander or pollen. As a result, you may see an increase in allergies, coughing or sneezing.
  5. Frozen evaporator coils. The evaporator coils are located just downstream of your fan/blower. A lack of airflow can cause the evaporator to freeze up during a cooling cycle.
  6. HVAC failure. Unfortunately, if this problem is ignored long enough your HVAC system will fail and it can be costly to repair.
  7. Short cycling. Does your furnace turn on and off frequently? This is a sign you may have a dirty filter.

As you can see, it’s important to replace your furnace filter at least once every 90 days.

How do I know if my furnace filter is dirty?

If you experienced any of the symptoms listed above then you may have a dirty furnace filter. However, the easiest thing to do is check your filter for signs of dirt, ware and tare.

Here’s what a dirty furnace filter looks like after being left in service for too long.

Clean vs dirty air filter

You can see the filter on the right is overdue for a filter change. The filter on the left is a new filter. Unfortunately, this filter was in service for seven months or about four months too long!

What factors contribute to a dirty furnace filter?

Causes of dirty air filters

Constantly running your HVAC unit is one of the fastest ways to dirty your filter. Your filter will continually trap dirt because the air is constantly cycling through the filter.

Another problem is the number of people and pets who occupy your home. People are surprisingly dirty, which means you’re constantly introducing dirt into your home. More people means you’ll have to change home air filters more often. Smoking homes will also foul up the filter quicker.

Buying cheap air filters also causes your filters to foul up sooner. Cheap filters aren’t designed to last long and should be replaced once per month. Unfortunately, cheap air filters can cause more damage to your unit over time if you forget to replace them consistently.

Owning a large home is another culprit for dirtying furnace filters. Larger homes require more volume of air to pass through the filter in order to service the entire house. Yep, more air through the filter means a dirtier filter!

Can a dirty furnace filter make you sick?

Did you know your home furnace filter is responsible for filtering out at least 10 airborne problems? A clean MERV 13 air filter will remove dust, lint, dust mites, mold, pet dander, smoke, smog, virus, and bacteria. All of which can help reduce cough and sneezing attacks.


Removal of these 10 pollutants is especially important to allergy sufferers. Improving your home’s air quality is important if you don’t want to become sick.

If your furnace filter becomes dirty then it can no longer remove these 10 airborne pollutants. Unfortunately, this can cause problems like mold to spread through your duct work.

So yes, having a dirty furnace filter can make you sick.

Not sure which filter is best for you? Check out our guide on selecting furnace filters and determining the right MERV rating.

When should I replace a home air filter?

Checking your filter for dirt is the best indication for replacement. However, there are some general rules of thumb that you can follow if you don’t want to go check the filter.

In general, replace your home’s air filter every 90 days unless you see significant dust. Families with a pet should consider changing their filter every 2-3 months. Allergy sufferers and multiple pet owners should change filters monthly.

What benefits will I see if I replace my filter?

When you replace your HVAC filter, you should expect better system performance. As a result, you’ll be more comfortable with quicker heating and cooling times. You’ll enjoy better indoor air quality and lower electricity bills. You will also be protecting your system and should expect a longer operating life.

Where can I get a replacement filter?

Nordic Pure makes it easy to find a replacement filter for your furnace. Enter in your filter size in the upper left corner of the website and search for your filter.

Select filter size

Hit “FIND YOUR AIR FILTER” and you’ll be matched with your furnace filter replacement options.

Nordic pure order screen

As you can see, Nordic Pure has filters ranging from MERV 7 to 15. I do not recommend anything MERV 10 and under or 14 and higher.

MERV 14 and higher may cause restricted airflow for some HVAC units and the cost is substantially higher. MERV 10 and under does not filter nearly as well as 11 or 12, but costs roughly the same amount.

Therefore, I highly recommend you purchase a replacement filter at MERV 11 or 12.


Questions or comments? Let me know in the comments below!

John is the founder of TightFist Finance and an expert in the field of personal finance. John has studied personal finance for over 10 years and has used his knowledge to pay down debt, grow his investment portfolio, and launch a financial based business. He is committed to sharing content related to personal finance based on his experience in his career, investing, and path towards reaching financial independence.