How do I know if my furnace filter is dirty?

Author: Mrs. Oma Hettinger DVM  |  Last update: Wednesday, July 9, 2025

While having a schedule in mind for changing your furnace filter helps keep your filter clean, you can also look for the following signs it needs replacement:
  1. There is noticeable discolouration on the filter.
  2. You can see the buildup of dirt and dust.
  3. You find your home needs dusting frequently.

What are the symptoms of a dirty furnace filter?

Warning Signs of a Clogged Air Filter
  • Increased Energy Consumption. When home air filters are clogged, furnaces have to work harder to meet the desired temperature of the thermostat. ...
  • Higher Electric Bills. ...
  • Uneven or Poor Heating. ...
  • Short On/Off Cycles. ...
  • Indoor Air Pollution. ...
  • Dust. ...
  • Burning Smells. ...
  • Getting Sick.

How to tell if your furnace filter needs changing?

However, there are some signs you can look for that indicate your filter needs to be changed.
  1. Dirt and Dust Build Up On Filter. ...
  2. More Dust Than Usual In Your Home. ...
  3. The Furnace Cycles Longer Than Normal. ...
  4. Current Filter Exceeds Manufacturer's Recommendation. ...
  5. Burning Smell or Overheating.

What does a dirty furnace filter look like?

The filter will be gray or black and bits of debris will be visible. But you may notice other changes that indicate the furnace is not operating properly.

What happens if you don't clean your furnace filter?

A dirty filter in your furnace will generally lead to the following consequences: The furnace can overheat and turn off as a failsafe. Your home won't achieve the warmth or comfort you want. You can severely damage your furnace's internal components.

Side effects of a dirty filter | Fire & Ice Heating and Air Conditioning

Can I clean my furnace air filter instead of replacing it?

Cleaning an air filter only has two additional steps from replacing one. Again, the first step is to turn off the HVAC system. Find the service panel, open the furnace, and then locate your filter and remove it from the furnace. From here, you'll want to place it dirty side up and gently vacuum the large debris.

Will my furnace shut off if filter is dirty?

If your filter becomes clogged, it can cause your furnace to shut off as a safety feature to prevent damage to the furnace. To fix this problem, simply replace your air filter on a regular basis. If you have pets, be sure to change your air filter more frequently.

Is no furnace filter better than a dirty one?

NO. A furnace filter has a critical job to perform and a filter cannot be run without one. For a furnace to work properly, it should always be equipped with a clean, working filter. As we'll see, a dirty filter can cause problems, and so can having no filter at all.

Where do I check my furnace filter?

Your furnace's air filter can generally be found in a rack at the return or the entrance to the blower compartment. You can confirm its location with your furnace owner's manual. Sometimes instructions are printed on the furnace itself. Turn off your furnace and slide the filter out.

Can a dirty furnace filter make your house dusty?

If your HVAC filter clogs and causes you to change it more often than every 3 months, your home may have high dust levels and poor air quality. You also may be running the system too frequently or using a low-quality filter.

What happens if furnace filter is not changed?

Your furnace filter collects debris, including allergens, mold spores, and dust. If your filter is too full, it will allow this debris through into the ducts, to be spread throughout your home. As a result, your air quality will drop, and you may notice that your allergies or respiratory symptoms get worse.

What does a dirty air filter look like?

A filter that is clogged with dirt is an obvious sign of a dirty air filter. Performing a visual inspection is an easy way to know if your air filter needs to be changed. A brand new air filter is white or off-white in color, while a dirty air filter will appear darker with the dirt and debris visible.

How do I know if my furnace filter is good?

The MERV rating system helps you choose the best furnace air filters to fit your home's needs. Each rating dictates the effectiveness of a filter. The higher the MERV rating, the more efficient the filter.

What happens if you wait too long to change the air filter?

If you don't change your AC filter, it will begin to fail. It will no longer be able to filter the air properly, letting dust and contaminants get into the HVAC system. Dust jams the moving parts of an AC such as fan motors and valves. Airflow is restricted which creates a strain on the system.

Does a dirty filter affect AC?

When the air filter becomes clogged with dust, dirt, and debris, it restricts the airflow passing through the system. This restriction causes the AC unit to work harder, leading to decreased efficiency and reduced cooling capacity. Restricted airflow can even cause the coils to freeze in your outdoor or window unit.

How many runtime hours to change furnace filter?

The good news is that you can expect to change your filters every six months or after 150 continuous hours of usage, whichever comes first with these types of systems. However, if the unit is at least 10 years old, then you may want to replace your filters every three months.

How do I know if my furnace filter is too dirty?

If the filter is still in good condition, light should shine through. If the filter is dirty or clogged with debris, the light will be obstructed, indicating it's time for a replacement. The presence of visible mold is another one of the dirty HVAC filter symptoms that indicates it's time for a replacement.

Do all furnaces have a filter inside?

Both gas and electric furnaces have furnace filters, so you also need to find the right type of filter for your furnace. Selecting the appropriate filter for your gas furnace is crucial.

Where is my furnace filter carrier?

However, common places filters are found can include: within a small slit on the bottom of the unit, on an external rack usually off the side of the furnace, in the ductwork as a cutout, or in the return air vents. If you are unable to locate your filter, please contact your dealer or installer.

What happens if you never change your air filter?

Poor Air Quality

If the filter is too dirty to do its job, those unwanted particles can collect in your ductwork and cling to the internal components of your HVAC system, ultimately diminishing air quality throughout your house.

How often should I change my furnace filter 16x25x1?

Generally, it is recommended that you replace a 16x25x1 furnace filter at least every 45-60 days. However, in certain cases, the replacement may need to be sooner. For instance: You should change your filter every 6 weeks if one of your family members suffer from asthma or severe allergies.

What color is a dirty furnace filter?

Whether black or gray, any dark discoloration entails a dirty furnace filter and should be cleaned. On top of particle pollution, combustion can turn regular build-up into black soot and saturate the entire furnace filter. Pet dander, dirt, and dust don't cause black soot per se, but enough buildup can lead to it.

Does a furnace filter go in the garbage?

Put this item in your black waste cart. Lid must be completely closed. This item and all the contents of your cart will be taken to a landfill. Ensure no organics (fruit and veggie peels, coffee grounds, paper towels) go into your Black Waste Cart.

Will air stop working if filter is dirty?

Is my AC not working because of the filter? Potentially, yes. While there are a whole host of issues that could cause problems with your air conditioning system, a dirty air filter should be the first thing to check if your central air conditioning unit isn't working as properly as it should.

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