Why Your Furnace Filter Is Black – And How To Fix It

More than half of all American homes use furnaces to heat the air in their main living spaces. Yet, after a few weeks or months of use furnace filters can turn black, with many people not knowing why or how to fix it.

Furnace filters can become black because of carbon monoxide, a clogged filter, soot, or mold. Other reasons include improper air flow, and excessive dirt and/or debris. Depending on the cause the filters may be cleaned or need immediate replacement. Regular maintenance is required to prevent a furnace filter from turning black. 

This guide will take you through all the possible causes of a black furnace filter, explain how to diagnose each reason, and, most importantly, provide step-by-step instructions on how to solve the problem.

Your subscription could not be saved. Please try again.
Welcome to Plentiful Air! Your subscription has been successful.


We use Brevo as our marketing platform. By clicking below to submit this form, you acknowledge that the information you provided will be transferred to Brevo for processing in accordance with their terms of use

5 causes of a black furnace filter

1. Carbon Monoxide

Carbon monoxide is one of the most prevalent and significant causes of black furnace filters. It is an odorless and colorless gas caused by incomplete combustion, and can be detrimental to human health in severe circumstances.

It’s widely known for causing soot accumulation on the furnace filter and throughout the home, and is usually recognized by a yellowish-brown stain on the surfaces surrounding the furnace, and as a black residue on the furnace filters.

There are many sources of carbon monoxide. Carbon monoxide production can come from the furnace itself or from elsewhere in the house.

Carbon monoxide can be produced by a leaking gas fireplace, smoking, or damage to the furnace. According to the Centers for Disease Control and Prevention (CDC), carbon monoxide levels that are too high are dangerous and can possibly cause death if not addressed quickly.

Diagnostic features for carbon monoxide.

Because carbon monoxide has no smell or color, you may not notice it unless you have a carbon monoxide detector installed. Although, you may be able to detect the presence of carbon monoxide by watching for physical signs in yourself and your family, such as:

  • Frequent headaches
  • Blurred vision.
  • Vomiting.
  • Weakness
  • Confusion.
  • Loss of consciousness.
  • Dizziness.
  • Chest pain.
  • Shortness of breath.
  • Nausea.


  • Clean and replace your furnace filter every 2 to 3 months.
  • If the leak is coming from the furnace system, such as through cracks in the heat exchanger, get it fixed as quickly as possible (contacting a professional is recommended).
  • Keep debris out of vents and flues, as this can clog ventilation systems.
  • Every year, have a skilled technician service your furnace, heating system, water heater, and any other gas, oil, or coal-burning appliances.
  • Invest in carbon monoxide sensors and position them strategically in your house to alert you in case of gas leakages.
  • Consider learning what safety steps to take upon discovering a carbon monoxide leak. Some of these safety tips include:
    • Being aware of carbon monoxide poisoning symptoms.
    • Moving out of the room immediately in case of carbon monoxide detection and performing an immediate counting to ensure everyone in the household is safe.
    • Opening windows in case of carbon monoxide leakage.
    • Contact a local fire department as soon as carbon monoxide is detected.

Also read about how to prevent dirty furnace filters releasing carbon monoxide here.

2. Clogged filter

A clogged filter is another reason your furnace filter is black. Over time, furnace filters naturally become clogged and can appear black.

The filter’s main purpose is to keep particulates from the heating system. When a lot of dirt, soot, and mold get trapped in the furnace filter, it becomes clogged.

When this happens, the furnace filter turns black, indicating that it needs to be replaced or cleaned. A blocked air filter will not keep any dirt out of the furnace, jeopardizing its proper operation. Blocked or clogged furnace filters can also lead to the release of carbon monoxide.

A lack of frequent maintenance typically causes a blocked furnace filter. Cleaning the furnace burners and filters is a critical component of routine maintenance. When these parts of the furnace are neglected, the filters quickly become clogged.

Diagnostic features for clogged furnace filters.

  • Turn off the furnace system.
  • Open it and access the filter.
  • Examine it closely; if it is clogged, you will see debris, dust, or a color change.
  • If you have not changed or cleaned a furnace filter in the last four months or more, it’s time to do so.
  • Other signs include;
    • Low air flow.
    • Higher energy bills.
    • Reduced furnace efficiency.
    • Poor indoor air quality.


  • Clean the furnace filter.
  • If it is totally worn out, consider replacing it with a new one.
  • Schedule regular maintenance and cleaning of your furnace filter.

How to clean and replace furnace filters

  • To clean it use a hose and spray it directly until it is clean.
    • Allow it to dry completely before using it.
  • To replace it, simply dispose of the old filter.
  • Remove the new filter from its packaging and slide it into the filter compartment location.
  • Then turn the power back on.

3. Soot

Soot can lead to furnace filters turning black. In fact, it’s one of the main reasons why a furnace filter can quickly turn black. There are various causes of soot that are everyday household items, including Candles, fireplaces, and gas water heater use.

Candles emit a lot of soot, and some of it sticks to the walls, furniture, carpet, and other surfaces. Some soot enters the furnace and sticks to the filters, which is why the furnace filters are black. Some will also wind up in your lungs as you breathe the soot-contaminated air, which can lead to breathing issues.

Some people use scented candles for meditation or to disperse aromas throughout their homes. Unfortunately, many scented candles are made with unsaturated oils, which generate soot when burned and may end up in the furnace filter.

Fireplaces, on the other hand, are another source of soot.. Routine maintenance, cleaning, and a system and air quality check can help prevent this problem, especially during the winter months.

Another primary cause of soot around the house is gas water heaters. This is usually apparent when the flame turns from blue, indicating a clean burn to flickering, yellow, or orange, resulting in soot production.

Diagnostic features of soot.

  • If you have a fireplace, use candles, or gas water heaters, among other things that cause soot.
  • If any of your gas appliances are creating a yellow flame indicating a soot production flame.
  • Yellowish-brown stains on walls and furniture.


  • Consider cleaning the filters or replacing them if you notice it is clogged due to soot.
  • Consider using high-quality natural candles made from soy or beeswax candles rather than those made from petroleum jelly or vegetable oil because they produce soot.
  • If you must use scented candles, trim the wicks to a quarter inch before lighting them each time to avoid soot.
  • Instead of using soft candles, think about using hard ones – e.g., beeswax.
  • To keep soot from spreading, utilize vases and hurricanes that are open on both sides.
  • Avoid excessive airflow around candles as this will cause soot to form.
  • Check to see if your gas water heater’s flue pipe, which vents exhaust outdoors and can contribute to soot build-up, is clogged or blocked, and have it fixed.

4. Mold

Mold can cause a furnace filter to turn black. Typically, mold grows and thrives anywhere that is damp or wet. While we may not think of furnaces as wet places, condensation can quickly build-up from the heat exchangers and can leak from the drain line or pump.

If your furnace has any leaks it quickly becomes a conducive environment for mold to infest and thrive, resulting in the black furnace filters that you are seeing. Usually, furnace filters are not supposed to become wet, but if they are it is a sign that your furnace system requires urgent maintenance.

Have a technician check the heat exchangers, condensate drain line, condensate pump, drain pan and/or coils for leaks (the area of the leak will depend on if you have a gas or electric furnace).

Different types of mold that can develop on a furnace filter

  • Stachybotrys 
  • Mucor.
  • Acremonium.
  • Chaetomium.
  • Cladosporium.
  • Alternaria.
  • Fusarium.
  • Penicillium

Diagnostic features of mold.

  • Turn off the entire furnace system.
  • Access the system’s filter.
  • Examine it for signs of mold, such as a musty smell coming out of the filter.
  • Does the filter look slimy and surrounded by black dust? That also indicates mold.

Some other signs of mold include:

  • Frequent allergic reactions to you or a family member who has allergies to mold.
  • Other signs of mold reactions include a stuffy nose, red and itchy eyes, coughing, and wheezing.


  • Discard the furnace filter immediately after you discover mold infestation.
  • Consider buying an air purifier that eliminates mold and its spores.
  • Make sure there isn’t any moist space surrounding your furnace system since this will encourage mold growth.
  • A mold professional should be engaged to clean the affected furnace system regions if the mold problem is extensive
  • Regular maintenance and cleaning of your furnace will help keep mold out of your system and prevent your filter from turning black due to mold exposure.

5. Improper Air Flow

Improper air flow is another common reason for black furnace filters. This occurs as a result of clogged air vents. At the end of a heating cycle, a venting system on a furnace usually guarantees that the hazardous gases are evacuated from the heat exchangers and burners. 

This means that clogged vents cause incorrect combustion in the furnace, sometimes resulting in black emissions or leaks landing on your filter, which is why your furnace filter is black.


  • Replace the furnace filters as needed and as directed by the manufacturer.
  • If the problem is clogged vents, get a professional repair provider to inspect and clean or repair the system.

Why is my furnace filter black after one week?

A furnace filter can become black after just one week because of extremely dusty conditions or high usage of soot-producing equipment. A cheap, low-quality filter type can also cause quickly result in a black furnace filter.

Why does my furnace filter get dirty so quickly?

A furnace filter can become dirty quickly because of a high concentration of contaminants in the air, or  the filter type being used is cheap and of low quality. Extreme temperatures and incorrect settings can also lead to a dirt furnace filter when there is an abundance of dirt, debris and/or soot in the home.

Russell Singleton

Russell holds a Bachelor of Science (Environmental and Marine Geoscience) with Class I Honors. He is currently completing his doctorate in science and is passionate about all earth processes, especially isotope geochemistry and paleohydrology.

Recent Posts