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Why Is My Furnace Filter Black? 6 Common Causes

A furnace filter is the most important part of your furnace when it comes to keeping out dirt and debris and ensuring they don’t compromise the efficiency of your heating system. Once you fix or replace it, a furnace filter is bound to turn brown with time, and that is normal. But if you open your furnace and find that your filter is all black, then something is amiss, and you need to act.

A black furnace filter is often caused by the accumulation of carbon monoxide (CO) and a clogged filter. Soot from candles, the fireplace, or mold are also common causes. Damaged vents are also a possible culprit.

Besides being a possible health hazard, a black furnace filter is also typically clogged, which means poor airflow, reduced furnace efficiency, and eventual furnace breakdown. We’ll tell you along the way what you should do to avoid damaging your furnace because of a clogged filter.

Your Furnace Filter Is Collecting CO From Home Emitters

The colorless, odorless, and poisonous carbon monoxide gas is the most common cause of a black furnace filter. According to the Minnesota Department of Health, Carbon monoxide in a home can come from adjacent garages or fuel-burning appliances such as water heaters, fireplaces, clothes dryers, boilers, stoves and ovens, power tools, and lawn equipment, furnaces themselves, as well as from smoking. 

While your furnace filter is meant to sieve out dirt from other furnace equipment, getting the dirt stuck on the filter compromises its function and the function of the entire furnace. In this light, an all-black filter has the least level of efficiency.

Solution for Carbon Monoxide on Furnace Filter

If your furnace filter is turning black quickly, you may have a serious issue with CO, and you should call an HVAC company to establish the cause and advise you on how to resolve it. Besides being bad for your furnace filter, carbon monoxide is also incredibly dangerous for you and anyone else in the house.

You should also clean and change your furnace filter every 30-90 days. I also recommend installing carbon monoxide sensors at strategic positions in your home, just to be sure you aren’t putting yourself in danger.

There’s Soot in the Surrounding Air

Whether it’s from burning candles, a fireplace, or from gas appliances burning incorrectly with an orange/yellow rather than a blue flame, soot is bad news when it accumulates on your furnace filter.

Solution for Soot on Furnace Filter

As much as you love the fragrance from your scented candles and the warmth of a lounge with a fireplace, some of the soot that is released into the air is bound to settle on your furnace filter. Reduce the amount of candle or wood-burning that you do and get into the habit of dusting and cleaning your furnace filter often.

There’s Mold on Your Furnace Filter

There are several conditions that can create moist environments for mold growth when it comes to your furnace. One such condition is the condensation from running an air conditioner that creates a damp evaporator coil. If left unattended, this condensation mixes with dirt and dust and eventually forms a black mold, which is likely to spread to your furnace filter.

Other favorable environments for mold growth around your furnace include ducts in damp areas of the building, such as the basement, or clogged exhaust pipes that aren’t draining properly.

Solution for Mold on Furnace Filter

Mold is stubborn and will not go away by just removing the mold on your furnace filter or the places that favor its growth. As such, professional cleaning for both your filter and the affected areas is advised.

You should also ensure you do not have damp space around your furnace as this facilitates mold growth.

Your Air Filter Is Clogged and Not Filtering Dirt

If you are sure that mold, soot, or carbon monoxide are not to blame for a black filter, then the filter itself could be clogged and accumulating any form of dirt that comes into contact with it. A clogged air filter will not keep any form of dirt from the furnace, and that compromises your furnace’s proper functioning. 

Solution for Clogged Filter

Checking your furnace filter every 30 days to make sure it’s clean and working properly is a recommended practice. A functioning filter is crucial in ensuring even house heating and quality airflow in your home. 

You should replace filters every 3 months to allow proper furnace functioning. You can follow these steps to do it yourself:

  1. Ensure the furnace is not charged or switch it off if it’s on.
  2. Open the panel cover of your furnace and spot the filter you want to remove.
  3. Hold the filter with each hand on the extreme ends and carefully slide it out.
  4. Slide the new filter into the place you have removed the old one.
  5. Replace the furnace cover.

For a visual demonstration, watch this video:

You Have Clogged Vents Causing Improper Airflow

A black furnace filter may at times be caused by a clogged or leaking venting system. This is because blocked vents cause incorrect combustion at the furnace, with possible black emissions or leaks landing on your filter.

See this case on Reddit where the venting system has a horrible-looking black substance. The substance turns out to be melted tape or thumbing compound used to seal the piping, which got overheated and melted. 

Solution for Clogged Vents

If you suspect that your vent system is clogged, call a professional HVAC repair service and have them checked and cleaned or repaired if need be.

Other Possible Causes of a Black Furnace Filter

A furnace filter can get black simply by doing its work, which is to keep dust and debris from other parts of the furnace.

So, if there’s plenty of dust and smog in the environment or the furnace room and other parts of your home are not dusted regularly, this dust could find its way to the furnace. With accumulation, the dust will make your filter extremely black. See from this thread on Reddit just how black your filter can get.


  • Chris Hewitt

    Chris is a Texas-based freelance writer who loves the outdoors and working in his garage. When he's not enjoying the Texas sun, he can be found tinkering with all sorts of things in his workshop.

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