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Why is My AC Filter Black?

Why is My AC Filter Black?

If your home has an HVAC system, it’s best to replace the air filter every month. In one of those check-ups, you may notice regular dust buildup. However, if your AC filter is entirely black, that’s never a good season. 

A black AC filter can be caused by soot, mold, or clogs. When left unattended, it can lead to bigger issues in your HVAC system, requiring costly repairs and replacements. Unfortunately, none of these problems can be fixed without the help of a professional HVAC technician, but you can prevent them. 

Keep reading to learn why your AC filter is black and how to prevent each of the three causes. 

3 Reasons Your AC Filter Is Black

Your air filter can turn black due to soot buildup, mold growth, and a clogged HVAC system.

1. Soot

Soot is the most common cause of a black air filter, and it works pretty rapidly to clog up your AC. It mainly comes from your gas water heater or candles around the house.

If you constantly have candles burning in your home, their smoke can create soot buildup in your air filters and reduce their efficiency. But, of course, giving up candles entirely is impractical, so you can prevent soot buildup by trimming the candle wick each time you light it.

If you have a propane water heater in your home, it could also be the reason your AC filter is turning black. A blue flame in your gas water heater indicates a clean burn, while others produce smoke. A flickering flame can also contribute to soot buildup in your air filter.

How to Prevent It

Here’s how you can prevent soot buildup in your AC filters:

  • Opt for candles that are manufactured in America, as US-made candles are created under certain strict guidelines. These regulations guarantee that candles don’t produce a dangerous amount of soot. Candles manufactured overseas are more likely to be created under free-flowing guidelines, so they may contain more soot-producing materials.
  • Some candles may include vegetable oil or petroleum jelly mixed into the wax, which you should avoid. These additives can cause the candle to produce more soot and clog the air filter.
  • Don’t put candles in places where there is regular air movement. When candle flames are repeatedly disturbed by air movement, they can produce 3000% more soot. So it’s best not to place your candles near the AC, directly under ceiling fans, or in frequently used areas.

Read More : What Is the Reason Behind a Black AC Filter?

2. Mold

Black soot is bad for your AC filter, but mold is much worse. Mold may grow on your AC filter or within your AC system due to condensation collecting on the evaporator coil. In ideal conditions, this moisture should drain the drain pipe or into the drip pan.

However, if any of those parts are cracked or clogged, the condensation goes to the air filter. A wet air filter creates an ideal environment for mold to grow, especially during humid days.

If your AC filter has mold, it’s crucial to replace it immediately and call a professional to ensure there’s no mold growth elsewhere in your home. Aside from being unhygienic and smelly, mold can be detrimental to your health. It may cause issues like pneumonitis, fungal sinusitis, asthma, etc.

Mold in your AC filter is a problem that requires immediate attention, so make sure to call your HVAC technician right away. They will inspect your HVAC system to ensure mold hasn’t grown to other AC parts.

How to Prevent It

Here’s how you can prevent mold from growing in your AC filter:

  • Maintain regular clean-ups for the evaporator coil in your AC system. This coil collects condensation, which should transfer to the drain in ideal working conditions. However, this moisture may build up due to a clogged drain and lead to mold growth.
  • Regularly drain and clean up the AC’s drip tray, which collects moisture from the condensation. It can transfer moisture to other parts of the system when it overflows and creates mold.
  • Change your filter at least once a month and call an HVAC technician if it gets wet frequently. 

3. Clogs

If you’ve checked your air filter for soot or mold and found none, you may be dealing with a clogged air filter. A clogged AC filter will lead to restricted airflow, which hinders the air quality within your home and damages your HVAC system simultaneously.

Plus, it will do a poor job of effectively stopping pollutants from infiltrating your home’s air quality. As a result, you and your family may be breathing in pet dander and other pollutants. At the same time, a clogged air filter can ruin the performance of your HVAC system. 

The clog prevents air from smoothly passing through your AC system, which causes it to run longer and ensures that your home reaches the desired temperature. Not only will this strain your HVAC system, but it will also raise your electricity bills.

How to Prevent It

Here’s how you can prevent your AC filter from getting clogged:

  • Check your AC filter every couple of weeks to ensure it’s not collecting an abnormal amount of dust and debris. This can block the airflow and cause unnecessary strain on your HVAC system. 
  • Replace your AC filter once a month, even if it seems in relatively good condition. Call your HVAC technician to guide you if your filter often gets fully clogged before the month ends. 
  • Invest in a high-quality, activated carbon filter for your AC, which uses a five-stage filtration system to purify your home’s air. The last stage in this system consists of activated carbon, which can trap tobacco smoke, toxins, fumes, gasses, and chemicals produced by gas water heating systems and candles.


Now that you know what’s causing your AC filter to turn black, you can fix the issue with its dedicated solution. Plus, you can take certain preventive measures to ensure it doesn’t happen again, as these problems can wear down your HVAC system unnecessarily and lead to failure.

If none of the above-mentioned issues are the reason behind your AC filter turning black, you can contact your HVAC technician to guide you.