Why Is My AC Filter Turning Black?

June 5, 2017

Your air conitioner filter is necessary to prevent dust and debris from settling on and possibly damaging delicate HVAC components. If you purchase a higher-efficiency filter, it also removes tiny airborne particles to clean your indoor air.

As your AC filter traps pollutants, it starts to become clogged. This blocks airflow, reducing system efficiency and possibly causing overheating or other issues. Plan to replace your air conditioner filter every one to three months or according to the manufacturer’s recommendations to avoid these problems.

Things That Cause an AC Filter to Turn Black

When you replace your AC filter, you may notice it isn’t just clogged—it has turned completely black! Under normal conditions, the filter should collect grey-colored dust and debris. If the filter is black, here are the possible causes.

Candle Soot

Black soot is a byproduct of burning candles, especially scented candles. If this is the reason your AC filter is turning black, you may also notice soot stains on the walls, carpet, and furniture.

The best way to prevent candle soot from turning your AC filter black is to stop burning scented candles altogether. But if you love your candles and want to continue enjoying their scent, follow these helpful tips:

  • Choose candles that feel hard to the touch at room temperature, a sure sign of fewer impurities and therefore less soot.
  • Trim the wick to one-quarter inch before lighting the candle each time.
  • Avoid buying candles with petroleum jelly or vegetable oil mixed into the wax.
  • Keep airflow around the candle to a minimum to reduce soot production.

Gas Water Heater Soot

Anything powered by natural gas—including your furnace, gas fireplace, and water heater—can produce soot if it operates improperly. Since you’re unlikely to run the furnace or fireplace in the summer, the water heater is the most likely appliance to cause a black AC filter.

Check the color of the water heater’s flames to see if it’s burning cleanly. You want to see a blue flame, which indicates a clean burn and no soot. A yellow or flickering flame could mean you have a problem. To reduce the soot coming from your gas water heater, call a technician to clean the burners and remove any debris blocking oxygen flow.


When you run your AC unit, condensation collects on the evaporator coil. This moisture can transfer to the filter where black, sooty mold can begin to grow. Replace the filter as frequently as once a month to prevent this.

Schedule Air Conditioner Services in Cincinnati

If you’re concerned about the condition of your air conditioner air filter, give Rick’s Heating & Cooling a call. We’ll assess the problem and possibly recommend a different type of filter to better meet your home’s air filtration needs. We can also tell you about our indoor air quality products that can help clean up your air even more, including air purifiers and whole-house humidifiers.

To work with a company that has over 30 years of experience serving Cincinnati and the surrounding areas in southwestern Ohio, contact Rick’s Heating & Cooling today. We deliver all the HVAC services you need to stay cool and comfortable this summer.