News and Helpful Tips

Why Does my AC Filter Turn Black?

Two-stage furnace

We don’t have to remind you how important it is to change your air filter at least every two months. The filter traps dust, dirt, pet dander, and other debris, preventing these particles from circulating throughout your home. Not only does a clean filter improve the quality of your indoor air, it extends the life of your air conditioner so you stay cool and comfortable during those warmer months. But what if the AC filter turns black despite changing it on a regular basis? Read our blog as we discuss reasons why your filter may not be doing its job and how this can affect your home’s HVAC system.


No homeowner likes to hear the “M” word unless it’s a gourmet French fromage. However, if your AC filter is black, that’s reason enough for concern, mon amie. Mold thrives in a moist, humid environment. When the air conditioner runs, condensation collects on the evaporator coil. Under normal circumstances, the excess moisture should drain to the outside of the unit. However, sometimes it deposits on the filter itself, causing black mold to grow. If you notice your filter is black, throw it out immediately and call an HVAC professional for an inspection.

Black Soot Deposition

Yes, this really is a thing and it’s due to burning none other than scented candles. Candles add warmth, ambiance, and mood lighting to your home, not to mention the delightful scents. But burning them often inside can result in black soot depositing on your walls, carpet, furnishings, and, you guessed it, the air filter. Scented candles are made from paraffin wax, which is a carbon-based material. As it burns, it releases tiny impurities into the air that get caught by the air filter. Over time, the excess deposits build up and clog the air filter, reducing airflow and affecting indoor air quality. If you must burn candles, choose ones made from natural materials such as soy. Also, trim the wicks to ¼” and keep the flame away from vents or ceiling fans.

The filter is clogged

Lastly, if the air filter looks black but you’ve determined it’s not from mold or soot, look a little closer. It may just be extremely dirty or clogged. Again, a dirty filter is unable to trap debris and prevent it from circulating in your home, putting excessive strain on your HVAC system. We recommend changing your air filter every one to two months. However, if you notice excessive debris on your filter despite changing it often, call a heating and cooling professional to schedule an inspection.

Family-owned and operated since 1980, Heating & Cooling Two can help service all of your HVAC needs. Contact us today to schedule a repair or routine maintenance.