Why Is My Furnace Filter Black?

January 5, 2020

Inside your home’s heating and cooling system, you’ll find an air filter that protects the unit and keeps the indoor air quality clean. Although the filter is out of sight and out of mind, it’s important to keep an eye on it. While it’s normal for a dirty air filter to look gray or off-white, a black furnace filter can indicate a serious problem. Our Guidry's Air Conditioning & Refrigeration, Inc technicians can help you get to the bottom of the problem with our reliable heating repair services.

If your air filter is black, it may be caused by one or more of the following:

Air Filter Needs to Be Changed

Your air filter is designed to trap dust, dirt, and other debris that otherwise enter your furnace. If you don’t change your filter regularly, the build-up of dirt will eventually turn the filter black. This effect is especially dramatic if you frequently burn candles or use your fireplace. You can keep your filter fresh by changing it every three months at a minimum, more often if you have pets.


When your air conditioner runs during the summer, condensation builds up on the evaporator coil, which becomes the perfect breeding ground for black mold. Given enough time, the mold can even spread to your furnace and air filter. This can spread mold spores throughout your home, which is a serious health hazard. Changing your air filter at the end of summer, when the weather is at its dampest and most humid, can stop mold for spreading.

Carbon Monoxide

Black filters are also a sign of a carbon monoxide leak. Carbon monoxide is a dangerous, odorless gas that can leak from old and malfunctioning furnaces. As exhaust fumes from the furnace leak into your house, it clogs the air filter with black soot. If you find soot in your air filter, it’s a good idea to have your furnace examined by a technician.

Look to Us for Furnace Service

If your furnace filter is black, but you’re not sure what’s causing it, we can help. At Guidry's Air Conditioning & Refrigeration, Inc, we understand that trouble-shooting a complicated furnace is no easy task. That’s why our Raceland technicians will examine your system and get to the root of the problem. Our team provides high-quality HVAC repair, air conditioning maintenance, and heating installation services in Raceland, LA.