What Causes a Black Sooty Furnace Filter?

Dirt

• Furnaces are generally located in basements and will naturally develop a layer of dust and dirt on them. The filter is especially susceptible to dirt buildup, as it is responsible for cleaning the air as it pushes it throughout the house. Prevent this by cleaning the furnace thoroughly every year. You can hire a professional to thoroughly clean it, or you can tackle the project yourself with the use of a detailed guide.

Mold

• In the summer months, the air conditioning coils in the furnace can cause the dirt and dust on the furnace to dampen. If you constantly use your air conditioner, the dirt will stay damp long enough for mold spores to land and begin to grow on them. Once this happens, the mold will quickly spread throughout the furnace and can completely cover the furnace filter. Mold can range in color from white to gray to black, and it often has a sooty appearance. Prevent this by cleaning the furnace before or during the summer or by installing a dehumidifier in your home to lower the amount of spores in the air.

Fire

• Soot is a byproduct of yellow flames, and it can build up over time and eventually cause sooty deposits on your furnace filter. This happens more commonly in homes that have a fireplace that doesn’t vent properly, but it can even occur if you use candles, especially scented ones. These candles produce a yellow flame and will create smoke. As the smoke rises, it cools and will deposit soot. This soot can eventually move to your furnace filter. If this is the cause of the soot on your furnace filter, it is likely that you will also see the soot deposits on your walls, floors and ceilings. This is also a more common problem for new homes that have very tight construction.

Filter Maintenance

• Once your filters become clogged with black soot, you’ll need to carefully remove them and install replacement filters. New HEPA filters need to be replaced every three months, while you should change non-HEPA filters every month. When doing this, immediately put the filter into an empty trash bag to contain the black soot so that it doesn’t spread throughout your home. Check the new filter every few weeks to ensure it is still intact and that it is filtering properly.