Why Is My Furnace Filter Black?

November 12, 2019

Floworx Mechanical LLC is a fully licensed and insured heating company serving Pueblo and the surrounding areas. One of the troublesome problems we encounter is furnace filters that develop a dark and even black color. Let us consider the reasons for why discoloration could occur.

Carbon Monoxide

If you do notice that your air filter is black, you should schedule an appointment with your trusted heating service provider right away. The reason is that this discoloration can actually be a worrisome sign. The black substance is actually soot, but in this particular case, it is soot created through inadequate combustion. Carbon monoxide is also a byproduct of that partial combustion.


Mold is another cause of a black furnace filter. This problem usually begins in summer when condensation occurs due to the evaporator coil. The moisture mixes with dust, soot, and other particles. Eventually, mold will develop, and as it does this, this soot-dust-mold mixture will spread throughout the furnace. When it reaches the filter, it’s likely that you already have a serious problem, which is another good reason to schedule a spring tune-up for your air conditioner.

Fireplace Soot

Do you have a fireplace? If so, a blackened furnace filter can be a sign that the fireplace is not properly vented. The soot is backing up into the home and being trapped by the filter. Seasonal inspections of your fireplace are a good idea, and you should schedule one as soon as possible if this occurs.

Other Soot

You can be introducing soot into your home in other ways. Do you burn a lot of candles? It is possible that you are overdoing it and/or lack adequate ventilation. Do you use an oil lamp or a similar product? While beautiful, such lighting can introduce a large amount of soot into a home.

Schedule a Furnace Tune-Up

If you are experiencing a black furnace filter, we recommend you schedule a tune-up as soon as possible. Floworx Mechanical LLC in Pueblo also provides services related to central air conditioning, ductless systems, and indoor air quality. Call today to schedule an appointment.

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