New Hampshire omeowners frequently ask about their HVAC system’s filter color.

Why is my furnace filter black?

Posted: January 27, 2023 at 11:40 am
By: Lakes Region HVAC

Regardless of what type of air filter your heating or cooling system uses, most are white when purchased and turn gray when it is time to replace them. If you remove your filter and see that it is black, that can be a sign that something is amiss.

Carbon Monoxide

If you notice a black furnace filter, one of the first things to do is test your carbon monoxide levels. According to the CDC, "Every year, at least 420 people die in the U.S. from accidental CO poisoning. Approximately 100,000 people in the U.S. visit the emergency department each year due to accidental CO poisoning." While carbon monoxide is colorless and odorless gas, it can leave a sooty residue and turn your filter black. Call a professional immediately so they can check for leaks and make sure your furnace is working properly. Click here to read more about keeping your family safe from carbon monoxide.

Ventilation Clogging

Your venting system can become clogged or start leaking. This leads to improper combustion which can result in a black buildup on your filter.

Old Air Filter

The longer you leave your filter in place, the more soot and debris it can collect. As the dirt and other pollutants pile up, your filter will turn a darker and darker shade of gray. For example, if you use your fireplace regularly or frequently burn scented candles, the soot from these sources will build up on your air filter and turn it black. An old filter can negatively affect your system’s efficiency, create uneven inside temperatures increase utility bills, and greatly decrease the indoor air quality of your home. Regularly changing your filter is essential to staying healthy and comfortable while saving money. Click here to learn why indoor air quality is so important.


If your home has high humidity levels or your system is faulty, condensation can form in and around your unit. If condensation is left unattended, it can begin to create black mold on your filter, in your ductwork, and even in your vents. Black mold can cause respiratory and other health issues, so it's essential to pay close attention to this issue.

It is important to call a trusted HVAC professional if anything seems off with your heating or cooling system.

If you notice a black air filter, or anything else that seems off with your HVAC system, it is always best to contact a professional and have them access the situation. Scheduling regular maintenance and cleaning can help keep your heating and cooling systems running efficiently, effectively and safely.

If you have any questions, one of our trained professionals can advise you further.

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Have another question? Contact the Lakes Region team, and we’ll find your solution.

Our clients are homeowners, small business owners, and corporations—and one could be you by calling 603-286-2290 or emailing

About Lakes Region HVAC
Since 1992

Lakes Region’s expert service technicians and installers are trained in the latest advancements in heating and cooling technologies. We'll help you select the correct comfort system for your home, ensuring that your equipment operates at the highest efficiency — giving you the lowest monthly operating cost.

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LRHVAC’s office hours: Monday - Friday 7:30-4:30
Phone: 603-286-2290
Mail: 86 Bay Hill Rd., Northfield, NH 03276
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