Why is my furnace filter turning black? - National Refrigeration Blog

Why is my furnace filter turning black?

Thinking about furnace filter turning black

The air filter in your HVAC system trap dust, pet hair and other particles so your system can deliver clean air throughout your home. When these filters are not changed regularly, it can cause your HVAC unit to work harder and breakdown more frequently. This inefficiency can also lead to higher energy bills as well as decreased indoor air quality.

The recommended frequency to change your air filter varies. Some filters may only need to be changed once per year while others may need to be changed every few months. Check with your HVAC technician, or the filter manufacturer, to ensure you are following the suggested air filter replacement schedule. It is important to note, there are some factors that can affect how often a filter needs to be changed. If you own pets or suffer from allergies you may want to monitor your furnace’s air filter more frequently to ensure airborne irritants are not circulating throughout your home.

Typically, air filters are white when purchased and turn gray when it is time to replace them.  But what happens if you check your filter and notice it is black?

Why is a black air filter in furnace more concerning?

A black air filter can be a sign that something is amiss with your HVAC system. This can be potentially hazardous for your household, so it is important to check for the following:

Carbon Monoxide:

One of the first things to check is your carbon monoxide levels. Carbon monoxide (or CO) is a colorless and odorless gas that leaves a sooty residue in air filters. According to the Mayo Clinic, in high concentrations, in humans and pets, carbon monoxide can lead to serious tissue damage or even death. Call a professional immediately so they can check for leaks and ensure your furnace is functioning properly. Also, having CO detectors can help prevent carbon monoxide poisoning.


When mold is present in or around your system, it will grow on your filter. Mold grows when there is condensation. Condensation can be caused by a high humidity level in your home or can be a sign of a faulty system. Catching this issue early is vital as the mold can easily spread through your ductwork and vents, blowing mold spores throughout your home. Black mold has numerous adverse health effects and shouldn’t be ignored. Consulting a professional HVAC technician will allow you to assess the extent of this problem accurately and address it appropriately. 

Dirt or Soot:

If you burn candles frequently or use a fireplace, the soot from these sources contribute to higher air particulate levels and may cause your air filter to turn black. If you have not changed your filter in a while, dust, pet dander, pollen and other debris can buildup and your filter will get darker and darker as these air pollutants collect. Regularly changing your filter is not only important for the health of you and your family but can extend the lifespan of your HVAC unit as well.

If you notice that your air filter is black, or there are any other signs that concern you, it is always best to contact a professional HVAC technician and have them assess the situation. Having your heating and cooling systems regularly maintained can help detect issues before they become a major problem.

If you’re looking for one-on-one expert advice about your HVAC options in New England, contact the National Refrigeration team, and we’ll find the right system for you.

Whether you need 24-7 emergency service or advice on improving your everyday air quality and comfort—at home and work—call us at 401-737-2000 or email us.


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