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Why Is My Furnace Filter Black

By October 12, 2022Furnace
Black air filter

When it comes to owning a furnace, you will want to ensure that all parts and pieces are maintained. Just like maintaining your AC unit and your appliances, you will need to check on your furnace. Some things can happen to your furnace if it is not properly maintained. Getting to know how your furnace filter can turn black is the most important part. Why is my furnace filter black? The accumulation of carbon monoxide (CO) can occur and this will clog the filter.

A clogged filter means the build-up of debris. This can become hazardous to your health. Poor airflow and the reduction of efficiency will create such a problem for your furnace, that it will break down.

Let’s take a look below at what this all means.

What Does It Do

The furnace filter is in place to collect all dirt and debris that is flying around your heating system. Replace the furnace filter regularly. You will want to swap it out before it turns black. Having a brown furnace filter is fairly normal and your first warning sign that a replacement is needed soon.

Soot on a wall

What Causes My Black Furnace Filter

There are a variety of things that can cause a furnace filter to turn black. Let’s take a look below at what those ways are:

  • Carbon Monoxide – this gas is odorless and colorless. It can create soot in your heating system. This soot is then trapped in your furnace filter. Soot is typically a black residue as a result of burning gas. If you have any gas-powered appliances or are located near a neighbor’s garage, the carbon monoxide gas could escape into your home and get sucked into your furnace filter.
  • Mold – when your evaporator coils are left damp due to condensation in your air conditioning unit, mold and mildew can begin to grow. Mold loves moist areas of a home and will not take very long to grow. Once the black mold arises, the spores are then blown through your air conditioning and furnace unit. The black mold then gets trapped in the furnace filter, causing it to become black.
  • Soot – when soot is created in your furnace, it has very limited areas where it could go. Soot will typically affix itself to the walls of your HVAC system. When the furnace is then turned back on and is blowing hot air through your home, the soot often becomes loose from the walls and flies through the air. The furnace filter will trap the soot and create a black filter.
  • Dirty filter – as the filter begins to get dirty, it will continue to collect more dirt and debris that is flying in the air through your air ducts. Dirty filters will slow the efficiency of your furnace and dirt and debris will begin to collect. The next time you run the furnace, there could be a much larger about of dirt and debris waiting to be blown through the system, thus creating a darker and black furnace filter.
  • Clogged Air vents – your furnace filter will turn black if the vents in your furnace are blocked. Your furnace combustion will not work properly and thus black emissions or leaks can end up landing on your furnace filter.

How Much Does It Cost

Depending on the type of furnace filter that you have chosen to purchase will dictate how much it costs. The national average for a standard furnace filter ranges from $10 to $50 depending on the make and model of the furnace.

Recommended Maintenance

Soon there will be cooler temps here in Florida and you may need to run the heater. If it smells like it is burning, it could be dust on the evaporator coils, HVAC belts are burning, electrical components are burning or there is a gas issue. Contacting a professional may be needed.

Next, you might see soot on the wall. This is called “ghosting” on the walls and can be caused by things such as burning candles, door mats being dirty, or drafty windows or doors to name a few. After you figure out the cause, wipe down the walls and buy paint to cover what you can’t erase.

Lastly, when the temperatures do cool, you don’t want cold air blowing out, you want warm air from your heater. If your heater is blowing cold air, then the pilot light might be out, the filter may be dirty, or the thermostat isn’t working.

A dirty air filter doesn't allow air to flow.

When to Call a Professional

Anytime you have an issue with your HVAC system, you will want to call a professional. If all you need is to change your filter, that can be done by any homeowner. However, if there are issues bigger than the filter, you will need a professionally licensed HVAC contractor.

If you aren’t sure who to use, call on your local home inspection team. They can inspect your entire HVAC system and offer up a reputable HVAC service repair contractor.


When your furnace or AC unit begins to malfunction the cost of repairs can mount. Making sure that you hire the right HVAC service company is key. You don’t want a contractor who has little to no experience working on HVAC issues. Call your local home inspection team.

They can inspect the HVAC system and recommend possible repairs. This way you have a list before the HVAC service contractor comes to the home and tries to sell you parts that you might not need. Reach out to Waypoint Inspection to conduct an HVAC system inspection during your home inspection in your home in Tampa and Central Florida.