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Why Is My Air Filter Black?

troubleshooting black air filter

If you’ve checked your air filter to find it black instead of the usual layer of grey dirt and dust, that’s not normal.

So, what’s causing this?

Soot build-up, also called “black soot deposition”, is most likely the reason behind a black filter. 

Black soot deposition inside your home is more than likely caused by:

  • Burning scented candles
  • A gas furnace that isn’t burning properly
  • A gas water heater that isn’t burning properly
  • Gas fireplaces

We’ll show you how to determine which one of these is producing soot in your home and what you can do to fix it.

But first, let’s take a closer look at what black soot deposition is and why it’s harmful.

The culprit: “black soot deposition”

Black soot deposition is really just a fancy name used to describe an indoor soot build-up problem.

Wondering why you suddenly have a large amount of soot in your home?

Well, soot is produced when fuel isn’t burning properly. So, if you have any fuel-burning appliances that aren’t working properly, they’re most likely producing a lot of soot in your house. And, over time, soot collects on the surfaces in your house (walls, carpet, curtains, air filter, etc.).

But soot doesn’t just dirty up your home. It can also make you sick. According to the EPA and the American Lung Association (ALA), breathing in soot particles can lead to lung and respiratory diseases and even premature death.

To help you get rid of harmful soot particles in your home, let’s look at some of the most common home appliances that produce soot.

What’s producing soot in my home?

Scented candles

If you regularly burn candles in your home, there’s a good chance this is the main reason your air filter is black, especially if you burn scented candles. You see, scented candles incorporate a lot of unsaturated oils that don’t burn “clean”.

You’ll know if your candle is producing a lot of soot if the flame is mostly yellow or orange instead of blue.

What to do:

The most effective answer is to stop burning candles. But if that’s just not an option for you, try these soot-reducing tips:

  • Keep candle wicks trimmed to ¼ inch.
  • Keep candle flames undisturbed if possible (i.e. away from vents, fans, hallways, etc.).
  • Don’t buy candles with petroleum jelly or vegetable oil.
  • Buy candles that are hard to the touch at room temperatures.

A gas furnace that isn’t burning properly

Problems with certain parts of a furnace can cause it to burn gas incompletely, causing soot. 

Just like a candle, you can tell if your gas furnace is producing soot by the color of the burner’s flames. If it’s yellow or orange, this is most likely what’s producing soot in your home.

What to do:

Have a professional inspect your gas furnace for the following issues:

  • Cracked heat exchanger
  • Combustion gases aren’t properly venting outdoors
  • Problems with your burners (misalignment, clogged with dirt or damaged)
  • Gas valves that are clogged with dirt or debris

Note: A furnace that isn’t burning properly could also lead to “flame rollout”, a dangerous condition where furnace flames “roll out” of its confined area in search of oxygen. So if you see yellow flames or signs of soot production, try to avoid using the furnace until it’s repaired.

A gas water heater that isn’t burning properly

If you have a gas water heater, an incorrect mixture of air and gas can cause the unit to produce soot.

Again, you can check the color of the water heater’s burner flames to see if it’s creating soot (yellow=improper burning and soot, blue=clean burning and no soot). You’ll also most likely see soot forming around the unit.


What to do:

Have a professional inspect the unit for the following unsafe conditions:

  • Dirty burners/debris blocking burners
  • Debris blocking the flue
  • Lack of oxygen in surrounding area

An improperly vented fireplace

Gas fireplaces are designed to burn improperly because yellow, red and orange flames are more inviting and “homey” than blue flames.

So if you have a gas fireplace, expect some soot. But how much soot a gas fireplace produces can be controlled to a certain amount (see tips below for reducing soot).

What to do:

  • Keep glass doors open at all times the fire is on.
  • Adjust any gas logs so that they steer clear of flame paths.
  • Remove old, cracked, sagging or broken logs.
  • Have a professional inspect the gas burner and gas pressure from time to time.

Want to remove more soot? Upgrade to a carbon activated filter

Soot is dangerous for your health and can cause expensive damage to your home.

If you want a filter that can trap and remove soot particles from your home, invest in an activated carbon air filter. These filters use a 5-stage filtration system that can catch gases, fumes, smoke and soot particles.

Live in the Twin Cities metro area? Contact us today to learn more about our filter and air purifying options.

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Categories: Air Conditioning