Why is my furnace filter black?

So, you’re changing your gas furnace’s filter and notice that it’s got nasty black sooty substance on it.

Should you be worried?


Gas-fired furnaces shouldn’t produce soot. And, when left alone, soot can cause pricey problems like:

  • Insulating the heat exchanger, lowering your furnace’s efficiency as a result (meaning bigger energy bills)
  • Clogging flue (exhaust) pipes
  • Creating a safety hazard since it’s very combustible. If the soot in a vent is ignited, the resulting fire in the vent can catch your house on fire.

So what’s causing this sooty mess?

Two common problems:

  • Burning scented candles
  • Improperly burning furnace

Burning scented candles: a problem you can solve

Everyone needs their pumpkin-spiced scented candles around October, right? Well, that may be the cause of the soot problem.

According to Certified Restorer, Chris Cole, scented candles are major soot creators because “waxes that have unsaturated fragrance oils mixed with them inherently produce more soot.”

Once the scented candle create the soot, your return duct (the metal register) sucks up the soot, depositing it on the filter.

How to fix it:

  • Stop burning scented candles.
  • Scented or unscented, you should, “Keep candle wicks trimmed to ¼ above the wax pool…longer wicks introduce more fuel and make the mix too rich, creating excess soot,” says Cole.

Improperly burning furnace: a problem a tech needs to solve

If you don’t burn candles, the situation probably lies within the furnace. Most likely the furnace is improperly burning the gas (also called “incomplete combustion”), creating soot as a result.

Incomplete combustion is caused by:

  • Too much fuel
  • Too little air
  • Improper mixing of air and fuel

To prevent soot from forming, your technician should do the following:

  • Ensure that the burner orifices are the correct size.
  • Check the manifold gas pressure and adjust if
  • necessary.
  • Check the combustion air openings in the appliance
  • room.
  • Adjust the air shutters on the gas appliance to produce
  • a blue flame.
  • Perform a combustion analysis check.

SourceFundamentals of HVACR (2nd Edition) (Page 805).

You don’t need to know what any of that jargony stuff means. You just need to keep your furnace tech accountable and make sure he or she does it.

Need furnace help in metro Kansas city area?

If you’ve ruled out candles and think your furnace is the issue, call Santa Fe Heating for help.

We guarantee our workmanship for 365 days, and you’ll get an upfront price to repair the furnace before the work is done (no surprises!).