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Why Is There Soot on My Furnace?

Why Is There Soot on My Furnace?

If you have a furnace, especially a gas-burning furnace, you may one day find soot visible on the outside of the unit. Though this may alarm you at first, don’t panic.

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Soot isn’t necessarily a bad sign, depending on where you find it. Let’s take a look at some of the places you’re likely to find furnace soot, and what it means.

Incomplete Combustion

First, you must understand where the soot is actually coming from. No matter where you find soot on or around your furnace, it always comes from the same place: the burners. As the furnace in your house burns fuel to create heat, especially natural gas, there are remnants left over after the process is completed. This is called “incomplete combustion.”

One of the combustion byproducts is carbon, the primary ingredient in black soot, which is sent up the heat exchanger and out of the house. However, if there is a problem with your furnace’s venting, you may start to find furnace soot in other places. This is where it can become a problem.

Oversized HeaterBurners

Soot that collects on your furnace’s burners can build up to the point where it starts to impede their ignition. A dirty burner may not burn as intensely as it should, lowering your furnace’s heat output. If there is enough black soot, the gas burners in your system may not light at all.

Pilot Light

Just like the burners, the pilot light can become caked with furnace soot over years of use, or if the combustion byproducts are being improperly vented. If you’re using a standing pilot light, this can be particularly troublesome, as it can prevent the light from igniting or staying lit. If your pilot light seems to be having trouble burning, especially if it is a primarily yellow flame instead of blue, you may want to check the pilot assembly.


This is the biggest issue. If black soot is found near where the furnace connects to the ducts, the heat exchanger may be leaking. This can cause combustion byproducts like soot to mix with the air in the house. Carbon monoxide can also be released in this fashion, which is highly toxic. If you see soot in this area of your house, turn the heat off and call a professional right away.

If you’d like to know more, contact Wm. Henderson Plumbing & Heating Inc. We provide dependable and safe heating repair services in Broomall, PA such as heat pump installation, boiler repair, furnace installationradiant heating system replacement. Give us a call by phone at (484) 206-8594 to receive assistance with an emergency heating or cooling issue.