Heating / December 1, 2020

Common Causes of Furnace Odors

A smelly furnace can mean anything from a minor to a serious heating issue. You need to diagnose the odor to help you determine how serious the problem is and how to solve it. Below are some of the reasons a furnace might cause odors.


Mold can affect your furnace in two main ways. First, condensation on some parts of the furnace, such as the coil, can encourage mold growth. The risk is especially high if you don’t use the furnace for some time. Secondly, mold can grow in the air ducts and circulate with the warm air as the furnace runs. Clean your furnace regularly to reduce the risk of mold growth.


Dust can accumulate on your furnace, including on the burners, during the cool season when you don’t use the furnace. The next time you start the furnace the dust might burn and fill your house with a burning smell. Fortunately, the smell only lasts for a short while before it dissipates.

Electrical Malfunction

Your furnace requires electricity for some of its functions, such as motor operation. As such, the furnace has electrical parts and cables that can malfunction, just like other electrical appliances. An electrical malfunction that burns components can also fill your house with a burning smell.

Such malfunctions require professional expertise to diagnose and solve. Shut down the furnace and contact a technician if you suspect an electrical malfunction.

Gas Leak

If you have a gas furnace, then a gas leak can also fill your house with a bad smell. Natural gas itself is odorless, but gas producers mix it with a smelly (but harmless) chemical. The chemical makes it easy to detect gas leakages. Here are some of the reasons your furnace might leak gas:

  • Accidental gas line damage
  • Gas line damage due to corrosion
  • Loose gas line fittings
  • Shutoff valve malfunction

Gas leakage is another emergency that requires you to turn off the furnace and all heat sources in the house. Contact a technician to diagnose and plug the leak before you use the furnace again.

Cracked Heat Exchanger

The heat exchanger is a metal contraption that allows the furnace burners to heat the air without contamination by flue gases. The heat exchanger can crack if it overheats. Causes of overheating include:

  • Clogged air filter
  • Blocked air ducts
  • Blocked air registers
  • Thermal stress (due to regular heating and cooling that causes expansion and contraction)

If the heat exchanger cracks, a mixture of flue gases and fuel might leak and contaminate your indoor air. Additionally, the cracked heat exchanger also lowers your furnace efficiency.


There is a limit as to how much heat your furnace can handle. Your furnace might overheat due to:

  • Dirty air filter
  • Motor damage
  • Dirt accumulation
  • Aging (wear and tear)

Overheating can burn different components of the furnace and release a burning odor into your house. You need to diagnose and deal with the overheating before the problem worsens.  

Intake Contamination

In some cases, the smell doesn’t originate from the furnace but rather from the supply air. The furnace draws air from outside the house. Thus, if the air is contaminated and foul, your indoor air will also smell. For example, an open sewer blowout can send sewer gases into the furnace.

You shouldn’t ignore furnace odors, especially if you don’t know their causes. Apart from the discomfort, the bad odor might also mean you have an underlying furnace problem. Mauzy Heating, Air & Solar has the expertise to help you diagnose and fix all manner of furnace issues. Contact us to get your furnace running efficiently again.

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