Published: January 11, 2018


Never ignore a weird smell coming from your furnace. One of the most obvious warning signs your furnace needs maintenance is the common smells of burning rubber or plastic and could be anything from a dirty filter which needs changing to a broken electrical component. Identifying the odor’s source and how it smells is the first step to catching the problem early and avoiding costly repairs.

The following odors are serious cause for concern and should be addressed immediately:

  1. Chemicals: a chemical smell similar to formaldehyde typically means your heat exchanger is cracked. Your heat exchanger cycles heat from the combustion chamber to the plenum. Damaged heat exchangers cause a high risk of fire and potential carbon monoxide leaks, and should be dealt with immediately. Call your HVAC professional right away if you smell chemicals coming from your furnace.
  2. Electrical burning: This smell usually seeps out if you have excessive heat inside your furnace, meaning either one of the metal components is overheating or you have a damaged rubber component. The risk of fire starting can be strong here, so it’s necessary you turn off your furnace and call a professional to identify and potentially replace the damaged parts.
  3. Rotten eggs: the smell of rotten eggs or sulfur generally means you have a gas leak. Natural gas is odorless and therefore can cause carbon monoxide poisoning, so gas suppliers add a strong odor to the gas, to warn homeowners there might be a gas leak. If you smell this odor and it isn’t super strong, turn off your furnace and ventilate your entire home. However, if the smell is strong, call a professional and ask for help.
  4. Smoke: Turn your furnace off immediately if you start smelling smoke or if your smoke detector alarm goes off. Smoke typically results from a blocked chimney forcing smoke to go through your ductwork instead of the chimney. If this is the case, professional chimney cleaning is recommended.

The next set of odors usually aren’t cause for concern and occur during normal heating processes. They may require you to call your HVAC professional but can be resolved on their own.

  1. Oil: An oily smell coming from your furnace usually means your filter is clogged and changing the filter should resolve this issue. If the smell persists after changing your filter, call your HVAC professional as you may have an oil leak, which could become dangerous.
  2. Must: The smell of must can manifest if your furnace is damp after a season of non-use. Again, if the smell doesn’t go away on its own after running your furnace for a few hours, call a professional. If the smell stays for several days, you may have mold in your HVAC system permeating throughout your home.
  3. Burning dust: Furnaces naturally become dusty over time, especially if they are located in your basement. Once you fire up your furnace after several months, dust may have accumulated inside the heater. As a result, you may smell burning dust during the first use of your furnace of the season. This isn’t a cause for concern unless it lasts more than one day.

As soon as you notice a foul smell coming from your basement – or rather, your furnace – first, identify what the odor smells like. Then, determine the danger of the situation. If the smell is vague, you can try opening your windows to ventilate your home and see if it goes away. Often, a small change such as replacing your filter can make the smell dissipate. However, if the smell comes on quickly and strongly, you and your family should leave your home immediately and call a professional.

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