7 Common Furnace Odors and Their Causes

When you use your furnace, the last thing you expect is for your nose to be treated to a variety of strange smells. Unfortunately, your furnace can produce odd odors — some of which are relatively harmless, but others could indicate an ongoing safety hazard and the need for immediate HVAC service.

So which odors are worth worrying about and which odors can you safely ignore? The following identifies seven of the most common odors you may encounter as you operate your furnace this season.

1. Burnt Dust

As you settle in for a spectacular fall in the Treasure Valley, chances are you’re starting your furnace for the first time in months. But just as unused items can gather dust in your home, your furnace collects layers of dust and debris during its dormancy. The accumulated dust eventually burns away when you finally fire up your furnace, producing a tell-tale burning odor.

Burnt dust isn’t a problem for your furnace in most cases. After a few heating cycles, the odor should clear up on its own as the last of the dust is burned away. You can dust and vacuum your furnace to remove some surface dust and minimize burning odors, but not much else needs to be done.

2. Musty Odors

Your furnace can also give off musty odors upon startup, especially if your furnace shares the same air handler as your split air conditioning system. Musty odors can manifest themselves from dark and damp environments, especially after long periods of inactivity. Clogged or otherwise unchanged air filters can also cause your furnace to smell musty.

Just like the burning dust odor, running your furnace for a few heating cycles will dry up any dampness and eliminate musty odors. If the musty odors don’t go away or they get stronger as time goes on, then you may be dealing with mold or mildew. Have a HVAC professional perform a thorough cleaning of your furnace and its ductwork.

3. Oil Odors

If you have an oil-fired furnace, it can occasionally give off unwanted odors. If your HVAC professional spilled any oil while refueling your furnace, for example, the smell can linger for a few days before dissipating entirely.

Oil leaks can also cause odors, so have a heating and cooling professional carefully inspect your furnace for leaks. Improperly burned oil caused by dirty or malfunctioning burners can also produce nauseating aromas.

4. Noxious Fumes

A partially or completely blocked exhaust chimney can redirect noxious exhaust odors back into your home. Chimney blockages can also cause carbon monoxide buildup inside your home, so have your furnace’s exhaust system thoroughly checked and clear all obstructions as soon as possible.

As a precaution against carbon monoxide poisoning, you should always check and maintain your carbon monoxide detectors.

5. Rotten Eggs

Natural gas and propane are odorless and colorless in their natural states, making them nearly impossible to detect. For this reason, natural gas and propane suppliers add methyl mercaptan, an organic compound, to their gases prior to distribution. Methyl mercaptan is also colorless, but it creates a very powerful odor similar to rotten eggs or cabbage.

The only time you should smell rotten eggs in your home is when there’s a gas leak. If you suddenly get a strong whiff of this highly unpleasant odor after you’ve started your furnace, shut it down and exit your home immediately. Once you’re safely outside, call the local gas company or fire department to shut off your home’s gas supply and trace the source of the leak.

If your furnace is the source of the gas leak, contact an HVAC professional and have them repair or replace your unit.

6. Metallic Odors

Electrical wiring inside your furnace can create a distinctly acrid, metallic odor as it burns. Electrical shorts caused by damaged or unshielded wiring can create sparks and melt wiring insulators. Furnace components made entirely from metal can also give off strange metallic odors as they overheat.

If you smell any metallic odors coming from your furnace, shut the unit down and have your HVAC technician search for any issues that could cause overheating. Operating your furnace in this condition could lead to further electrical damage and the risk of electrical shock.

7. Burning Plastic

Like metallic odors, the smell of burning plastic usually indicates the damage or failure of a plastic furnace component. Heat-damaged circuit boards can also give off acrid plastic odors. Whenever you smell burning plastic from your furnace, have your HVAC technician check the unit for any signs of damage.

No matter what odors your furnace makes, you can count on your HVAC professional to sniff them out. Contact our team of knowledgeable experts at Advanced Heating & Cooling for your heating needs throughout Boise and the surrounding area this season.


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