What Causes Furnace Odors From Vents: 10 Possible Reasons


Smelly furnace odors are problematic. They could be indicative of something very serious or a simple repair. It is key to determine what the cause of a smell is, to ensure you aren’t putting yourself at risk health-wise as well as to confirm there isn’t a risk to your property.

These are the possible reasons on what causes furnace odors from vents:

1. Gas Leak

A gas leak is the most common reason for what causes furnace odors from vents. If your furnace runs on a natural gas system and you’re getting a smell like rotting eggs, unfortunately, that’s a decent sign there’s a leak. If you suspect a gas leak, shut off your furnace immediately.

Action is needed as soon as possible. Take you, other people in the house, and any pets and leave. Call your gas company and notify them of the issue. They should send a representative to inspect your home for a gas leak.

2. Dusty Furnace Odors

A dusty smell sometimes comes in the fall when you first turn on your furnace after months of shutdown. Any sort of odor like a dirty sock or dust is common and nothing necessarily to worry about. The cause is dust and mold burning off the vents and/or off the coil.

You know this to be the case if the smell disappears after a few minutes. To avoid this odor, simply get the coils cleaned, filters changed, and a tune-up done every fall before starting up your furnace. If you have allergies, this can help save you a bit of hassle.

3. Musty Furnace Odors

A musty smell is different from dust. Dust and debris accumulate during dormancy and that is its own smell. Musty odors are common in ductwork that is shared between the furnace and air conditioner. Musty odors come from dark and damp environments.

Clogged or unchanged air filters are likely the source. Especially if they’ve accumulated moisture, musty odors can present. After running your furnace a few times, if the musty smell remains, it may be more complicated, which brings us to our next furnace odor source.

4. Mold or Mildew

Mold grows anywhere there’s food for it to nourish. Furnaces and ductwork are common areas for mold to grow because in these areas, moisture traps and builds. Thereby, you have spores feeding on dust, dirt, and debris inside these dirty ducts.

Daily exposure to some types of mold can lead to brain issues and even have fatal consequences. Fortunately, the fix is easy. Clean the ductwork and remove the moisture source.

5. Burning Metal

There’s electrical wiring in your furnace. When it burns, it can create a very acidic, metallic smell. Any damaged or unshielded wiring cause electrical shorts which means sparks, melted insulators, and a variety of hazardous issues. As they burn or overheat, the cutting metallic odor is unmistakable.

If you smell any sort of metal odor coming from your furnace vents, shut it down immediately and call a technician. Continuing to operate your furnace could lead to electrical shock and/or permanent damage.

6. Burning Plastic

There likely isn’t burning plastic in your furnace but even so, it can smell like burning plastic. A furnace component may have failed, such as a heat-damaged circuit board.

This could also be coming from another part of your electrical work rather than sourced in the ductwork. The only way to know for certain is with an inspection from an HVAC technician.

7. Noxious Fumes

Your exhaust chimney can become partially obstructed or blocked. If this happens, it will redirect noxious exhaust fumes back into your home. This is incredibly dangerous and at times, difficult to detect. This can also cause carbon monoxide buildup although, as you might already know, it’s impossible to smell it.

This is all why it’s imperative to ensure you have working carbon monoxide detectors installed and get regular checks done on your furnace.

8. Sewage Smells

Sewage smells can come up through your furnace vents. It isn’t common but it’s something to be aware of. It could be a sign a sewer line has opened near your system or that there’s a broken wastewater line somewhere near it. An inspection of external vents can usually identify the problem.

If not, however, you may either need an HVAC technician or a plumber to fully resolve the issue.

9. Oil Odors

In oil furnaces, they require a fuel refill from time to time. The person doing the refill may have spilled some oil. This is the source of most oil odors from furnace vents. If this proves to be the source, the smell should fade after a few days at most.

That said, you may also have malfunctioning burners. If the aroma is intense enough to be nauseating or if the odor remains after a few days, have a professional evaluate the situation.

10. Dead Animal Smells

Hope that you don’t have to deal with a dead animal that’s passed in your ductwork. It doesn’t happen often but it does happen. When it does, there’s a pretty clear smell that comes across. Sometimes rodents find their way inside ventilation systems and cannot find their way out.

Eventually, they die. Once again, the fix is easy – remove the animal. This likely requires a professional to inspect and identify where the animal is.