Bad Furnace Smells and What They Mean

For most homeowners, a furnace isn’t necessarily high on the list of priorities. As long as it’s working and puts out warm and comfy heat, a furnace doesn’t capture the attention like, say, an immaculate kitchen or an en suite retreat. After all, the furnace is something that works in the background. Typically, it’s hidden out of the way in your basement, garage or in a utility closet, and as long as it provides heat when you need it, your furnace doesn’t really deserve a second thought.

But if after a long summer you flick on your home’s furnace and you don’t get the warm heat you expect, or if you’re hit with bad furnace smells and you’re wondering what they mean, you may need to call a professional. Not only can a bad furnace smell be dangerous, but a malfunctioning furnace could leave you and your family without heat when you need it most. 

Some people think that they need to call 911 or the gas company if they encounter an odd smell, but while that can sometimes be the case, odds are you just need to call an HVAC pro. They’ll be able to determine whether the smell is normal or if you need to engage in proactive or preventative maintenance — or if you need a new furnace altogether. 

Normal Smells

While it can be disconcerting to fire up your furnace and be hit with bad furnace smells, that doesn’t mean something’s wrong. Some smells are actually normal, especially after not using your furnace for an extended period of time. 

A musty or burning smell could just be your furnace burning off the dust and other stuff that has settled during the off-season. When the furnace clicks on, dust in the heat exchanger is burned off in about an hour or so, and your ducts could also have a build-up that gets disturbed and thrown about as the furnace runs. 

However, if the bad furnace smells don’t subside in quick order, or if it gets any worse or another smell starts to develop, you have reason to be concerned. Remember, just because your furnace was running fine for the first hour doesn’t mean that it’ll run that way later in the day or tomorrow or forever. 

That said, a typical furnace has an expected operational life of about 10 to 20 years. If your furnace is older than that but it hasn’t given you any problems yet, you may want to be proactive and have it checked out or replaced. Not only could your aging furnace be on the verge of failure, but newer furnaces are much more energy efficient, so you could actually save money and make up the difference in a year or two with reduced energy bills.

Bad Furnace Smells To Look Out For

While some smells are harmless, others can be outright dangerous. The important thing is knowing the difference and whether a strange smell warrants concern or if you can get away with waiting it out. So without further ado, here are the smells that may need some extra attention from an HVAC professional.

1. A Burnt Dust Smell

While we already covered this smell earlier, it bears repeating if only because it’s the most common bad furnace smell you’re likely to encounter. Fortunately, this type of smell is mostly harmless, if not a little annoying. It’s because after the warmer season, you’re likely starting up your furnace for the first time when the mercury dips and your home needs help keeping the air warm during the colder months.

Just like how dust settles on furniture in an unused room, your unused furnace can accumulate quite the dust storm after months of non-use, sending an odd burning smell throughout your home the first time you fire it up. The good news is that this burning dust smell is temporary, and after a few heating cycles the odor should clean up on its own as the dust burns off. If you want, you can actually dust and vacuum your furnace before starting it up to avoid sending the acrid smell throughout your home, but there’s no real harm in just waiting it out.

2. Musty Smells

Like the burnt dust smell, your furnace can also emit a musty, bad furnace smell — and it doesn’t necessarily mean that you’re looking at repair or replacement. Sometimes this is the result of sharing an air handler with a split air conditioning system, or it can be a function of where your furnace sits. Dark, damp environments such as basements and some garages can exacerbate this, especially when they sit unused for extended periods of time like over the summer. 

A clogged air filter can also cause a musty smell, which is easily remedied by swapping it out and seeing if the smell subsides. Running your furnace for a few cycles can help eliminate dampness and odors, but if they don’t fade after a few days or it gets worse, you could have a bigger problem on your hands. A mold or mildew infestation could actually be taking hold in your furnace or ducts, and for the health and safety of all inhabitants it’s best to call a professional to come take a look.

3. Smelling Oil

For oil-fired furnaces, odd smells aren’t that out of the ordinary. If your furnace was recently refueled, it could be that a small oil spill is responsible for an oil smell around your furnace and in your ducts over the next few days. However, if the smell is still hanging around for a week or longer, you could be facing a larger issue. 

A continuous smell of oil could point to an oil leak, which may be sending noxious fumes throughout your home each time the furnace kicks on. Dirty or malfunctioning burners may also be producing bad furnace smells, and that may mean you need to call a professional out to remedy the issue.

4. Exhaust Fumes

If you have a partially or fully blocked chimney, harmful and noxious fumes may be redirected back into your home instead of safely exhausting outside. Worse yet, a chimney blockage may lead to a build-up of dangerous carbon monoxide fumes in your home whenever the furnace is running, so it’s important to periodically check your furnace’s exhaust system for any developing obstructions.

If an obstruction is found or you’re noticing an increase in exhaust fumes around your furnace or throughout your home, it’s important to call an HVAC professional. Exhaust fumes are poisonous in large doses due to the presence of carbon monoxide, and if you don’t have working carbon monoxide detectors in your home you may not even be aware of the problem until it’s too late.

A Rotten Egg Smell

As anyone that’s kept a carton of eggs well past their expiration date can attest, a rotten egg smell is one of the most off-putting smells out there. But it’s also an indicator that you might have a gas leak. Because natural gas and propane is odorless, suppliers add methyl mercaptan to the gas that is sent off to homes. This ingredient is colorless like natural gas and propane, but it puts off a strong odor akin to rotten eggs or cabbage.

If you smell this particularly bothersome bad furnace smell, it means you probably have a gas leak. Especially if the smell starts shortly after you fire up your furnace, it’s important to shut the furnace off and to exit your home immediately. Once you’re outside, place a call to your gas company or the local fire department to turn off the gas supply to your home and to trace the source of the leak. If your furnace is deemed the culprit, you’ll need to call out a HVAC pro to repair or replace it.

Electrical Burning Smells

If you detect any electrical or metal burning smells while your furnace is running, you likely have a failing furnace, or at least some kind of overheating or an electrical short issue. That’s because when electrical wiring burns, it gives off a pungent, metallic odor as shielding melts and wires spark. Furthermore, furnace components that are made of metal can also give off bad furnace smells when they overheat.

Just like a rotten egg smell, if you detect any electrical burning smells, it’s important to shut down your furnace immediately and call an HVAC technician out to determine what’s wrong. If you neglect to do so and continue to run your furnace in this state, you may cause further damage to your furnace and raise the risk of electrical shock or even fire, which could end up spreading throughout your home.

Burning Plastic Smells

Similar to any metallic or electrical burning smells, a plastic burning smell could indicate a significant problem with your furnace. Burning plastic is usually the result of a plastic component failing, or at least suffering significant damage, and it’s best to call an HVAC pro out to take a look. Circuit boards that have been heat-damaged can also produce a burning plastic smell, so it’s important to be diligent if you notice any of these bad furnace smells.

Unlike some of the other minor smells, a burning plastic smell is a serious issue. You’re either at or beyond the point of failure of an important component, and it would be inadvisable to continue using your furnace in the hopes that it’ll solve itself. It won’t, and you’ll want to call an HVAC technician out as soon as possible so you can repair or replace your failing furnace.

From the HVAC Pros at Gillette HVAC

If you’ve just detected some bad furnace smells and you’re wondering what they mean, you need the help of Gillette HVAC. No matter the issue, we’ll come in and diagnose your furnace, advising you on the best course of action and helping you regain control of your home’s heating system. We provide first-class residential HVAC service from Salt Lake to Nephi, and with over 15 years of experience, we have the know-how and expertise to fix any issue. Call us today!