Apart from the occasional smell of burning dust, the air that comes from your vents should be odor-free. Other smells can sometimes be hard to narrow down, but they usually point to an issue that needs to be addressed quickly to prevent health hazards and potentially serious damage.
Carbon Monoxide
If you've started smelling something like rotten eggs, your furnace's issue is likely due to poor ventilation. Normally, gases produced as a byproduct of the heating process are filtered harmlessly away from your home. If this venting process is no longer happening, which can sometimes happen if your vents are clogged or blocked, these gases can instead back up into your ducts and make their way into your home. Carbon monoxide is harmful to breathe, so shut your heater off, open some windows to filter out the gas, and make sure the issue is fixed as soon as possible before using your furnace again.
Pest and Pest Waste
Some pests, such as insects and rodents, find ducts an appealing method of travel. Ducts let them easily travel to rooms with food and nesting materials, and are usually safe from predators. If these pests make regular use of your ducts, especially if they start making nests or leaving droppings, you will start to smell it whenever the air kicks on. Like gases, the source of these odors can be dangerous to breathe, especially for those with allergies. On top of making sure the pests are completely removed, you may also need your ducts to be cleaned to remove any waste and trace of the pests.
Mold Growth
The inside of your ducts are rarely damp enough to promote mold growth, as moisture is pulled from the air before it travels into your house. If water is getting into your ducts by other means, such as a leak of some kind, then mold can start to grow inside your ducts. Because of the regular air flow, mold can easily spread as spores are carried through your ducts and from room to room. Mold will usually give off a strong musty odor, so if this smell becomes more prominent when your heater is running, mold could be the culprit. This issue will also likely require duct cleaning, and you should avoid using your furnace in the meantime to prevent the mold from spreading any farther.
Burning Plastic
If you start smelling something like burning plastic or rubber, this often indicates a potentially serious problem with some part of your furnace's electrical components or wiring. This smell usually comes from something like a protective coating starting to melt. If this problem is left alone, you run the risk of electrical fires and other damage, which can be very dangerous. As soon as you start noticing this smell, shut off your furnace and avoid using it again until it can be inspected and repaired by a professional.
Contact your local HVAC contractor for more information.
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