What’s That Smell? – Various Furnace Smells and Their Meanings

June 17, 2020

Strange Smells Have Meaning:

Sometimes during the start of the winter months, you may notice a strange smell when you finally fire up your furnace. Sometimes you may even notice a swathe of other kinds of bizarre smells that don’t quite make sense; Smoke and dust are logical, but sometimes you may be greeted with a pungent mustiness, foul-smelling garbage, plastic, or even rotting eggs.

What’s worse is that these smells are constantly being circulated around your central ventilation system, engulfing your entire home with the stench. However, the fact that these smells are so prevalent and appear instantaneously is a blessing disguised as a nuisance. The fact is that these smells do carry significant warnings with them, and should not be ignored.

Common Furnace Smells and Their Meaning-

Commonly reported smells from furnaces have been shown to represent specific malfunctions in the furnace, and thus can provide enough details to spot the source of the problem.


If you suddenly notice a musty, damp, pungent, or stale smell coming from your furnace, then it may be time to get your home/ventilation system inspected for mould. This type of smell may not come from your furnace itself, but the transfer of heat will bring the odour throughout the house. On the other hand mould can still grow inside your furnace, and distribute possibly noxious spores that can be harmful for your health.

What to do:

Do not ignore this sign, and get a mould inspector to check this out. Some species of mould are an acute hazard to your health, potentially causing respiratory problems for anyone exposed to it. There are specialists who will find, remove, destroy, and then remediate the problem for you.


A burning smell from your furnace can be normal, especially at the beginning of winter when some of the excess dust gathered around the filter is burnt off. However, a strong smokey smell can indicate a fire hazard. If a furnace filter is blocked completely, it will cause heat to build up and eventually force the system to turn itself off to prevent an explosion/ overheating. This process repeating itself is called ‘short cycling’ and it can not only threaten the safety of your home, but it can also potential rack up the energy bill.

Sometime furnaces with oil filters will produce an oily/ smokey smell too, which either means the filter must be replaced, or there is a leak.

What to do: A overheating furnace or potential oil leak is dangerous, and should be reported to a technician immediately.

Rotten Eggs/ sulphur-

This is perhaps the most serious smell on this list. If you notice the smell of rotting eggs, sulphur, or just a foulness in general, it could be a gas leak. The natural gas that is supplied into homes and utilized by furnaces is odourless, however, companies install the chemical mercaptan in order to give it a distinct presence. In other words, the sulphuric smell is a safety mechanism.

An unresolved gas leak can result in serious consequences. If the natural gas is only partially combusted by the furnace, then CO (carbon monoxide, which is actually odourless) can be filtered through the ventilation system. Carbon monoxide gas can cause immediate symptoms including dizzines, headache, vomiting, confusion, loss of consciousness, and death within minutes of exposure.

Other problems can occurring if a large amount of gas is able to collect around the home, as any spark or ignition can cause an explosion leading to loss of property, and death.

What to do:

Turn the heating system off and shut off any gas valves if possible. Otherwise, it is best to call emergency services and evacuate the home.

Burning Plastic-

Burning plastic or ‘gunpowder’ smells indicate short-circuiting within the furnace. The distinct smell is caused by the plastic wires melting, which can eventually cause an electrical fire.

What to do:

Have a technician inspect the furnace for electrical problems or anything else that may be structurally wrong with the system.

Furnace Filters:

Furnace filters are an integral part of the home heating system by serving as a filter of pollutants. Often times this cotton/polyester blend of pleated fabric will catch dust that circulates through your home and keep things running normally. However, they are also the first indicator that something is wrong.

As seen above, when something is wrong with the furnace, it usually has to do with the filters ability to block objects. Burning/ strange smells are often the result of blocked/ burning dust excessively covering the filter. As a general rule of thumb, it is best practice to change a disposable furnace filter every 1-2 months of possible.

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