Short answer: A dusty, slightly burnt smell the first time you run your furnace in the fall is usually normal. Dust settles on the burners and heat exchanger over the summer and burns off. It should fade within the first few heating cycles. A smell that lingers past a day, smells electrical or like hot plastic, or smells like rotten eggs is not normal. Each of those has a different next step, set out in the table below.
Why the first run of the season smells
Over the summer, a thin layer of dust settles on the burners, the heat exchanger and the inside of the cabinet. The first time the burners light in the fall, that dust heats up and burns off. The air from the vents can smell dusty or slightly burnt for a short time. Opening a window and running the fan for a while helps it clear faster.
Two things make it worse:
- A filter that was not changed since spring. Dirty filters hold dust that the blower pushes past the hot parts.
- Items stored against the furnace. Boxes, laundry or paint cans close to the cabinet can heat up. Keep the area around the furnace clear.
What the smell is telling you
| Smell | Likely cause | What to do |
|---|---|---|
| Dusty, slightly burnt, first run of the season | Summer dust burning off | Normal. Change the filter, open a window, let it run. Should fade within the first few cycles |
| Burning dust that lasts more than a day | Very dirty filter, dust in the ductwork, or something sitting on or near the furnace | Switch the furnace off, change the filter, clear the area. If it persists, book a service call |
| Electrical, hot plastic or burning rubber | Overheating motor, wiring or a failing part | Switch the furnace off at the thermostat and the service switch. Do not keep running it. Book a repair |
| Rotten eggs or sulphur | Possible natural gas leak (gas has an added odour so leaks can be smelled) | Leave the home now. Do not use phones, light switches or appliances inside. From a safe distance call Enbridge Gas at 1-866-763-5427 or 911 |
| Musty or damp | Moisture in the ducts, a humidifier pad or a drain | Book a service visit; check the humidifier and drain |
| Exhaust or fumes | Combustion gases not venting properly | Switch it off and book a same-day check. If anyone feels unwell, get everyone outside and call 911 |
Carbon monoxide has no smell
The burnt dust smell is harmless. The dangerous gas from a furnace has no smell at all. Carbon monoxide cannot be detected by your nose, so a furnace that smells fine is not proof that it is safe.
Since January 1, 2026, Ontario requires a carbon monoxide alarm on every storey of a home with a fuel-burning appliance, and next to each sleeping area. If an alarm sounds, get everyone outside and call 911 from outside. Our guide to furnace carbon monoxide in Ontario explains the symptoms and the alarm rules.
Before you switch the heat on for the season
- Put in a new filter.
- Clear anything stored within a metre of the furnace.
- Test your carbon monoxide alarms.
- Make sure the outside intake and exhaust pipes on a high-efficiency furnace are not blocked.
- Book the yearly inspection if it has been more than a year.
When to call us
If a smell does not fade after the first day, or it is electrical, chemical or exhaust-like, switch the furnace off and book furnace repair in Mississauga. Our technicians are TSSA certified and serve Mississauga, Brampton, Etobicoke, Milton, Georgetown, Caledon and Bolton. Call 905-564-8545.
Frequently asked questions
Is it normal for a furnace to smell like burning when first turned on?
Yes, a dusty burning smell the first time the furnace runs after summer is normal. Dust on the burners and heat exchanger burns off and the smell usually fades within the first few heating cycles.
How long should the burning smell last?
Usually no more than the first few cycles, often less than an hour of running. If it is still there the next day, switch the furnace off, change the filter and book a service visit.
What does a gas leak smell like?
Natural gas has an added odour that smells like rotten eggs or sulphur. If you smell it, leave the home without using phones, switches or appliances, and call Enbridge Gas at 1-866-763-5427 or 911 from a safe distance.
Can a dirty furnace filter cause a burning smell?
Yes. A clogged filter traps dust that can be pushed over hot parts, and it makes the furnace run hotter. Replacing the filter often clears the smell.
Can I smell carbon monoxide from my furnace?
No. Carbon monoxide has no smell or colour. Only a working carbon monoxide alarm can warn you, which is why Ontario requires one on every storey.
Sources and references
- Enbridge Gas, Safety (what to do if you smell gas; Ontario gas leak line 1-866-763-5427), read October 26, 2026.
- TSSA, Changes to Ontario Fire Code: New Requirements for CO Alarms, read September 3, 2026.
- Health Canada, Carbon monoxide: Overview.
- Aire One Peel, Furnace carbon monoxide in Ontario, and fall start-up service calls by our technicians in Peel Region.