If the smell is electrical, smoky or does not fade, switch the system off at the thermostat, and at its breaker if the panel is safe to reach. If you smell gas, leave the building and call from outside.
Usually harmless
Dust settles on heating surfaces over the months the system sits idle. The first time it runs again, that dust burns off with a dry, dusty smell that should clear within a short time. A clogged filter can make it last longer.
Not normal
- A sharp electrical or hot-plastic smell, which can point at an overheating motor or wiring
- Smoke from the vents or the equipment
- A burning smell that comes back every time the heat runs
- A rotten-egg smell near a gas furnace, which is a gas leak and not a burning smell
Smell burning? Switch it off
Switch the system off at the thermostat, and at its breaker if the panel is safe to reach. Leave it off until a technician has looked at it. If there is smoke or a fire, leave the building and call 911.
When professional help makes sense
Any smell other than a brief dusty one at the start of the season is worth a diagnosis. A technician checks the blower motor, electrical connections, heating elements and, on a gas furnace, the burners and venting.