Decide which one this is first
If none of those apply, read on.
The normal one
The first time the heat runs each autumn, dust that settled on the heat exchanger over the summer burns off. It smells hot and slightly acrid, it is strongest for the first few cycles, and it clears within an hour or two of running. If the system has not run since spring and the smell is fading rather than growing, that is what you are smelling.
When it is not normal
- It is mid-season. The system has been running for weeks and the smell is new.
- It smells electrical. Hot plastic or burnt insulation, rather than warm dust.
- It is getting stronger rather than fading over a few cycles.
- It comes with a noise. A shriek, a grinding, or a buzz that started around the same time.
- There is a visible mark. Soot around a register, scorching on the cabinet, discoloration.
Those point at something overheating: a blower motor failing, a seized bearing, a wiring fault or a control.
What causes it
- A blocked filter. Restricted airflow overheats the furnace and bakes the dust on the filter. Free to check.
- A failing blower motor. Overheating windings smell of hot varnish. See blower not working.
- A seized bearing in the blower or the draft fan, smelling of hot metal.
- Wiring or a control overheating. Sharply plastic.
- Something on the ductwork: an item stored against a warm duct, occasionally a rodent.
What to do right now
- Decide whether this is the emergency version. Use the list at the top. If in doubt, treat it as the serious one.
- If it is not, turn the system off at the thermostat. Use the breaker instead if the smell is electrical.
- Look at the filter. If you cannot see light through it, that may well be the answer.
- Do not keep running it to see whether the smell goes away.
- Call, and describe the smell. Dusty and electrical are different problems.
Carbon monoxide
If you burn gas, propane or wood in your house, keep a working carbon monoxide alarm whether or not anything smells wrong: carbon monoxide has no smell at all. Idaho's amendment to the residential code requires an approved alarm outside each separate sleeping area when permitted work is done in an existing home with a fuel-fired appliance, and it is a sensible standard for any home that burns fuel.
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