Furnace Smells Like Burning

Dust accumulates on the heat exchanger over months of inactivity. When you fire up the furnace for the first time in fall, the dust burns off, creating a dusty/slightly acrid smell. Completely normal.

Is this the first time you've run the furnace this season? Smell is dusty, slightly burnt, but not electrical or chemical. No smoke visible from vents.

Motor windings or bearings are overheating, producing a hot oil or burning electrical smell. Often caused by restricted airflow (dirty filter, closed vents) forcing the motor to work harder.

Smell is sharp and electrical, not dusty. Strongest near the indoor unit. May coincide with blower being slower than usual or making a different sound.

A toy, paper, dead insect, or other foreign object has fallen into ductwork or the burner compartment and is scorching or burning.

Smell came on suddenly. Children in the house? Recent renovations? Power off and inspect supply vents (with a flashlight) and the burner compartment.

A wire has frayed, a connection is arcing, or insulation is degrading and burning. SERIOUS — fire risk.

Sharp ozone or plastic/electrical smell. Sometimes visible smoke or scorch marks at the control board, wire bundles, or transformer.

A cracked heat exchanger can produce a slightly burning or chemical smell along with combustion gases leaking into your air supply. SERIOUS — CO risk.

Smell persists beyond first cycle. Yellow burner flames instead of blue. Headaches/dizziness in occupants. CO detector activation.