Furnace smells: burning dust, plastic, gas — which are normal and which are emergencies

One furnace smell is completely normal, one means shut it down, and one means leave the house. Know which is which.

Burning dust on first start of the season

Normal. A summer of dust on the heat exchanger burns off in the first hour of the first cold day. If it fades the same day, no action.

Electrical or plastic burning

Not normal. Overheating blower motor, failing capacitor, or wiring insulation cooking. Shut the furnace off at the switch and call — this is the precursor smell to an electrical fire.

Rotten eggs / sulfur

Gas. Do not flip switches, do not restart the furnace. Leave the house and call the gas utility's emergency line from outside. They come free and fast.

Chemical or formaldehyde smell

The classic warning sign of a cracked heat exchanger letting combustion fumes into house air. Shut down the furnace, ventilate, and get a technician with a combustion analyzer. Take this one seriously — it is the carbon monoxide pathway.

Musty or dirty-sock smell

Biological growth on the coil or in ducts, common when heat first pushes through a damp AC coil. Unpleasant, not dangerous. Coil cleaning fixes it.

Is the furnace flashing a light?

If the burners will not light, the control board is almost certainly flashing a diagnostic code behind the little window. Count the flashes and look the code up by brand here — walking into the phone call knowing the code saves diagnostic time.