Why does my furnace smell when it first turns on?

The first heat of fall almost always comes with a smell. All summer, dust settles on the heat exchanger and burner assembly; the first burn cooks it off. If the smell fades within an hour or so and does not return, that is the furnace equivalent of a new-oven smell — harmless.

Three smells break that rule. An electrical or hot-plastic smell that persists points at a failing blower motor or wiring insulation — shut the system off and have it looked at. A musty smell every time the blower runs is usually the coil or ductwork, worth addressing but not an emergency. And the smell of gas — rotten eggs, sulfur — is the one that skips the queue entirely: do not flip switches, leave the house, and call the gas utility and then us from outside.

Waco-area furnaces spend nine months asleep, so first-fire problems cluster into the same week the first cold front lands. If your furnace made it through last winter on grit and prayers, a fall check costs far less than the January emergency visit.