Why does my furnace smell like rotten eggs when I turn on the heat?

When your furnace emits a distinct rotten egg or sulfur smell the moment you turn on the heat, it is a critical warning sign indicating a potential natural gas or propane leak. Utility companies deliberately add a harmless chemical odorant called mercaptan—which smells strongly of sulfur or rotten eggs—to otherwise odorless utility gas so that people can detect leaks instantly. Alternatively, if you smell rotten eggs but professional tests confirm there is no gas leak, the odor can sometimes be caused by sulfur gases rising from a dry plumbing P-trap or biological waste trapped in nearby floor drains. If you ever smell rotten eggs, treat it as an emergency: never use open flames or light switches, evacuate all occupants immediately to a safe distance, and contact your local gas utility provider. You can verify safety once professionals inspect and clear the gas lines.