Abstract: An automatic safety system for a fluid fuel air heating furnace. The safety system utilizes a double-sided carbon monoxide sensor for detecting unsafe levels of carbon monoxide interiorly and exteriorly of the furnace's heated air duct and a single-sided carbon monoxide sensor for detecting unsafe levels of carbon monoxide interiorly of the furnace's flue pipe. The sensors employ carbon monoxide detector chips which are electrically conductive in normal conditions, but become non-conductive when exposed to levels of carbon monoxide that are deemed unsafe for humans. The sensors are incorporated in an electrical circuit for energizing a solenoid valve which valve, when energized, opens to permit fuel flow to the furnace. Detection of unsafe levels of carbon monoxide will result in the sensors becoming non-conductive, thereby causing the solenoid valve to become de-energized and to close thereby preventing fuel flow to the furnace.