A carbon monoxide (CO) detector should be placed at least 15 to 20 feet away from a furnace to ensure accurate detection of CO levels and to avoid false alarms caused by small, harmless amounts of CO that may be emitted when the furnace starts up. Placing the detector too close to the furnace could result in frequent, unnecessary alarms. Instead, position the detector near sleeping areas or on the same level as the furnace, ensuring that it will alert you to any dangerous buildup of CO. If your furnace is located in a utility room or basement, make sure the detector is placed outside of this room but still close enough to detect CO if a leak occurs. The detector should be placed in a central location where it can monitor CO levels effectively, but not directly in front of the furnace or near vents where airflow might interfere with its readings. Proper placement ensures that the detector will alert you in time if carbon monoxide from the stove becomes a serious issue.