How many feet must the flue gas exit of a chimney be above the highest point on a roof?

The correct answer indicates that the flue gas exit of a chimney must be three feet above the highest point on a roof. This height is crucial for ensuring that the emissions from the chimney are safely discharged away from the building and do not enter occupied spaces. The additional height helps prevent down drafts and ensures proper ventilation of the exhaust gases, which is vital for the safety and efficiency of heating systems.

The requirement for this height stems from building codes and safety regulations designed to protect people and property from the potential hazards associated with improper flue gas dispersal. Adequately raising the flue gas exit reduces the risk of carbon monoxide and other harmful substances re-entering the building. It's also important for ensuring that smoke and other emissions rise above the roof line, minimizing nuisance and health risks to nearby occupants or neighboring structures.

The other options suggest lower heights which may not provide adequate clearance for safe operation and proper venting, emphasizing the importance of adhering to the three-foot requirement for the flue gas exit.