What is A good rule of thumb is that the exhaust flue for an oil furnace should?

A good rule of thumb is that the exhaust flue for an oil furnace should be at least 6 inches in diameter and made of a non-combustible material, such as stainless steel or galvanized steel, to ensure safe venting of combustion gases. The flue should also have a slope of at least 1/4 inch per foot to facilitate proper drainage of condensate. Additionally, it is important to maintain the flue to prevent blockages and ensure efficient operation. Always consult local building codes and manufacturer guidelines for specific requirements.