What happens after the furnace warranty ends? If there is a major problem can the defective part, such as a water jacket, be replaced by itself or does the complete furnace need replacing? Don?t get sold on the modular parts pitch. Usually modular parts means small parts and small parts mean more seams and more seams mean increased chances of corrosion. Water jackets are the main component of all furnaces. It is faster and cheaper to repair them on-site rather than exchange them. Modular parts furnaces are not a good investment and selling prices are excessive. As for any other parts, they can easily be replaced on-site.