Industrial boilers are welded from sturdy steel plates that are engineered to withstand intense heat and pressure - as a result of this thick steel, boilers can sometimes weigh as much as 165 tons! The construction of hot water boilers and steam boilers is very similar. They both feature a cylinder tube, otherwise known as the pressure vessel. The pressure vessel contains something called a flame tube, which is fired through a burner and a reversing chamber that feeds flue gases back through a second smoke tube. These flue gases are reversed again via an external reversing chamber. This reversing chamber sends the flue gases to the end of the boiler, in the third smoke tube pass. A major difference between hot water industrial boilers and the steam versions? Hot water boilers, as their name implies, are usually completely filled with hot water during their operation, while steam boilers are filled with water only until the ¾ mark, with the top fourth of the boiler reserved for steam.