Designed water heating boilers can be vertical or horizontal. Vertical boilers are three-pass boilers with a minimum capacity of 8 MW equipped with moving grate furnaces. The main element of these boilers is the drum with mounted boiler shells. The installations on its bottom protect from overheating. Capacity of the horizontal boilers is up to 5 MW. Furnace is installed under the boiler. This type of boiler comprises a cylindrical shell with chamber and two vertical flue gas convection sections.