Cast iron boilers are a type of firetube boiler as well. In most original boiler rooms, the building was built around or on top of the boiler room. When it came time to replace many of these boilers, there was no convenient way to bring in a new boiler. The cast iron sectional boiler provided a solution to this problem.

Cast iron sections are taken individually into the boiler room and field assembled at the jobsite. While the initial cost is low, much on-the-job labor is required to make the boiler fully operational.

Cast iron boilers are used for closed, low pressure heating systems only. These boilers also have small steaming areas so that they are very sensitive to changes in demand or water levels.

Cast iron boilers have rough surfaces that require a lot of water treatment to maintain proper operation. They are sensitive to scale and require care in blowing down. It is difficult to remove sediment and sludge that form in the waterside of this design. That debris becomes encrusted causing the surfaces to overheat and crack. If a section becomes cracked, the entire boiler must be disassembled.