A steam boiler or boiler is a closed tank in which water vapor is produced for use outside of it by the heat caused by fuel combustion. Inside the steam boiler, there are two parts: the fire side and the water side. The heat level of the boiler refers to the sum of all levels on the fire side of the boiler. All internal and pressurized parts of a boiler are made of iron alloys. Steam boilers are divided into two types, cast iron and steel. Cast iron boilers are made to produce low-pressure steam, and for this reason, they are mostly used in cases such as central heating systems. Most of the steam boilers are made of steel. Cast iron boilers are also produced. Also, two types of boilers are generally made in the world, which are water tube and fire tube.