Boilerwater or another liquid is heated under pressure. The steam or hot liquid is then circulated out of the boiler for use in various process or heating applications. A safety valve is required to prevent over pressurisation and possible explosion of a boiler.
Sources of heat for the boiler can be the combustion of fuels such as oil, wood or natural gas. Electric boilers use resistance-type heating elements. Waste-heat boilers use the heat rejected from other processes such as gas turbines.
See also: Furnace
A boiler (colloquial) is also an old hen whose flesh is too tough to eat roasted or grilled, and is therefore only suitable to be boiled in a stew.