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A boiler is a closed vessel in which water 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.

Construction of boilers is mainly limited to steel and cast iron.

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.

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.