vessel in which water or other fluid is heated. The fluid does not necessarily boil. (In North America, the term “furnace” is normally used if the purpose is not to boil the fluid.) The heated or vaporised fluid exits the boiler for use in various processes or heating applications, including water heating, central heating, boiler-based power generation, cooking, and sanitation.
In much of the temperate climate zone, most detached housing has had central heating installed since before the Second World War. Where coal was readily available (i.e. the Anthracite coal region in northeast Pennsylvania) coal-fired steam or hot water systems were common. In more recent times, these have been updated to use fuel oil or gas as the source of combustion, eliminating the need for a large coal storage bin near the boiler and the need to remove and discard ashes after the coal is burned. Coal-fired systems are now mostly reserved for larger buildings.
A cheaper alternative to hot water or steam heat is forced hot air. A furnace burns fuel oil, which heats air in a heat exchanger, and blower fans circulate the warmed air through a network of ducts to the rooms in the building. This system is cheaper because the air moves through a series of ducts instead of pipes, and does not require a pipe fitter to install. The space between floor joists can be boxed in and used as some of the ductwork, further lowering costs" />