Steam boilers are closed systems containing water that is heated to produce steam. This steam spreads through pipes that, when heated, radiate the heat throughout a structure.
Steam boilers are typically powered by the combustion of a fuel. While coal boilers were popular options in the past, natural gas boilers are most common in the United States as of 2015. Oil-fueled boilers are still in use due to their efficiency, and some homes, particularly those in remote areas, are heated using wood combustion.
Boilers that do not convert the water to steam but instead warm it and pass it through pipes are inherently more efficient than steam boilers, but the efficiency difference can be minimal. Steam boilers provide simpler operation; steam expands and fills the pipes automatically, while hot water boilers must rely on pumps. This simple design also makes it easier to switch to a different type of fuel, and many natural gas and oil boilers were converted from older coal-based designs.
While steam boilers are not as efficient as hot water boilers, they still provide more efficient operation than most furnaces, which use air instead of water or steam to spread heat throughout a home. Natural gas and oil boilers are most often placed indoors, while wood-fueled boilers are generally placed outdoors, where the smoke and chemicals generated are easier to disperse.