Furnaces heat air and distribute the heated air through a building using ducts; boilers heat water, providing either hot water or steam for heating.
Furnaces are the most common heating systems used in homes in the United States. They can be all electric, gas-fired (including propane or natural gas), or oil-fired.
Boilers consist of a vessel or tank where heat produced from the combustion of such fuels as natural gas, fuel oil, or coal is used to generate hot water or steam. Many buildings have their own boilers, while other buildings have steam or hot water piped in from a central plant. Commercial boilers are manufactured for high- or low-pressure applications.
Most medium-to-large facilities use boilers to generate hot water or steam for space heating, domestic water heating, food preparation, and industrial processes.
In homes with boilers, steam is distributed via pipes to steam radiators, and hot water can be distributed via baseboard radiators or radiant floor systems, or can heat air via a coil.
Steam boilers operate at a higher temperature than hot water boilers, and are inherently less efficient, but high-efficiency versions of all types of furnaces and boilers are currently available.
Visit the Energy Saver website for more information about furnaces and boilers in homes, including efficiency ratings, retrofitting or replacing systems, maintenance, and ventilation.