Furnaces heat air and push the heated air through a duct system in a building. Boilers heat water and provide warmth to a home either through steam or through water traveling in pipes, as in a radiant floor system.
New boilers and furnaces are comparable in terms of energy efficiency. All heating systems are rated by annual fuel utilization efficiency (AFUE). This rating is determined by the percentage of heat created by the system in relation to the amount of energy used. Today's furnaces are between 78 and 96 percent AFUE. Boilers made after 1992 have at least 80 percent AFUE.
Boilers usually take up less space, as the piping is smaller than furnace duct work. However, boiler pipes are constantly filled with water, which could freeze in extreme weather conditions.