Yeah.. but the hot water never boils. Your right though. Hot Water (Hydronic) Heat Water is heated in boiler, usually to between 160 and 180 degrees Fahrenheit. Pumps circulate the hot water through pipes in the building. This heated water warms radiators placed in the rooms. These radiators, in turn, warm the air in the room. In some homes, pipes are buried in the concrete slab. This is type of system is called radiant heat. Many people prefer (hydronic) hot water heat, because the radiators are small, the system typically quiet, and it can be easily divided into multiple zones. Hot water heat has made steam heat obsolete in homes and smaller buildings. Cherry picked from here. http://www.heimer.com/information/building_codes.html