Cast iron baseboard heaters radiate heat using hot water that continually flows through the systems. The cast iron heater emits heat from the top, replacing the cold air near the top of the room. The cold air sinks and is drawn into the bottom of the heater, providing circulation.
The water is heated in a boiler, located somewhere on the property. Boilers can burn oil, natural gas and wood or use electricity. An intricate piping system is used to move the hot water into the baseboard system. As the water cools, it is pumped back into the boiler for reheating.
Cast iron baseboard heaters are usually preferred over the thinner metal varieties that have fins. The cast iron heater sections are cast in one piece, leaving no seams to wear out. The cast iron is also thicker, holding the heat longer than the finned variety. The chamber for the water tends to be larger, allowing the heat to be dispersed more easily from all sides of the baseboard.
The all-in-one design also minimizes the clanging sound heard when the heater is first turned on. Usually this sound occurs when the metal starts expanding with the heat. Cast iron expands very little.