A hot water radiator is an integral part of a central heating system in which water is used as the heat transfer medium. The heated water flows through pipes into the radiator, where it transfers heat to the surrounding air. Hot-water radiators come in a variety of shapes and sizes and are made from materials such as steel, cast iron or aluminium. They offer efficient and even space heating and their output can be controlled by thermostatic valves. They are suitable for homes, offices and industrial buildings.