A hypocaust (Latin hypocaustum, Greek hypocauston) was an ancient Roman system of underfloor heating, used to heat houses with hot air. The word derives from the Ancient Greek hypo meaning "under" and caust-, meaning "burnt". The Roman architect Vitruvius, writing about the end of the 1st century B.C., attributes their invention to Sergius Orata, but this is not fully confirmed.
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Hypocausts were used for heating hot baths (thermae), houses and other buildings, whether public or private. The floor was raised above the ground by pillars, called pilae stacks, with a layer of tiles then a layer of concrete then another of tiles on top; and spaces were left inside the walls so that hot air and smoke from the furnace would pass through these enclosed areas and out of flues in the roof, thereby heating but not polluting the interior of the room. Ceramic box tiles were placed inside the walls to both remove the hot burned air, and also to heat the walls. Rooms requiring the most heat were placed closest to the furnace, whose heat could be increased by adding more wood to the fire. It was labour-intensive to run a hypocaust as it required constant attention to tend the fire, and expensive in fuel, so it was a feature of the villa and public baths.
Vitruvius describes their construction and operation in his work De architectura in about 15 BC, adding details about how fuel could be conserved by designing the hot room or caldarium for men and women to be built next to one another, adjacent to the tepidarium so as to run the public baths efficiently. He also describes a device for adjusting the heat by a bronze ventilator in the domed ceiling.
Many remains of Roman hypocausts have survived throughout Europe, western Asia, and northern Africa. The hypocaust was an invention which improved the hygiene and living conditions of citizens, and was a forerunner of modern central heating.
Excavations at Mohenjo-daro in what is now Pakistan have unearthed what is believed to be a hypocaust lined with bitumen-coated bricks. If so, the structure would pre-date the earliest Roman hypocaust by as much as 2000 years.
With the decline of the Roman Empire, the hypocaust fell into disuse, especially in western provinces. In Britain, from c. 400 until c. 1900, central heating did not exist, and hot baths were rare.[1]
In Spain the Roman system was adopted for the heating of Hispano-Islamic (Al Andalus) baths (hammams). A derivation of hypocaust, the gloria, was in use in Castile until the arrival of modern heating. After the fuel (mainly wood) was reduced to ashes, the air intake was closed to keep hot air inside and to slow combustion.
Korean traditional houses sometimes use an ondol, which is similar to a hypocaust, drawing smoke from a wood fire typically used for cooking.