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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" (as in caustic). The Roman architect Vitruvius, writing about the end of the 1st century B.C., attributes their invention to Sergius Orata.
Roman operation 
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 so 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.
Non-Roman analogs 
Excavations at Mohenjo-daro in what is now Pakistan have unearthed what is believed to be a hypocaust lined with bitumen-coated bricks. If it fulfilled a similar role, the structure would pre-date the earliest Roman hypocaust by as much as 2000 years.
In the Republic of Georgia, excavations in the ancient settlement of Dzalisa uncovered a large castle complex in 1984–1985 featuring a well-preserved hypocaust, built between 200–400 BC.
Dating back to 1000 BC, Korean houses have traditionally used ondol to provide floor heating on similar principles as the hypocaust, drawing smoke from a wood fire typically used for cooking. Ondol heating was common in Korean homes until the 1960s, by which time dedicated ondol installations were typically used to warm the main room of the house, burning a variety of fuels such as coal and biomass.
After the Romans 
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.
In the Iberian Peninsula 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.
See also 
- De Architectura
- Roman engineering
- Roman technology
- Gloria (Spanish heating system)
- Masonry heater (similar to Kachelofen in the German Wikipedia).
- Kang bed-stove
- Underfloor heating, (history)
- Winston Churchill (1956), A History of the English Speaking Peoples: The Birth of Britain, Dodd, Mead & Company, p. 35
|Wikimedia Commons has media related to: Hypocausts|
- About Roman baths (referring to Sergius Orata), by William Smith.
- Disputing the priority of Sergius Orata Garrett G. Fagan's paper "Sergius Orata: Inventor of the Hypocaust?" published in Phoenix, Vol. 50, No. 1 (Spring, 1996), pp. 56–66
- Hypocaust heating Wordreference.com
- Diagram of a Roman hypocaust system
- How to build your own hypocaust Channel 4 Time Team project