Hypocaust

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Caldarium from the Roman Baths at Bath, England. The floor has been removed to reveal the empty spaces which the hot air would flow through.
Caldarium from the Roman Baths at Bath, England. The floor has been removed to reveal the empty spaces which the hot air would flow through.
Ruins of the hypocaust under the floor of a Roman villa. The part under the exedra is covered.
Ruins of the hypocaust under the floor of a Roman villa. The part under the exedra is covered.

A hypocaust is an ancient Roman system of central heating. The word literally means "heat from below", from the Greek hypo meaning below or underneath, and kaiein, to burn or light a fire. They are traditionally considered to have been invented by Sergius Orata, though this is not fully confirmed.

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[edit] Operation

Hypocausts were used for heating public baths and private houses. The floor was raised off the ground by pillars, called pilae stacks, and spaces were left inside the walls so that the hot air and smoke from the furnace (praefurnium) would pass through these enclosed areas and out of flues in the roof, thereby heating but not polluting the interior of the room. Rooms requiring the most heat were placed closest to the furnace, whose heat could be increased by adding more wood. 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 25 BC, adding details about how fuel could be conserved by designing the hot room or caldarium for men and women should 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.

The hypocaust is generally regarded as a major Roman invention which improved the hygiene and living conditions of citizens, and was a forerunner of modern central heating.

[edit] After the Romans

A derivation of hypocaust, the gloria, had been in use in Castile until the arrival of modern heating. After the fuel (mainly wood) has been reduced to ashes, the air intake is closed to keep hot air inside and slow combustion.

Detail of flues in a Roman wall.

Partially intact hypocaust floor at Bignor Roman Villa.

Partially intact hypocaust floor at Bignor Roman Villa.

Korean traditional houses use an Ondol which is similar to a hypocaust, drawing smoke from a wood fire typically used for cooking.

[edit] See also

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