hypocaust

thumb|300px|right|Hypocaust under the floor in a Roman villa in Vieux-la-Romaine, near [[Caen, France]]

A hypocaust () is a system of central heating in a building that produces and circulates hot air below the floor of a room, and may also warm the walls with a series of pipes through which the hot air passes. This air can warm the upper floors as well. The word derives from Ancient Greek hupó and kaustós (compare caustic). The earliest reference to such a system suggests that the Temple of Ephesus in 350 BC was heated in this manner, although Vitruvius attributes its invention to Sergius Orata . Its invention improved the hygiene and living conditions of citizens, and was a forerunner of modern central heating.