Hypocaust

A hypocaust (Latin hypocaustum) 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 Vitruvius, writing about the end of the 1st century BCE, attributes their invention to Sergius Orata.

Roman operation

Hypocausts were used for heating hot baths, 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 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.