hypocaustum

Hypocaustum refers to the Roman underfloor heating system used to warm rooms and baths. The term derives from Latin hypocaustum, meaning “burning from below.” A furnace (praefurnium) located in a lower chamber or outside space burned fuel to generate hot gases. These gases were guided through a network of flues and hollow spaces beneath the floor, which was raised on brick or tile pillars called pilae to create a gap for heat to circulate. The circulating hot air would heat the floor surface and radiate warmth into the room; in many installations, vertical flues embedded in walls carried heat upward to warm adjoining spaces as well. Some systems used tubuli, small ceramic tubes running through walls to distribute heat more evenly.

In baths (thermae), hypocausta could be large and complex, heating caldaria (hot rooms), tepidaria (warm rooms), and sometimes frigidaria through connected floor and wall channels. The construction relied on fired clay, brick, and plaster, and required regular maintenance to keep flues clear of ash and soot. The hypocaust system is considered an early form of centralized heating and is among the best-documented features of Roman engineering.

Evidence comes from archaeological remains in places such as Pompeii, Herculaneum, Ostia, and various villas, where collapsed floors and exposed pillars reveal the layout. The use of hypocaustum declined in the late Roman period, though related concepts influenced later heating technologies.