Noun
A furnace, esp. one connected with a series of small chambers and flues of tiles or other masonry through which the heat of a fire was distributed to rooms above. This contrivance, first used in bath, was afterwards adopted in private houses.
Source: Webster's dictionaryGarrett G. Fagan, "The Genesis of the Roman Public Bath: Recent Approaches and Future Directions," American Journal of Archaeology 105.3 (2001), p. 404. Baths had hypocaust heating: the floors were suspended over hot-air channels that circulated warmth. Source: Internet
Central heating Ruins of the hypocaust under the floor of a Roman villa. Source: Internet
Lullingstone Roman Villa, Michael Fulford, page 18 Enlargement Around AD 150 the villa was expanded and a heated bath block with hypocaust was added. Source: Internet