The hypocaust system (hypocaustum in Latin) was a heating system used in (wealthy) Roman homes and Roman baths, and the closest thing to central heating today. The hypocaust was a system that circulated hot air under the floor and surrounding walls.
Bath, UK CC BY-SA 3.0
Roman houses with a hypocaust system had a furnace that would heat the air. The floor was raised above the ground by pillars that were called pilae, so that hot air could circulate under it. The floor consisted of a layer of titles, followed by a layer of concrete, then another layer of tiles. There were also tile or clay flues under the walls which circulated the hot air to the floor and walls of the rooms above to finally escape from the roof. The walls had ceramic tiles in them in order to maintain the heat. The Romans also made sure that the hot air and smokes did not leak from the floor and walls which was quite a feat of engineering considering the materials used at the time. It is worth noting that a leaking floor was very dangerous and could even cause death as CO2 is odorless and toxic.
Rooms that required the most heat were placed close the furnace and the heat would be increased by adding more wood. For the baths this room was called the caldarium. In his book De architectura, Vitruvius describes the construction of hypocaust for the public baths and how fuel could be saved by building the hot room for men (the caldarium) next to that for women, with both rooms adjacent to the tepidarium.
It is worth noting that most Romans could experience the joys of such a heating system by simply visiting the thermae or the Roman hot baths where walls and floors were heated. The Pompeii Old Baths is a good example of such public baths.
Interesting facts about the hypocaust system