The word literally means "heat from below", from the Greek hypo meaning below or underneath, and kaiein, to burn or light a fire. They are traditionally considered to have been invented by Sergius Orata, though this is not fully confirmed.
Hypocausts were used for heating public baths and private houses. The floor was raised off the ground by pillars, called pilae stacks, and spaces were left inside the walls so that the hot air and smoke from the furnace (praefurnium) would pass through these enclosed areas and out of flues in the roof, thereby heating but not polluting the interior of the room. Rooms requiring the most heat were placed closest to the furnace, whose heat could be increased by adding more wood. It was labour-intensive to run a hypocaust as it required constant attention to tend the fire, so it was only the wealthy that could afford to have it.
A derivation of hypocaust, the gloria, had been in use in Castile until the arrival of modern heating. After the fuel (straw, paper, refuse) has been reduced to ashes, the air intake is closed to keep hot air inside and slow combustion.
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