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View looking south at the well-preserved and partially reconstructed Hot Room (caldarium). The wooden suspend flooring is of course modern. On the wall notice the black lines—everything above the black lines is reconstructed.
On the left side of the image note the small columns that supported the floor—a portion of the floor is a modern reconstruction. On the lower far left note the very bright area. Outside of this was the furnace and the heated air entered the caldarium from there, circulated under and heated the floor before passing up clay pipes next to the walls—this is called a hypocaust system .
Thus the floor and the walls of this structure were heated.