The calidarium or caldarium was the smallest and most intimate area of the thermal bath complex; round or quadrangular in shape, with hot water pools for several guests or even individuals.
The room was positioned on the warmer sides, from east to south, to make the most of the sun’s heat. In the construction evolution of the baths, large wood-fired ovens were used to obtain a heating system with the circulation of hot air, at high temperature, under the floor and through the walls. The flooring of the Calidarium was in fact formed by a light floor, laid on brick pillars called suspensurae, under which circulated the hot air that was brought back up along the walls by clay pipes, called tubules.