The Calidarium, or hot room, was heated in one of two ways. Baths that were near natural hot springs (as the one in England) were heated by the water naturally flowing from the hot springs. The baths in not as fortunate locations were a bit more difficult to build as they included a subfloor (hypocaustum) were slaves kept fires burning to heat the water and floors. The floors of the Roman baths were heated throughout so that the people enjoying their baths did not get cold feet. These fires also helped to keep the rooms warm on cooler days and heated the water. The picture above shows how these fires kept the rooms warm.
