Roman Baths and Hypocausts
Roman Baths consisted of a number of rooms that were used by the public for various functions considered important for personal hygiene. The caldarium or hot room was one of them. It was normally built on the south side to take advantage of the sun and its heat.
Ancient Roman Baths used under floor heating to heat public rooms used in the baths for what could be nowadays called saunas. They were called hypocaust from the Latin word hypocaustum. The word ‘hypo’ in Greek means from below and this was the main method of operation by lighting fires from below the floor. Raised floors were created on a series of pillars and this hollow was used to light fires that were constantly attended to by slaves and attendants. The heights of the pillars were such that there was enough space for a slave to enter the floor below for cleaning or other reasons.
The hot air and smoke was led out through flues in the roof of the structure above this hollow heating space. Adjustment of heat was done by bronzed ventilators placed in the roof of the room being heated. Hollow walls were also quite commonly used as a means to preserve the heat and prevent condensation. Temperatures in such hypocausts had wall surfaces being heated up to 100º F. Floors at times were so hot that users of the bath wore wooden sandals before entering the heated spaces. Roman Baths can be seen today in well preserved sites like in the English city called, strangely enough, Bath.
While the hypocaust is credited to the Romans, such heating devices were common in the Hellenic times and evidence has even been found in Mohenjo-Daro, a city of the Indus civilization, of such heating devices. This could predate the Roman era by over 2000 years. Similar concepts have been found in traditional houses in Korea where the kitchen fire or "ondol" was used to heat the house. Houses in Spanish areas used such systems till very recently and also had systems where they could close the air entering the furnace so that fires died out gradually, while the heat was still trapped within the heated area.
Image Source: Wikimedia: Hypocaust