Underneath the the polished floors of the bath houses was an ingenious system used to heat the baths above.
The main source of heat came from a furnace in which a fire was kept burning by a slave. Romans mainly used wood to fuel the fire.
Flamma ventum inflammat.
Connected to the furnace was a low lying space where columns of tiles, usually half the furnace's height, held up the floor of the bath house. Air heated by the fire would circulate throughout this area, because the air was hot it would rise, heating the floor of the bath house.
Caldius ventus per ordinibus colunatarum tegularum fluit.
The hot air circulated through the wall, via hollow bricks and channels. The heated air would heat the wall. The combination of the warm wall and floor helped to keep the environment inside the bath house pleasantly warm. The hollow bricks in the wall also help to reduce condensation and increase air flow.
Ventus murum et pavimentum inflammat.
The floor would not get too hot because the floor was several layers thick. The different materials and layers helped keep the floor from becoming too hot to walk on.
As air traveled further away from the furnace, it would cool. This cooling caused rooms further away from the furnace to be milder in temperature, increasingly getting cooler the further from the furnace they were. This was why the rooms would have varying temperatures.
Air that had cooled became denser, sinking to the ground. This cooled air would then flow through the tiles, gradually being heated by the hot air or the fire itself.
Frigus ventus fluit. Frigus ventus ad flammam fluit. Flamma ventum inflammat. Calidum ventus crescit.
A boiler, large tank of water, usually was fitted above the furnace. The fire heated the water in the boiler, which was carried throughout the baths through lead pipes. The water, like the air, also lost heat as it traveled away from the fire. This cooling was what caused the different temperature ranges in the baths.
Olla surpa flamma. Aqua in tubum plumpi fluit. Aqua in caldrium fluit.
The cooled water became denser, moving downward. The water then was either heated by the incoming hot water or was heated as it moved back to the furnace, through convection.
Friga aqua ad flammam fluit. Calida aqua fluit.
The system was not perfect. The fires were hard to control and there was usually poor air quality in the rooms. There was also the threat of carbon monoxide from the fire.
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