Romans heated the floors and walls of the bath by lighting a fire in a fireplace that opened underneath the building.
Hot air from the fire would be drawn into the open space beneath the bath's floor. The hot air would then rise up into tubuli, hollow clay boxes Romans built into the bath's walls. Hot air and smoke would exit from chimneys at the top of the building.
After wood is brought to the site, the fire is lit. For half an hour the smoke does not draw into the bath. Then the fire is pushed deeper into the building. Smoke begins to billow out of the smokestacks.