The Romans? I didn't know that. What method did the Romans use? It is amazing how much knowledge the Western World lost after the fall of Rome and Greece into chaos. The dark ages truly were quite a few steps backwards in regards to science, philosophy, and art.
They called it a hypocaust. Due to the expense, It was limited to the homes of the wealthy, certain public baths, temples and the like. But they'd build concrete floors raised above the ground by a few feet via numerous small columns. A furnace is built in such that the hot air and smoke would accumulate in the spaces beneath, slowly venting through the hollow bricks forming the interior walls. Heat would be continually transferred to the floors and walls of the building. And the furnace would be located closest to the rooms they wanted kept the warmest.
The catch is, it basically required a skilled slave/servant on hand to monitor the thing constantly, especially if running it at night.
Edited: daishain on 1st Mar, 2017 - 1:44pm