The floor is lava! Heating techniques in ancient Carnuntum

When we think of life in ancient Rome, images of southern landscapes, mild winters, and sun-drenched cities often come to mind. However, this idea falls short. Even in the Mediterranean region, and especially in the northern provinces of the Roman Empire, freezing temperatures in winter were not uncommon. For cities such as Carnuntum, a reliable heat supply was therefore a fundamental prerequisite for living, working, and social life.

Braziers were not only used as a source of heat, but also in rituals and ceremonies - © Römerstadt Carnuntum

The simplest forms of space heating were based on open fires. In kitchens, stove fires were used for both cooking and heating, while metal wood or charcoal basins were often used in living rooms. Although these were easy to regulate, they were not particularly efficient: they only heated their immediate surroundings, deteriorated air quality due to smoke gases, and left the floors cold. Permanently installed fireplaces were therefore uncommon in living rooms. 

Underfloor heating in the adjoining room of the red salon in Villa Urbana in the civilian town of Carnuntum. The hypocaust pillars are made of sandstone - © Römerstadt Carnuntum 

The introduction of underfloor heating, known as hypocaust, represented a decisive step forward in development. Ancient authors attribute its “invention” to C. Sergius Orata around 80 BC, but archaeological findings show that comparable systems already existed in the 3rd century BC. By the Roman Empire at the latest, the hypocaust had become an integral part of prestigious architecture.

Praefurninum in the kitchen of Lucius' house, where the underfloor heating in the adjoining room is fired up - © Römerstadt Carnuntum 

The functional principle is as simple as it is effective: the floor rests on a substructure of small pillars. Wood is burned in a firebox located on the side, the praefurnium; the hot smoke and combustion gases flow under the floor, heating it, and are then discharged to the outside via chimneys or hollow bricks (tubuli) embedded in the walls. This allowed floors and walls to be heated. The resulting heat was even, mild, and almost draught-free – comparable to the radiant heat of modern tiled stoves.

Hollow tile (tubulus) made of ceramic (inv. no. CAR-K-1063) and roof attachment (presumably ventilation attachment) made of ceramic (inv. no. CAR-K-1532) - © Landessammlungen NÖ

These heating systems can be found in various forms in Carnuntum. While the large public baths were fully equipped with hypocausts, private residential buildings were much more economical. Many houses had only one heated room, often without full underfloor heating. Instead, duct heating systems were used, in which a covered heating duct led from the praefurnium into the room and branched out there. This construction method, as used in the House of Lucius, for example, was inexpensive and particularly widespread in the northern provinces, but did not achieve the performance of fully developed hypocausts. Charcoal braziers also remained in use.

Roman heating technology was most significant in the thermal baths. Public baths were central places for personal hygiene and social life. Here, technical sophistication was combined with ostentatious luxury. Heated rooms such as the caldarium, tepidarium, and sudatorium required precise coordination of air and water temperatures. Reconstruction and measurement experiments show that floor temperatures of 20 to 50°C and wall temperatures of up to 30°C were achieved. In the caldarium, air temperatures of around 32°C prevailed with very high humidity; the bath water could be up to 48°C hot and had to be mixed before use.

Channel heating with covered heating channel in the civil baths in the Roman quarter - © Römerstadt Carnuntum 

From today's perspective, this form of heating appears to be energy inefficient. Hypocaust systems had long heating times, could only be regulated to a limited extent, and consumed enormous amounts of firewood. Large thermal baths often had to remain in operation continuously to compensate for temperature losses. Studies on fuel consumption in the civilian city of Carnuntum illustrate the ecological consequences: wood was the most important energy source in ancient times, and the high demand for heating and bathing contributed significantly to the deforestation of entire regions.

On closer inspection, the comfort of living is also put into perspective. Heating analyses of reconstructed houses show that the high heat losses through uninsulated walls and windows could only be compensated for by very high floor temperatures, which often exceeded today's comfort and health limits. In addition, there were significant temperature differences between the floor and walls, as well as limited possibilities for regulating individual rooms independently.

Roof structure in the shape of a house on the thermal baths in the civilian town of Carnuntum - © Römerstadt Carnuntum 

Carnuntum in particular provides an excellent example of how people in ancient times responded to climatic challenges, based on archaeological findings and modern reconstructions. Simple braziers, economical canal heating systems, and monumental hypocausts span a wide range—from pragmatic necessity to luxurious staging. In ancient times, warmth was not merely a matter of comfort, but a central factor in the quality of urban life, traces of which remain visible to this day.