A monastery under Basel heated its hall through eight openings in the ceiling

Fire in a stone channel, a bed of fieldstones above it and eight closable openings in a vaulted ceiling: that was how the medieval women’s convent of Gnadental in Basel warmed its dining hall. Archaeologists working at Petersgraben 52 have uncovered the system nearly intact, and the cantonal archaeological service calls it unique in Switzerland.

The fire channel and the stone-filled chamber

According to the September 23, 2026 release from the Canton of Basel-Stadt, the heating chamber lies directly below the refectory of the former convent. It was found during renovation work at Petersgraben 52, beneath the courtyard of a former vocational school, which is one reason the medieval remains stayed hidden for so long. The release describes the warm-air system as remarkably well preserved. Below the chamber ran a fire channel built from stone blocks and brick, where the fire was lit.

The chamber above the channel was filled with fieldstones for heat storage, so the heat from a single firing was held in the stone mass and released upward over time. Two pointed arches of ashlar and brick remain in their original positions, and together with the channel and the stone fill they make up the parts of the installation that survive.

Eight closable openings into the refectory

The sequence the archaeologists reconstruct has several steps, and the layout is vertical: fire channel at the bottom, stone-filled chamber in the middle, and the closable ceiling openings at the top, directly beneath the floor of the dining hall. A fire was kindled in the channel, the smoke was allowed to clear, and only then did warm air rise from the heated stones through the openings in the vaulted ceiling into the hall above. The release specifies eight closable openings, so the air could be admitted or shut off, and the archaeology department’s page on the find describes the same arrangement. The point of closing the openings was to regulate how much warm air entered the dining room, so the hall could be warmed in a controlled way, an arrangement the release counts among the most significant of its type in Europe.

Stone-chamber air heating of this type is “very rare in Switzerland,” as Bluewin’s report on the announcement quotes the archaeological service. The release itself goes further: because of its almost complete preservation the system is unique across Switzerland and counts among the most significant known examples of this heating type in Europe.

The dating is based on architectural and historical analysis, according to Bluewin’s summary, which is why the sources give a range of the 13th to 14th centuries and not a year. A range that wide still places the system inside the working life of the convent, which stood from the 13th century until the Reformation.

Gnadental from 1231 to the Reformation

The convent was founded around 1231 for Franciscans, outside Basel’s walls, and later passed to other religious communities; Bluewin’s report lists Cistercian nuns and Poor Clares among them. The Clare Sisters lived there from 1279 until the Reformation. Experts date the heating system to the 13th or 14th century, a range wide enough to cover both the Franciscan and the Clare periods. After the Reformation the convent was abandoned, and Archaeology magazine’s September 24 account notes that some structures were repurposed or dismantled for building material, before the site became a vocational school that hid the medieval remains.

The excavation also produced a green-glazed ceramic stove tile from the 14th or 15th century, likely a decorative roof finial. The release does not say where on the site it was found relative to the refectory.

Preservation at Petersgraben 52

The archaeologists documented the chamber in conventional drawings and as 3D digital models, so the structure can be studied even if parts of it have to be taken away. The 3D models sit alongside the conventional documentation of the structure, which means a record of the arches, the stone fill and the fire channel exists regardless of what happens to the physical remains. Officials are working out solutions for its long-term preservation in coordination with the real estate and transport departments, and aim to integrate it into the Petersgraben 52 renovation project. Guido Lassau, the cantonal archaeologist, is listed as the contact for inquiries.

HeritageDaily’s October 3 report ties the preservation question to construction of the new University Library in Basel, and says the heating chamber dates to the 13th or 14th century and is exceptionally rare in Switzerland. The cantonal release itself names the renovation of Petersgraben 52 and says nothing about the library, so the official text leaves the chamber’s final form and location to the continuing talks between the canton’s departments. What it does commit to is a count: eight openings, two arches, one fire channel.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

More from Morning Overview