A remarkably well-preserved medieval stone-chamber heating system has been uncovered during property renovations in Basel, Switzerland, the Canton of Basel-Stadt announced in late September.
The system once served the Gnadental convent, a women’s religious house founded around 1231 just outside Basel’s city walls. It was discovered beneath the former dining hall in the complex’s east wing.
The structure remains almost entirely intact, representing one of the most significant examples of this heating technology known in Switzerland and wider Europe.
Based on the site’s architectural history, researchers date the system’s construction to between the 13th and 14th centuries.
The subterranean chamber contained heat-retaining fieldstones characteristic of such systems. Once these were removed, archaeologists could examine the firing channel below.
Constructed from ashlar blocks, the channel features two pointed arches still standing in their original positions.
Additionally, eight closable vents remain carefully preserved. Warm air traveled through these openings from the heating chamber into the dining hall above, after smoke from the initial fire had dissipated.
Following the excavation, the Archaeological Soil Research Basel-Stadt produced a 3D model of the find for research documentation. The remains will be integrated into the ongoing Petersgraben 52 renovation project for further study.
The History of Gnadental Convent
Founded around 1231 outside the city walls for Franciscan friars, the Gnadental convent later served as temporary lodging for various orders awaiting the completion of their own houses within the city.
Approximately two decades later, the Franciscans relocated to the newly established Barfüsser Monastery at what is now Barfüsserplatz. Thereafter, the complex housed several women’s orders, including Cistercian nuns and the Order of Poor Clares.
In 1279, lay sisters from Gnadenthal near Niederwil (Aargau) took up residence, giving the convent its enduring name. They were later incorporated into the Poor Clares and remained until the Reformation led to the convent’s dissolution.
Following its abandonment, the buildings saw continued partial use. In 1573, materials from the complex were repurposed to construct a granary.
Watercolors by Johann Jakob Schneider from the 19th century depict the former convent structures. However, the granary was demolished late in that century to make way for a vocational school.