Archaeologists in Basel have uncovered an almost completely preserved medieval hot-air heating system during renovation work at the site of the former Gnadental monastery. The rare installation, dating to the 13th or 14th century, is considered unique in Switzerland and one of the most significant examples of its kind in Europe.

The heating system was discovered in the courtyard of the former vocational school at Petersgraben 52, beneath walls and mortar floors belonging to the medieval convent. Archaeologists believe that it was probably used to heat the community’s dining hall.
Based on architectural and historical evidence, specialists have dated the installation to the 13th–14th centuries. It consisted of an underground chamber lined with stones that stored heat, with a firebox channel situated below. Warm air was then directed into the room above through eight openings that could be closed when necessary.

Stone-chamber hot-air heating systems of this type are extremely rare in Switzerland. What makes the Basel example particularly important is its exceptional state of preservation: the installation has survived almost intact. It is regarded not only as a unique find in Switzerland but also as one of the most significant surviving examples of such a heating system in Europe.

The archaeological remains are to be incorporated into the renovation project at Petersgraben 52.

The history of the site stretches back to the early 13th century. Gnadental Convent was established around 1231 outside Basel’s city walls for the Franciscans. About 20 years later, however, the Franciscans moved to the newly founded Barfüsser Convent, and the buildings were subsequently occupied by Cistercian nuns and Poor Clares.

In 1279, the monastery took the name Gnadental after lay sisters from Gnadenthal near Niederwil in the canton of Aargau joined the community. They were incorporated into the Order of the Poor Clares, which remained at the site until the monastery was dissolved following the Reformation.

Some of the former convent buildings continued to be used after the Reformation. In 1573, a granary was constructed using building material from the monastery, particularly from its church. The granary itself was demolished at the end of the 19th century to make way for the vocational school, removing the last visible remains of the medieval complex.
Recent archaeological excavations have now brought parts of the long-lost monastery back to light, including its remarkably well-preserved medieval heating system.