From the SAP News Center:
The Viessmann Group which has its headquarters in Allendorf, Germany, is a global player and is regarded as an internationally leading producer of energy systems such as heating, industry, cooling, photovoltaic, and electric. Viessmann is among other several companies which have chosen to switch to SAP S/4HANA.
Read here more: https://news.sap.com/germany/2019/09/erp-migration-s4hana-viessmann/ .