In the project LegioSafeCheck 40-50 domestic drinking water systems with and 40-50 systems without solar water heating are tested for Legionella, and system parameters such as temperatures in the week before sampling, hydraulics, storage tank design and mode of operation, as well as properties of the hot water distribution, are recorded. Preparations for the field studies were made in 2018. Temperature measuring instruments were procured, a meeting with the Cantonal Laboratory St. Gallen took place, a checklist for the recording of system data in the field was drawn up, and 88 houses with solar heating systems in the Rapperswil-Jona area were identified and the owners or residents were contacted by letter and asked to participate. The follow-up by telephone and the planning of the first sampling dates in January began.