Between 1 November and 12 December 2019, a Legionnaires" disease (LD) outbreak occurred in the Strasbourg metropolitan area, France. Epidemiological, environmental and genomic investigations (nested sequence-based typing and whole genome sequencing (WGS)) were undertaken to locate the outbreak source, characterise the causal Legionella strain, and understand its spread. Through a positive urinary antigen test, 28 cases (14 male; 14 female) with 70 years median age (range: 42-88 years) were diagnosed. Two died. For nine cases, typing revealed L. pneumophila serogroup 1 (Lp 1) sequence type (ST) 62 infection. Mapping where cases were during their incubation period pointed to a place south-west of Strasbourg city as the outbreak epicentre. There, in the biomass condensing boiler of a heating plant, high Lp 1 contamination levels (105-106 CFU/L) were discovered. Eight environmental Lp 1 isolates from inside the plant were ST62. Analysing WGS data from these isolates and the nine Lp 1 ST62 clinical isolates, found their sequences clustering with zero to two single nt polymorphisms. Sub-optimal maintenance and warm weather before the boiler started operating may have allowed Lp 1 proliferation within, with boiler fumes subsequently disseminating Lp 1, thereby exposing people. This outbreak highlights potential LD risks arising from innovative energy-saving heat production processes designed to reduce greenhouse gas emissions.