Potable water systems are often contaminated with the opportunistic pathogenic bacteria Legionella pneumophila. Growth of the pathogen in buildings is affected by operational settings, system design, water use patterns, and unplanned disruptive events.The impact of technical interventions on Legionella is often studied after detection of Legionellosis, or in buildings with at-risk occupants that prioritize rapid reduction of Legionella, limiting the number of studies that evaluate more moderate interventions. The aims of this work were to evaluate (1) boiler operational settings that attempted to balance reduced energy demands with Legionella levels and (2) the impact of stagnation events on Legionella. Samples were collected throughout a contaminated building supplied with non-chlorinated water. Over 2.5 years, the boiler temperature was systematically changed and the impact on L. pneumophila was quantified. The study period spanned holidays and the Covid19-related lockdown. Our results show that a boiler temperature of 45 °C combined with 70°C superheating cycle(s) was ineffective in reducing Legionella concentrations. A constant boiler temperature of 60 °C effectively reduced pathogen numbers, and stepwise elimination of 60 °C operational days during the week had variable success in controlling Legionella. Stagnation had variable impact on the bacterium; while a two-week holiday led to an increase in concentrations in 2019, the same break in 2020 and the 7-week Covid-19 related lockdown led to a decrease in Legionella. Importantly, we demonstrated that rapid water heater turnover temporarily increased Legionella numbers, presumably due to disturbing biofilms in the plumbing system. These findings suggest that straightforward technical interventions on the boiler temperature can be sufficient to control Legionella. Moreover, our results emphasize that stagnation alone does not necessarily lead to proliferation of Legionella, and that there are unintended consequences of some interventions that are broadly recommended.
