Abstract
Pathogenic Legionella bacteria can thrive in building water systems. In Norway, drinking water often
lacks residual disinfectant and can sustain bacterial growth when environmental conditions like
temperature are favorable. The primary preventative tools against Legionella pneumophila in Norwegian
buildings are hot water systems at 60°C or more and periodic hot-water flushing of showers. These
management strategies are water and energy-intensive, with flushing requiring substantial personnel
time.
In this work, field monitoring revealed the presence of Legionella in both hot- and cold-water
systems. Cold water likely represents the source of Legionella into a building water system via the
municipal water network, but most prevention and monitoring focuses on hot water because Legionella
thrive at 25 to 45°C. Thermal contamination of the cold water, however, was prevalent in buildings.
Therefore, we examined how different design and operational practices in building water systems could
compromise Legionella prevention. Specifically, we focused on thermostatic mixing valves, serially
connected hot-water heaters with preheating below 50°C, and heat transfer from hot- to cold-water
pipes in e.g. shafts and manifold cabinets.
We found that cold-water pipes and plumbing components often exceeded 25°C, providing suitable
conditions for Legionella growth. Legionella spp. were prevalent in cold water at concentrations higher
than in hot water, as confirmed by water samples analyzed with digital PCR. Heating water to 70°C or
higher effectively eliminated Legionella. However, to prevent scalding, hot water is mixed with cold water
to reach temperatures of 60°C or less. This mixing process may not fully inactivate cold-water bacteria,
posing a potential risk for Legionella contamination during hot water production.
This work demonstrates that the cold-water system is just as vulnerable to Legionella contamination as
hot water, if not more so due to the lack of design and operational controls managing the cold water.
Interactions between the hot and cold water—both direct due to thermostatic mixing and indirect via, for
example, thermal transfer—can compromise water in both systems. Future work will focus on how
relatively minor or cost-effective changes to new and existing water systems might help mitigate
Legionella growth in buildings.