At a glance
- Growth between 25 and 45 °C
- Die-off from 60 °C, growth inhibited above 55 °C
- Hot water ≥ 60 °C, circulation ≥ 55 °C in line with DVGW W 551
- Stagnation favours growth; regular water exchange counteracts it
- Biofilm as a refuge and protection against brief heating
The temperature range
| Temperature | Behaviour of the Legionella |
|---|---|
| below 20 °C | virtually no growth |
| 25 to 45 °C | optimal growth |
| 45 to 55 °C | growth slowed |
| above 55 °C | growth inhibited |
| above 60 °C | die-off within minutes |
| above 70 °C | die-off within seconds |
This is why the technical standards require hot water to be stored at a minimum of 60 °C and kept from falling below 55 °C in the circulation loop (DVGW W 551), while cold water should stay permanently below 25 °C (VDI 6023 Part 1 and DIN 1988-200). The transition zones are the critical ones: uninsulated cold water pipes running next to warm risers, or hot water pipes that cool down between two uses. This is exactly where water sits in the ideal growth range for hours.
60 °C at the outlet, 55 °C in the circulation
The rule follows from the biology: hot water leaves the tank at at least 60 °C, and on its round through the building it must not drop below 55 °C. It should not lose more than five degrees on the way. Lowering the tank temperature to save energy pushes the system into exactly the range where Legionella grow best.
The hot water tank
The tank is where the most heat and the most water meet. If it is larger than it needs to be, part of the contents sits for a long time, and the lower layers are cooler than the upper ones. So what counts is not only the temperature the sensor reads at the top, but whether the whole volume reaches it. How long it takes for that temperature to actually arrive at the tap is covered under how long to let the water run.
Where the 60 °C must not arrive
At some outlets the water is not allowed to come out that hot. Where people could scald themselves, a mixer limits the temperature right at the tap. That is as it should be, and it has a consequence almost nobody says out loud: the temperature safeguard ends there. Past the mixer it is no longer heat that keeps the water clean, only regular exchange.
Why stagnation is a key factor
When water flows regularly, it is exchanged before any significant populations can build up. If it sits in the pipe for days, it takes on the ambient temperature and almost inevitably ends up in the critical range. That is why the empty room, not the occupied one, is a key risk factor.
The role of the biofilm
A biofilm of microorganisms forms on the inner wall of every pipe. Legionella live within it, protected, and multiply inside amoebae that colonise the biofilm. The biofilm is therefore less a by-product than a precondition: it makes Legionella resistant to brief temperature spikes and to disinfection.
That explains why a single heat treatment rarely solves the problem for good. Thermal disinfection lowers the bacterial count in the short term, but as long as the biofilm stays intact and the water keeps stagnating, the population builds back up within a few weeks. Only the combination of the right temperature and regular water exchange has a lasting effect.
Where Legionella sit in a building
Not every point in a drinking water system is equally susceptible. Growth is particularly favoured wherever water stands still or turns lukewarm:
- Rarely used outlets such as showers in empty rooms or guest bathrooms.
- Dead legs and dead-end pipes, meaning pipe sections without regular flow, often the remnants of decommissioned connections.
- Hot water storage below 60 °C or circulation loops that do not hold the temperature everywhere.
- Oversized or uninsulated pipes, in which water moves or warms up slowly.
- Shower heads, hoses and aerators, where deposits and biofilm collect.
In practice, a single neglected shower is enough to cause elevated readings in an otherwise well-maintained system.
How transmission happens
Legionella do not spread through drinking. They spread through aerosols, the fine water droplets a few micrometres across that you breathe straight in. Only these reach deep into the lungs when inhaled, where the bacteria can trigger pneumonia. This is why the shower is the most critical point in a building, because it atomises the water into droplets of exactly this size. Taps with aerators, whirlpools or air conditioning systems with evaporative cooling also produce such aerosols.
Far more people fall ill than the figures suggest. The Robert Koch Institute reports, in its RKI guide on legionellosis (2021 edition), a notified incidence of around 1.7 cases per 100,000 inhabitants (2018). On the basis of studies, the actual incidence of non-hospital-associated (community-acquired) cases is estimated at 18 to 36 per 100,000. The majority of cases therefore go undetected. The RKI publishes current notification figures in its infectious-disease epidemiology yearbook.
Symptoms and course of the illness