Legionella testing: What landlords and property managers need to know

Increased risk of Legionella during the warmer months

Legionella are tiny rod-shaped bacteria that occur naturally in bodies of water, but are anything but harmless to humans.

They enter the drinking water systems of residential buildings via groundwater. Studies show that approximately 15 percent of tested drinking water systems are contaminated with Legionella. Showers, faucets, and humidifiers are the primary sources of infection for humans. When finely atomized water enters the air, it can be inhaled and cause infection.

According to the Robert Koch Institute, a total of 2.745 cases of Legionnaires' disease were reported in Germany in 2024 alone – and the trend is rising. Infection can have serious consequences, as the mortality rate is around 5 percent. To prevent illness, consistent drinking water hygiene with appropriately safe storage temperatures is essential.

The risk of Legionella increases during the summer months and travel season.

The risk of Legionella growth increases, especially during the summer months and vacation periods. High ambient temperatures and extended periods of unused water pipes, such as when tenants are away, provide optimal conditions for Legionella bacteria to multiply. Property managers and landlords are therefore advised to remind tenants to flush the pipes regularly upon their return and to ensure water circulation even during extended periods of vacancy. Humidifiers and air conditioners should also not be neglected. Devices that contain or circulate water and release it into the environment should be cleaned regularly.

An often underestimated risk factor is the unintentional warming of cold water pipes. In poorly insulated parts of buildings or shafts, stagnant cold water can heat up to over 25°C in summer – a temperature at which Legionella bacteria can already multiply. The temperature of cold water should remain permanently below 25°C, ideally below 20°C.

Storage temperature and Legionella prevention

Especially in view of rising heating costs It might seem tempting to lower the storage temperature of the hot water system to save costs. However, this can be risky: if the temperature in the water tank consistently falls below 60°C, it promotes the growth of Legionella bacteria throughout the entire drinking water system.

What does the legislator stipulate?

For approximately 13 years, legionella testing has been mandatory in the housing sector. The legal basis for this is the EU Drinking Water Directive 98/83/EC and the German Drinking Water Ordinance (TrinkwV). With the amendment to the Drinking Water Ordinance in 2023, the so-called technical action level was slightly tightened: A test result is considered positive if 100 CFU (colony-forming units) of legionella bacteria per 100 milliliters of water are reached. Previously, this was only the case if the value was exceeded. The obligations for landlords and property managers remain unchanged: Operators of central drinking water systems must have the drinking water tested for legionella regularly. The legislator already established all the framework conditions for legionella testing with the "Second Ordinance Amending the Drinking Water Ordinance" of December 13, 2012. For example, the initial sampling of central drinking water systems in apartment buildings had to be carried out by December 31, 2013, at the latest. Since then, the test must be repeated every three years. Anyone who had their systems inspected for the first time in 2013 will be required to have them inspected for the fifth time by December 31, 2025. For landlords, this also applies in 2025: even initial inspections of the drinking water system must be carried out without being requested.

Who is required to have legionella testing performed?

The obligation to conduct regular legionella testing applies if a property meets the following criteria:

• The building has at least three residential units, and at least one of them is rented.
• There is a central drinking water heating system.
• The storage volume of the water heater is more than 400 liters or more than 3 liters of volume in a drinking water pipe between the outlet of the water heater and the furthest draw-off point.
• Shower facilities are available.

If all criteria are met, a Legionella test is mandatory every three years. The operator of the drinking water system is responsible. This is usually the landlord or property manager. Samples may only be taken by accredited testing laboratories. The water samples must be taken at precisely defined sampling points in the drinking water system, in accordance with the specifications of DIN EN ISO 19458:2006-12. Minol has been supporting the housing industry with Legionella testing since 2012 and is thoroughly familiar with all requirements. Thanks to our collaboration with leading, accredited laboratory partners, we offer nationwide service availability. Important for landlords: The costs for the recurring Legionella test, also known as a "preliminary test," can be passed on to tenants as operating costs.

More information about Legionella testing

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