Legionella testing: who must test, and how often?
The operator of a drinking water installation must have it tested for Legionella when three conditions are met together: a drinking water heater with a capacity of more than 400 litres, or more than 3 litres in a hot water pipe, showers or other aerosol-generating fixtures, and the water is supplied as part of a commercial or public activity. Single and two-family houses are exempt (Section 31 TrinkwV).
When does the testing obligation apply?
Section 31(1) of the Drinking Water Ordinance sets out the conditions. All of them must be met:
- Size of the hot water system. A storage-type drinking water heater or a central instantaneous drinking water heater with a capacity of more than 400 litres, or more than 3 litres in at least one drinking water pipe between the outlet of the drinking water heater and the point of use. The content of a circulation pipe does not count towards this.
- Aerosol formation. The system has showers or other fixtures in which the drinking water is dispersed into a fine mist.
- Not a single or two-family house. Systems in single and two-family houses are exempt.
- Commercial or public supply. The drinking water is supplied as part of a commercial or public activity.
The 2023 Ordinance no longer uses the former term "large-scale drinking water heating system" as a defined term in its own right; the criteria now appear directly in Section 31.
How often must testing take place?
| System and activity | Interval | Reference |
|---|---|---|
| Building, commercial activity (for example letting) | at least every three years | Section 31(2) No. 2(a) |
| Building, public activity | at least once a year | Section 31(2) No. 2(b) |
| Building, public activity, after three years without findings | the health authority may set an interval of up to three years; not for facilities under Section 23(5) of the German Infection Protection Act (IfSG), care facilities and facilities with at-risk patients | Section 31(3) |
| Mobile and temporary systems | frequency is set by the health authority | Section 31(2) No. 1 |
| Newly commissioned system | first test within three to twelve months of commissioning | Section 31(4) |
Under Section 2 No. 8, "commercial" means supply as part of letting or of any other independent, regular activity carried on for profit. Under Section 2 No. 9, "public" means supply to an indefinite, changing group of people who are not connected by personal relationships.
Who takes the samples?
Testing under the Drinking Water Ordinance, including sampling, may only be carried out by accredited laboratories (Section 39(1)). The operator commissions such a laboratory; the federal states publish lists of accredited laboratories (Section 40(2)). Samples must be taken at several representative points (Section 41(4)).
Where are the samples taken?
The German Environment Agency (UBA) recommendation on systemic testing, dated 13 October 2025, names three types of sampling point: the outlet of the hot water pipe at the drinking water heater, the point where the circulation pipe re-enters the drinking water heater, and outlets in the periphery connected, with as short a connecting pipe as possible, to the end of the risers. Sampling follows DIN EN ISO 19458, purpose b. More on this under Sampling and measurement.
Frequently asked questions about Legionella testing
As the landlord of a multi-family building, do I have to have testing done?
Yes, if the building meets the criteria in Section 31: a drinking water heater with more than 400 litres, or more than 3 litres in a hot water pipe, plus showers. Letting is a commercial activity, so testing is then due at least every three years.
Does the obligation also apply to single and two-family houses?
No. Section 31(1) No. 3 TrinkwV expressly exempts systems in single and two-family houses.
Does the circulation pipe count towards the 3 litres?
No. Under Section 31(1) No. 1 TrinkwV, the content of a circulation pipe is not taken into account. What matters is the content of the pipe between the outlet of the drinking water heater and the point of use.
Can the testing interval be extended?
Yes, for public activity: if the annual tests show no findings for three years in a row, the health authority may set an interval of up to three years. This does not apply to facilities under Section 23(5) of the German Infection Protection Act (IfSG), care facilities and facilities with patients at higher risk of infection.
When is the first test due for a new system?
Within three to twelve months of commissioning, under Section 31(4) TrinkwV.
What happens if testing finds Legionella?
From 100 CFU per 100 ml, the obligations under Section 51 apply: notification, establishing the cause, a risk assessment and measures. The steps in detail are set out under Positive Legionella result: what to do now.
Sources
- Section 31 TrinkwV: testing obligations in respect of Legionella spec.
- Section 2 TrinkwV: definitions
- Section 39, Section 40 and Section 41 TrinkwV
- German Environment Agency (UBA): systemic testing of drinking water installations for Legionella, dated 13 October 2025
This page does not replace legal advice or information from the health authority. The version of the Drinking Water Ordinance in force at the time is authoritative.