Hot water temperature against Legionella: what applies?

Legionella multiplies very well at around 25 to 45 °C; from 50 °C its growth is inhibited, and above 60 °C it dies off quickly. That is how the DVGW describes it. In systems with a central drinking water heater and circulation, the technical rules therefore require at least 60 °C at the outlet of the drinking water heater and at least 55 °C throughout the circulation.

Which temperatures apply?

These figures belong together but must not be mixed up: 50 °C is the technical lower limit for small systems, while 55 to 60 °C is the recommendation. The German Environment Agency (UBA) generally recommends a temperature level of consistently above 55 °C.

What is a small system, and what is the 3-litre rule?

A system counts as a small system if it has a drinking water heater of no more than 400 litres and no more than 3 litres in any pipe between the outlet of the drinking water heater and the point of use. The same limits of 400 and 3 litres determine, under Section 31 of the Drinking Water Ordinance, whether a system must be tested for Legionella. The Ordinance does not count the content of the circulation pipe towards this. More on this under Legionella testing.

Heat pump and Legionella: what to do?

The German Environment Agency (UBA) often describes heating drinking water with a heat pump as a challenge for Legionella protection, because of the high temperature this requires. In its example calculation for a typical single-family house, the seasonal performance factor for drinking water heating falls from 3.22 to 2.19 when the temperature rises from 48 to 60 °C and a circulation is operated; final energy demand and energy costs rise as a result by around 15 per cent.

There are no fixed requirements for private single and two-family houses; at least 55 to 60 °C is recommended. The principle that the DVGW adopts from the German Environment Agency is: health protection comes before energy saving.

Does a Legionella cycle help?

With a Legionella cycle, the storage tank is not kept hot permanently but is only heated briefly above 60 °C periodically, for example once a day. The German Environment Agency (UBA) states that such cycles are neither anchored in the technical rules nor proven effective in real operation, and rates them as not a safe way of protecting against Legionella. The DVGW writes that the Legionella cycle can be switched off, because there is no scientific evidence that it reduces the Legionella concentration. The German consumer advice centres (Verbraucherzentralen) recommend running the tank permanently at 60 °C.

What is thermal disinfection?

Thermal disinfection is a targeted measure once Legionella has already been detected: every hot water outlet is flushed at full flow with water of at least 70 °C for at least 3 minutes; times and temperatures are measured and documented. The rules for this are set out in DVGW W 551 and W 551-3. It only works where the temperature actually reaches, and does not replace the correct temperature in day-to-day operation.

Why stagnant water is dangerous

In rarely used pipes and dead legs, hot water cools down and falls into the temperature range in which Legionella grows well. A biofilm easily forms there, from which germs repeatedly contaminate the water. VDI 6023 Part 1 requires regular water exchange at all outlets; specialist publications on the guideline cite a limit of 72 hours for this. How biofilms form is explained on the page Fighting biofilms.

Frequently asked questions about temperature and Legionella

Which hot water temperature prevents Legionella?

In systems with a central drinking water heater and circulation, at least 60 °C at the outlet of the heater and at least 55 °C throughout the circulation. According to the DVGW, Legionella dies off quickly above 60 °C, and its growth is inhibited from 50 °C.

Is 50 °C enough in a single-family house?

Under the technical rules, 50 °C is the lower operating temperature for small systems without circulation. 55 to 60 °C is recommended. There is no statutory requirement for private single and two-family houses.

How often must a Legionella cycle run?

The German Environment Agency (UBA) rates the Legionella cycle as not a safe way of protecting against Legionella, because its effectiveness in real operation is not proven. A permanently adequate temperature level is safer.

Can I use chlorine dioxide in hot water?

Under the dosing instructions, biostream® ZERO may only be used at up to 40 °C, because the active substance breaks down at higher temperatures. Chemical disinfection of the hot water system with biostream® ZERO therefore requires the water to be below 40 °C. In buildings, system disinfection is carried out by a specialist contractor under DVGW W 551-3.

What can I do as a guest in a hotel, clinic or care home?

Ask those responsible whether the drinking water is tested regularly for Legionella and what protective measures are in place. For facilities with public activity, the Drinking Water Ordinance requires an annual test where the conditions of Section 31 are met.

Sources

• DVGW: energy savings in the hot water sector of drinking water installations, March 2023
• DVGW, Gerhardy: technical article on W 551 (2012)
• German Environment Agency (UBA): hot drinking water concepts for buildings with heat pump heating (11/2023)
• Bavarian State Office for Health and Food Safety: Legionella
• German consumer advice centre (Verbraucherzentrale): hot water with a heat pump

Read more on Legionella

• Legionella: the overview
• Legionella: action value and assessment levels
• Legionella testing: who, how often, where
• Positive Legionella result: what to do now