Hygiene flushing: how long and how often to flush?

A hygiene flush is an automatic exchange of the water at a point of use, replacing the standing water before Legionella can multiply. During a hygiene flush, a complete water exchange is required at least every 72 hours. You flush until the temperature holds steady, meaning the stagnant water has been fully pushed out. Depending on the pipe volume, this takes between about 30 seconds and several minutes per point of use.

At a glance

  • Water exchange at the latest every 72 hours per point of use (VDI 6023 Part 1, proper operation)
  • Flush until temperature stability, point of use fully open
  • Exchange cold and hot water separately
  • Over 7 days of stagnation: complete water exchange in accordance with DVGW W 551-3, over 4 weeks also a microbiological follow-up test
  • Duty to operate at least in accordance with the generally accepted rules of technology (§ 13 TrinkwV 2023)
Water flowing from a shower, a symbol of regular water exchange
Regular water exchange keeps the system in proper operation and prevents stagnation.

How often is a hygiene flush needed?

Under VDI 6023 Part 1 (section 5.3.1), going more than 72 hours without a water exchange counts as an interruption of operation. The interval applies per point of use, not per building: a shower that has not been used for four days is due, even if the building is well occupied.

How long do you need to flush?

Which measure is due depends on how long the water has been standing. The following classification summarises what to do from each stagnation duration onward:

Classification based on VDI 3810 Part 2 / VDI 6023 Part 3 (Table 2), DIN EN 806-5 and DVGW worksheet W 551-3 (A). Own illustration.

Temperature constancy means that the water at the point of use has reached the same temperature as the fresh water in the supply line. Cold and hot water must be considered separately: cold until it stays reliably cold, hot until it comes out hot.

Why the point of use must be fully open

VDI 6023 Part 1 requires a turbulent flow for stagnation flushing. A half-open tap does not reach that flow rate, and the water is not completely exchanged. In large or sprawling systems, several points of use must run at the same time to cover the distribution pipes.

In which order do you flush?

As a rule, you flush from the supply line outward to the most distant point of use, so that fresh water passes through the entire network. Cold and hot water are handled separately. With several risers, you start at the point closest to the building connection and work your way toward the peripheral points of use.

What matters is that every single point of use is actually reached. One forgotten shower is enough to leave a stagnant zone in which Legionella can multiply undisturbed. That is exactly why each outlet has to be logged on its own.

Why manual flushing fails in practice

Without measurement, the end of the flush cannot be identified, so people round up, every day, at every shower. And there is a prior question: which point of use is even due? No occupancy system records when someone last showered.

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The interval applies equally to every facility; the effort differs by type of operation.

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Frequently asked questions

Does every hygiene flush have to be documented?

The operator must be able to demonstrate the regular water exchange. That is why each flush is logged per point of use with a date, because without this record the proof cannot be provided in an inspection.

What applies after a longer vacancy or holiday?

After an interruption of operation, the measure depends on the duration of stagnation. Up to three days, flushing until temperature stability is sufficient; from 72 hours to seven days the water has to run for at least five minutes at several points of use at once. For a longer standstill, a complete water exchange in accordance with DVGW W 551-3 is provided for. After more than four weeks, microbiological control tests in cold and hot water are added. In detail: what is due after a longer shutdown.

Does the normal use of a shower already count as flushing?

Yes. If a point of use is used at least every 72 hours, the water exchange counts as met as intended and an additional flush is not needed. If use does not happen, stagnation favours the growth of Legionella.

Who is liable if flushing is not carried out regularly?

The operator of the drinking water installation is responsible and must operate it at least in accordance with the generally accepted rules of technology (§ 13 TrinkwV 2023). If water exchange and flushing are neglected, the operator bears the consequences, for example when the Legionella test shows an exceedance.

Is there a legal obligation to carry out a hygiene flush?

The Drinking Water Ordinance does not name the hygiene flush explicitly, but requires operation at least in accordance with the generally accepted rules of technology (§ 13 TrinkwV 2023). The decisive water exchange every 72 hours under VDI 6023 Part 1 makes regular flushing effectively binding as soon as normal use does not ensure it.

The content of this page is for general information and does not replace legal, technical or health advice in individual cases. Information on Legionella and the associated health risks is general in nature and constitutes neither medical advice nor an assessment of a specific drinking water system. For implementation in your operation, coordinate the specific situation with the responsible health authority.