How Long Should Water Run During a Hygiene Flush?
You walk through the building, open every shower, count in your head. At some point the water feels colder, you turn it off, done. Or are you? The question of how long water must run during Legionella flushing — known in Germany as Hygienespülung — is asked thousands of times in the hotel industry and rarely answered clearly. As soon as nobody knows the duration precisely, people round up generously, because nobody wants to take the risk of having flushed too briefly. That costs time, staff, and energy.
VDI/DVGW 6023 requires complete water exchange at least every 72 hours. How long the water must run depends on pipe volume, flow rate, and temperature. Flushing ends when temperature constancy is reached, meaning the stagnant water has been fully exchanged. For a standard line to a shower, this usually takes between 30 seconds and several minutes.
What the Standard Actually Requires
According to VDI/DVGW 6023, lack of water exchange for more than 72 hours is classified as an operational interruption. If you don't sell a room for longer than that, close a floor in winter, or simply have no guests for three days, you must exchange the water to keep it fresh. The interval is clear. The duration is not.
The guideline says: flush until temperature constancy is reached, ideally until reasonably cold water flows. What that means in practice is never stated in seconds. It is the time water needs to flow from the supply line through the riser, the floor distributor, and the individual line to the shower, pushing out everything that stood there before. Every system is different, and therefore every flush duration is different.
In a compact hotel with short lines, 30 to 40 seconds might suffice. In a sprawling older building with long horizontal lines, it could be two minutes or more. Nobody can estimate that by eye. And that's exactly where the problem begins.
Why Staff Round Up Generously
You send your maintenance technician to flush the showers. She opens the tap, water runs. When does she turn it off? When it feels cold enough. Or when the time she counted in her head is up. Or when it feels right.
None of these reference points is reliable. So she rounds up, to be safe. If she's uncertain, she lets it run half a minute longer. If the facility manager said last time it was too short, she lets it run twice as long next time. The imprecision accumulates, and it always accumulates upward, never downward.
And the imprecision costs twice over. Not only because the water runs longer than necessary, but because every flush costs travel: walk there, open, wait, close, log it, move on. Each tap point must be operated individually. Anyone working through 20 bathrooms spends most of that time not flushing, but walking and waiting.
Work time that is missing elsewhere.
What Temperature Constancy Really Means
Temperature constancy means: the water now coming from the shower has the same temperature as the water standing in the supply line. It's fresh. Everything that stood in the line before has been flushed out. You can measure that with a thermometer, but not by feeling or counting.
This applies to both sides. On the cold line you flush until the water is cold and the temperature stays constant. On the hot line the opposite: you flush until hot water arrives at the tap. Both lines must be exchanged, because the Legionella risk sits precisely in the range between them, roughly 25 to 55 °C.
Once the 72-hour limit is missed, stricter measures apply. Up to seven days, it is sufficient to let water run for at least five minutes at several fully opened taps simultaneously. Beyond seven days, VDI 3810 Part 2 / VDI 6023 Part 3 require a complete water exchange by flushing according to DVGW W 557 (A): at least 2 m/s in the largest-diameter pipe, with aerators and shower heads unscrewed. That is a cleaning procedure and a job for a specialist contractor, not a routine flush.
Which is exactly why it pays not to miss the 72-hour rhythm in the first place. If you flush without measurement, you cannot recognize the endpoint. So you round up. Every day. At every shower. That adds up.
Temperature Constancy Alone Is Not Enough
VDI 6023 Part 1 additionally requires that stagnation flushing achieve at least turbulent flow. It is not enough to crack the tap open and wait for the water to turn colder. The tap must be fully open so that enough volume flow is generated in the individual supply line at all. In sprawling systems, several taps must run simultaneously to move the distribution lines as well.
Done by hand, this requires considerable staff time. An automatic fitting, by contrast, opens fully every time, at the same volume flow, without anyone having to remember.
What That Means in Practice
Take a hotel with 50 rooms. Occupied rooms exchange their water through normal use, so those need no flushing. At 70 % occupancy, an average of 15 rooms sit empty; in low season it is quickly 30.
The real problem, then, is not the flush itself but the question that comes before it: which rooms are actually due this week? When did the shower in room 214 last run? No PMS holds that data. If you cannot answer it reliably, you flush all of them to be safe, and you round up on every single one.
Then there is the walking: 30 bathrooms to visit, 30 waits, 30 log entries. And the risk that an entry is missing because someone was sick, on vacation, or because the room that hasn't sold in three weeks was simply overlooked.
That's not a reproach. It's the logical consequence of a task that must be precise but cannot be done precisely once it runs manually.
| Criterion | Manual Flushing | Automatic Surface-Mounted Shower Fitting |
|---|---|---|
| Trigger | Flush plan, memory | 72 h since last actual use |
| Flush duration | Estimated, rounded up to be safe | Defined, identical on every flush |
| Documentation | Handwritten, added afterward | Automatic, retrievable anytime |
| Precision | Varies by person and by day | Consistent, logged to the second |
How to Solve This
You can continue flushing manually, with a stopwatch and thermometer. You can install flush stations that are centrally controlled but do not measure at the tap point itself. Or you go where the risk is highest: to the shower.
That is where the aerosol forms through which Legionella reach the lungs in the first place. German law takes the same view: under § 31 TrinkwV (Section 31 of Germany's drinking water ordinance), a drinking water installation is subject to mandatory Legionella testing when three conditions coincide — a large water heating system (a tank over 400 litres, or more than 3 litres of volume in a pipe between heater and tap), showers or other fixtures where drinking water is aerosolized, and supply in the context of a commercial or public activity. A hotel almost always meets all three. The shower is therefore not only the most critical point in the room, but also the one that triggers the testing obligation in the first place.
A retrofit solution like the Aqvior surface-mounted shower fitting automates the flush directly at the shower. The flush duration is calibrated once during commissioning against the actual pipe volume — measured properly once, instead of estimated afresh every time. As soon as 72 hours have passed since the last actual use, the flush starts and runs for exactly that duration. Identical on every flush. No rounding up, no uncertainty, no staff who must remember. Every flush is logged in the web dashboard, and the compliance report is available at the click of a button.
Installation takes under an hour, without construction noise, without breaking tiles. The fitting is self-powered and requires no electrical connection. Other tap points keep their own flush routine, but the shower, the most critical point in the room, is covered.
Precise Flushing Without Guessing — Automatically Documented
Aqvior automates Legionella flushing directly at the shower and logs every flush completely. Inspection-ready, without anyone having to remember.
Request Pilot Slot Now Retrofit Without Bathroom Renovation · Fully Documented · Installation Under One Hour
Frequently Asked Questions
How long must water run during a hygiene flush?
Until temperature constancy is reached, meaning until all stagnant water has flowed out of the line. How long that takes depends on pipe volume and flow rate. In hotels, that's usually between 30 seconds and several minutes per shower. Those who flush without measurement round up to be safe, and that costs time and energy.
What does temperature constancy mean?
Temperature constancy means the water now coming from the shower has the same temperature as the fresh water in the supply line. That's the point where the old, standing water has been completely exchanged. You can measure that with a thermometer, but not by counting or feeling.
Why are 30 seconds often not enough?
Because in many hotels the lines are longer than they appear at first glance. From the riser through the floor distributor to the individual line, the volume adds up. In a sprawling building or with a T-piece installation, it may well be that you need two minutes or more until fresh water arrives. Those who apply a blanket 30 seconds may not completely exchange the water.
Sources
- VDI (2022): VDI/DVGW 6023 Part 1 – Hygiene in Drinking Water Installations (in German)
- VDI/DVGW: VDI 3810 Part 2 / VDI 6023 Part 3 – Operation and Maintenance of Drinking Water Installations (in German)
- Federal Republic of Germany: Section 31 TrinkwV – Legionella Testing Obligations (in German)
- Federal Ministry of Health (2023): Drinking Water Ordinance and Legionella – Reference Text (PDF) (in German)
- DVGW (2020): Technical Rule W 557 (A) – Cleaning and Disinfection of Drinking Water Installations (in German)
- DVGW (2024): twin – Drinking Water Installations: Stagnation and Recommissioning (PDF) (in German)
The content of this article is for general information and does not replace legal, technical, or health-related advice in individual cases. Information on Legionella and associated health risks is general in nature and constitutes neither medical advice nor an assessment of a specific drinking water installation. For implementation in your operation, clarify your specific situation with the responsible Gesundheitsamt (local public health authority). This article contains links to Aqvior products.