On a DHW system, sanitary safety is not decided by a device but by temperatures made enforceable by France's order of 23 June 1978, as amended by the order of 30 November 2005. The water must be at 50°C minimum at every point of the distribution system, and the water in storage vessels of 400 litres and above must be at 55°C or more at the outlet at all times, or raised to a sufficient temperature at least once every 24 hours. The same text caps draw-off at 50°C in rooms intended for washing and 60°C in other rooms, which makes downstream mixing compulsory as soon as production is held high. UV, filtration and chlorination come after those values, never in their place.
Understanding legionella in DHW: risks, obligations and points of vigilance in 2026
Why legionella develops in domestic hot water networks
Legionella thrives in lukewarm networks with poor renewal, scale and biofilm. As soon as water stagnates (dead legs, unoccupied dwellings, poorly set loop circulation), it finds what it needs to multiply. Mixing valves, showerheads and aerators add zones of mixing and fouling. The main risk then comes from inhaled aerosols, especially in the shower. Inhalation risk.
Temperature thresholds and risk zones: tank, loop, draw-off points
The zone most favourable to growth sits around 25 to 45°C, and that is precisely the band the text forbids you to let settle. Here are the values to record at inspection, as the order states them.
| Requirement | Enforceable value | Scope |
|---|---|---|
| Water temperature in distribution | ≥ 50°C | at every point of the distribution system, loop return included |
| Water in storage vessels of 400 litres and above | ≥ 55°C at the outlet at all times, or raised to a sufficient temperature at least once every 24 hours | preheating tanks excluded |
| Temperature at the draw-off point | ≤ 50°C | rooms intended for washing |
| Temperature at the draw-off point | ≤ 60°C | other rooms |
| Volume of the final feed pipework | ≤ 3 litres, and as low as possible | between the point of distribution and the draw-off point |
Sensitive points to record on the survey: undersized tank, loop that runs too cold or badly balanced, network lengths, scaled showerheads. Lukewarm zone.
What you must record on site: readings, instructions and maintenance
In 2026, vigilance means traceability. Note down the settings (tank setpoint, loop, mixing valves), the flow and return temperature readings, commissioning, flushing and rinsing. Hand the client simple instructions: temperature boost, use after absence, annual servicing, descaling of draw-off points. Job site traceability. Those readings and instructions do not have to go back out as an attachment: with Argile, the client finds the project documents and the follow-up of the installation in a dedicated space.
Identifying the most exposed DHW installations: field diagnosis before choosing a device
Spotting stagnation and poor balancing: loop circulation, dead legs, low usage
Start with a "pipes in hand" visit. A poorly set loop, dead legs (unused fittings, old branch pipes) or rarely used draw-off points create stagnation. This is the ideal breeding ground for legionella. On site, spot the zones where hot water takes a long time to arrive, and check that loop return is indeed present and consistent on every riser.
Checking network insulation and losses: avoiding lukewarm temperatures
Measure the flow, return and draw-off point temperatures. Risk increases when the network stays "lukewarm" over long lengths. Degraded insulation, open ducting or cold plant rooms bring the temperature down and lengthen wait times. Fix visible losses first, then adjust flow rates and setpoints. To dig further into heat loss, refer to DHW storage losses.
Auditing sensitive points: showers, mixing valves, aerators and filters
Showers and thermostatic mixing valves should be inspected first. Remove and clean aerators and filters. Look for scale, deposits, worn seals. Schedule a flush of rarely used points and, if needed, replace equipment that is hard to disinfect. A good diagnosis avoids overequipping and secures operation.
Choosing a suitable anti-legionella device: simple, effective solutions
Thermal cycle: temperature boost, frequency and anti-scald precautions
The simplest approach remains a thermal cycle. The idea is to limit the lukewarm zone where legionella develops. Schedule a regular temperature boost of the tank, then check that the hot water actually stays hot all the way to the draw-off points. Protect occupants with a thermostatic mixing valve and suitable settings, because water that is too hot burns quickly.
Complementary treatments: filtration, UV, controlled chlorination depending on context
When the network is complex or sensitive, you can add a barrier. Fine filtration retains the deposits that feed biofilm. UV is effective on clear water, preferably on a dedicated branch. Chlorination can be justified, but only with monitored dosing, to avoid corrosion and odours.
Regulation and scheduling: securing without overconsuming
Good regulation does the work in the background. Set the setpoints, schedule cycles during off-peak hours and monitor loop return temperatures. The goal is clear: sanitary safety without heating permanently more than necessary.
Implementing and securing DHW: settings, tests and good installation practice
Key settings: tank setpoint, return temperature, timers and valves
Set the tank setpoint to 60°C for hygiene, then bring draw-off back under the regulatory caps with a mixing valve set according to what each room is used for. On the loop, the return has to stay at 50°C minimum, which is checked with a thermometer and recorded. Adjust pump and backup timers to avoid short cycling. Check check-valves, limiters and flow direction before firing up.
Loop balancing and dead volume reduction: quick actions
For a loop that runs unevenly everywhere, start by measuring the returns and opening balancing valves where needed. Reduce dead volumes by removing unnecessary branch connections, shortening runs and insulating lukewarm sections. An oversized circulator cools more than it helps. Reduce the speed if needed.
Recommissioning protocol: flushing, disinfection, temperature checks
Fill, flush at every high point, then rinse until the water runs clear. If the network has been modified or left idle, plan for disinfection and a gradual temperature boost. Check at the furthest points and record the values. The goal is simple: no zone below 50°C when you hand it back. On tankless production the reasoning shifts to the distribution instead, see the instant DHW heat exchanger.
Proving safety and sustaining it: monitoring, maintenance and client communication
Maintenance plan: periodic checks, descaling and cleaning of components
A clear plan sets who does what, when and how. You schedule visual checks, cleaning of filters and heat exchangers, then descaling justified by the recorded water hardness if the water is hard. Goal: keep flow rates and temperatures stable, limit wear and reduce the risk of breakdown.
Logbook: intervention sheets, readings and alerts in case of drift
The logbook gathers intervention sheets, settings, readings (temperatures, pressure, flow rates) and before/after photos. In case of drift, a simple alert triggers a visit. This is your proof of thoroughness and a useful support for after-sales service.
Justifying your price against a cheaper bid: what your quote covers
A quote that prices the temperature survey, mixing by room use and the handover of a logbook is not comparable to a quote that fits a cylinder and stops there. Name the regulatory values on the quote, state what you record at handover and what you keep on file: that is what makes the price gap legible and what protects you if compliance is challenged later. An installation holding 50°C at every point stays efficient because it was balanced, not because it was overheated. To frame good practice and check points, also refer to the obligations and good maintenance practice.
