Anti-legionella: securing your DHW effectively

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 most favourable zone sits around 25 to 45°C. In practice, you target storage close to 60°C and a distribution temperature high enough to stay above 50 to 55°C, depending on the building type. Sensitive points: undersized tank, loop that runs too cold, 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.

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 protect users with a mixing valve set according to the site. On the loop, aim for a return that is warm enough and stable. 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 lukewarm zone favourable to the lukewarm zone favourable to legionella.

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 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.

Client pitch: sanitary safety, comfort and cost control in 2026

For the client, you connect monitoring and health. Suitable temperatures and regular servicing reduce legionella risk while ensuring stable hot water and controlled noise. In 2026, this monitoring avoids overconsumption and secures grants thanks to an installation that stays compliant and efficient. To frame good practice and check points, also refer to the obligations and good maintenance practice.