Legionnaires' disease at home: how can you make your hot water safe?

When we talk about water quality at home, we often think of the water we drink. Yet the hot water we use every day to shower also deserves particular attention.

Under certain conditions, domestic hot water systems can encourage the growth of bacteria called Legionella, which cause a potentially serious lung infection: Legionnaires' disease (legionellosis).

Contrary to popular belief, this risk does not only concern hospitals, hotels or large shared buildings. It can also exist in a detached house, particularly when water stagnates, when the temperature is poorly controlled or when equipment is not properly maintained.

How does Legionella grow? Can you catch Legionnaires' disease in the shower? What temperature should you set your water heater to? And which everyday habits help reduce the risks?

Here is our essential advice to prevent Legionnaires' disease at home and better understand how your domestic hot water system works.

What is Legionnaires' disease?

Legionnaires' disease is a lung infection caused by bacteria of the genus Legionella, the species most frequently involved being Legionella pneumophila.

These bacteria are naturally present in certain aquatic environments. They can also colonise man-made installations containing water when conditions are favourable to them.

Domestic hot water networks are among the installations that can allow them to multiply.

According to Santé publique France (the French public health agency), around 1,800 cases of Legionnaires' disease have been recorded each year in France on average since 2017. The disease can be serious, with a fatality rate of around 10% among reported cases.

These figures cover all possible sources of exposure and do not mean that all cases were contracted at home.

How do you catch Legionnaires' disease?

Infection mainly occurs through inhaling fine droplets of contaminated water, known as aerosols.

These droplets can be produced by various types of equipment, including showers, hot tubs and some installations that use water in the form of mist.

Legionnaires' disease is generally not transmitted by drinking contaminated water. The risk is mainly respiratory, although accidentally inhaling water into the airways can also be a cause.

Person-to-person transmission is exceptional.

Why can Legionella grow in domestic hot water?

Legionella multiplies more easily when several conditions are met.

The main factors that encourage its growth include:

• A water temperature favourable to its growth.
• Insufficient circulation or prolonged stagnation.
• The presence of deposits, limescale or corrosion.
• The formation of biofilms on the internal surfaces of installations.
• Insufficient maintenance of the network and equipment.

A temperature between 25 and 45 °C encourages its growth

According to Santé publique France, Legionella can proliferate in man-made water systems when conditions are favourable, particularly between 25 and 45 °C.

This temperature range can occur in parts of a hot water network that is poorly adjusted or insufficiently insulated.

Hot water stored at too low a temperature can therefore encourage bacterial growth.

Conversely, a sufficiently high temperature in the circuit helps limit the growth of Legionella.

However, a distinction must be made between the water storage temperature and the temperature delivered at the shower, as water that is too hot carries a significant risk of scalding.

Stagnant water encourages bacterial proliferation

A pipe left unused for several days or weeks can become an environment favourable to the growth of micro-organisms.

This particularly concerns secondary bathrooms, rarely used taps and second homes.

It can also occur in sections of pipework where water barely circulates, sometimes called "dead legs".

Regular water circulation is therefore an important element of prevention.

Limescale and biofilms make maintenance more difficult

Inside pipes and equipment, certain micro-organisms can form biofilms: bacterial communities that stick to surfaces.

These biofilms can help protect the bacteria present and make them harder to eliminate.

Limescale, deposits and corrosion can also create conditions that help them persist in some installations.

This is why prevention relies on temperature, water circulation and regular maintenance of the network all at once.

Can you catch Legionnaires' disease in the shower?

Yes, the shower is one of the possible points of exposure when the water contains Legionella.

When it is running, it produces many water droplets, some of which can be fine enough to be inhaled.

If the water is contaminated, these aerosols can carry bacteria into the air you breathe.

This obviously does not mean that taking a shower is dangerous in itself.

The risk depends in particular on whether bacteria are present in the installation, their concentration, the conditions of exposure and the vulnerability of the person.

Why do shower heads and hoses need to be maintained?

Over time, shower heads and hoses can accumulate limescale and deposits.

These can encourage the formation of biofilms and make cleaning more difficult.

French health authorities therefore recommend regularly cleaning, descaling and disinfecting taps and shower fittings.

It is also advisable to replace old, damaged or hard-to-maintain parts.

What temperature should you set your water heater to in order to limit Legionella?

Temperature is one of the main levers for preventing Legionnaires' disease in domestic hot water systems.

But be careful: raising the temperature without precautions can cause serious scalding, particularly in children and elderly people.

Three elements need to be distinguished:

• The temperature at which hot water is produced or stored.
• The temperature of the water circulating in the pipes.
• The temperature of the water at the tap or shower.

Why keep the water hot enough in the network?

French health recommendations stress the importance of maintaining a sufficiently high temperature to limit the multiplication of Legionella.

In its advice on prevention at home, Santé publique France recommends in particular checking that the hot water temperature reaches at least 50 °C in the network.

For production and storage installations, settings must be adapted to the type of equipment, its volume and the applicable requirements.

It is therefore better to have your water heater settings checked by a professional rather than changing the thermostat arbitrarily.

Beware of the risk of scalding

Water at 60 °C can cause serious burns in a few seconds.

French recommendations provide for limiting the water temperature at points of use, particularly in rooms used for washing.

A properly installed thermostatic mixing valve can help deliver water at a suitable temperature, while allowing the network to operate in conditions that help prevent Legionella.

The right approach is therefore to combine bacterial prevention with protection against scalding.

6 simple steps to prevent Legionnaires' disease at home

You do not need a complex installation to apply the main prevention recommendations.

1. Have your water heater serviced regularly

A poorly maintained water heater can have deposits, temperature problems or malfunctions that lead to a deterioration in water quality.

Santé publique France recommends having individual hot water systems serviced annually by a professional.

The type of work depends on the model: electric hot water cylinder, boiler, heat pump water heater or other equipment.

It is important to follow the manufacturer's instructions and the applicable maintenance obligations.

2. Clean shower heads and taps

Shower heads, hoses and tap aerators should be cleaned regularly.

Santé publique France's recommendations include cleaning, descaling and disinfecting them at least once a year.

White vinegar can be used for descaling when the materials allow it.

However, descaling with vinegar does not necessarily replace proper disinfection.

Only use products that are compatible with the equipment, following the manufacturer's instructions. Never mix different household products, especially bleach and acidic products.

3. Run water through rarely used outlets

A second bathroom, an extra shower or a rarely used tap can encourage stagnation.

Santé publique France recommends running cold and hot water at least once a week at rarely used outlets.

When doing this, it is best to limit exposure to aerosols, for example by standing back from the shower head.

4. Flush outlets after a long absence

After several weeks of holiday or a long period without occupancy, the water in the pipes may have stagnated.

Before using showers and taps again, run the cold and hot water in line with health recommendations.

Avoid standing in the spray or right next to the aerosols during this operation.

If the home has been closed for a very long time or if the installation is not working properly, ask a professional for advice.

5. Check that the installation is working properly

Hot water that takes an unusually long time to arrive, large temperature variations or heavily scaled equipment may warrant an inspection.

These signs do not prove that Legionella is present, but they may reveal an operating problem.

A plumber or a water hygiene professional can check the hot water production, the circulation and the equipment concerned.

6. Avoid unused sections of pipework

Pipes that are no longer used but are still connected to the network can contain stagnant water.

During plumbing work or a renovation, it is best to have these unused sections inspected in order to limit areas of stagnation.

Removing or modifying a circuit must be carried out by a qualified professional.

Second homes: a risk not to be overlooked

Second homes have one particular feature: the water installations can remain unused for several weeks, or even several months.

This prolonged stagnation can encourage the growth of micro-organisms when the other conditions are met.

Holiday homes, seasonal accommodation and rarely used bathrooms therefore deserve particular vigilance.

What should you do before using a shower again after several weeks away?

When you return, start by checking that the water heater is working and that there is no visible malfunction.

Then run the water at the various outlets, avoiding direct exposure to the droplets produced by the shower.

Also check the condition of the shower heads, hoses and tap aerators.

If the installation is old or has been shut down for a very long period, a professional inspection may be worthwhile before putting it back into service.

Who is most vulnerable to Legionnaires' disease?

Anyone can be exposed to Legionella, but some people are at higher risk of developing a severe form of the disease.

According to Santé publique France, the main risk factors include:

• Advanced age.
• Smoking.
• Chronic respiratory diseases.
• Diseases that weaken the immune system.
• Immunosuppressive treatments.

Vigilance is particularly important when vulnerable people live in the home.

What are the symptoms of Legionnaires' disease?

The disease can begin with flu-like symptoms: fever, fatigue, muscle aches or headaches.

It can then progress to pneumonia, with symptoms such as coughing and breathing difficulties.

The incubation period is generally two to ten days, although longer periods have been observed.

If fever is accompanied by respiratory symptoms, especially in a vulnerable person, it is important to see a healthcare professional quickly.

In the event of respiratory distress, contact the emergency services.

Can a water filter protect against Legionnaires' disease?

Water filtration can play a role in improving water quality, but it is essential to distinguish between the different technologies and their uses.

A system installed on the main water supply does not perform the same function as a specific microbiological filtration device fitted directly to a shower.

Above all, preventing Legionnaires' disease does not depend solely on the quality of the water entering the home.

Why is filtration at the water inlet not enough?

Legionella can grow inside installations, particularly in hot water networks, when temperature, stagnation and maintenance create favourable conditions.

Water filtered upstream can therefore be exposed to contamination or bacterial proliferation further along the circuit.

This is why no whole-house filtration system should be presented as a substitute for temperature control, regular water circulation and maintenance of the installation.

FLX Compact: a complementary approach to water quality

At FLX Water, we have developed the FLX Compact, a whole-house filtration solution designed for homes supplied by the mains water network.

Installed on the main water supply, it filters the water before it is distributed to the various points of use in the home.

Its technology combines 0.1 µm hollow fibres, an electro-adsorption mechanism and high-density activated carbon.

It operates with no power supply of its own and no chemicals added in its process.

This solution is part of an overall approach to improving the quality of household water.

However, it should not be regarded as a guarantee of protection against Legionnaires' disease, particularly because Legionella can multiply downstream of the unit, in the domestic hot water network.

Preventing this disease therefore requires maintaining good operating and maintenance conditions throughout the installation.

How can you make your domestic hot water safe in the long term?

To limit the growth of Legionella, French health authorities highlight several complementary measures.

Risk factor	Prevention measure
Lukewarm water in the network	Have production and distribution temperatures checked
Prolonged stagnation	Run water regularly at rarely used outlets
Scaled shower heads and hoses	Clean, descale and disinfect the equipment
Poorly maintained water heater	Follow the manufacturer's maintenance recommendations and call in a professional
Unused pipes	Have sections of the network that encourage stagnation inspected
Long absence	Check the installation and flush the outlets before using them again
Risk of scalding	Control the temperature at points of use, in particular with suitable equipment

These actions address the main factors that encourage bacterial proliferation, without confusing health prevention with water treatment.

Conclusion: well-maintained hot water is an essential habit at home

Legionnaires' disease remains a relatively little-known risk in the home environment.

Yet domestic hot water systems can, under certain conditions, encourage the growth of Legionella.

The good news is that several simple steps can reduce the risks: maintaining suitable temperatures, avoiding stagnation, cleaning shower equipment and having your water heater serviced regularly.

These precautions are particularly important in homes occupied by vulnerable people and in second homes.

At FLX Water, we advocate an overall approach to water quality at home: understanding its composition, choosing the right equipment and making sure the entire household network works properly.

Because improving water quality is not just about what arrives at the meter: it also means taking care of the installations that deliver it every day.

Frequently asked questions about Legionnaires' disease at home

Can you catch Legionnaires' disease in the shower?

Yes, when a shower sprays water droplets containing Legionella, inhaling them can be a route of infection. This does not mean that all showers present a high risk: the risk depends in particular on the condition of the installation and the vulnerability of the person.

At what temperature does Legionella grow?

Legionella multiplies particularly in water at a temperature between 25 and 45 °C. Maintaining suitable temperatures in the hot water network is therefore an important prevention measure.

What temperature should you set your water heater to?

The setting depends on the type of appliance and installation. A temperature high enough to limit bacterial growth must be maintained while avoiding scalding at points of use. A professional can check the appropriate setting.

Can an electric water heater encourage Legionella?

Yes, like other hot water systems, an electric hot water cylinder can present favourable conditions if the temperature is too low, if the water stagnates or if maintenance is inadequate. The risk depends on the installation and how it is used.

Should you run the water after several weeks of holiday?

Yes. Santé publique France recommends running cold and hot water after a long absence, before using the outlets again, especially showers. It is advisable to stand back from the shower head during this operation to limit the inhalation of aerosols.

Is white vinegar enough to eliminate Legionella?

White vinegar can help remove limescale from certain compatible equipment, but it should not be regarded as a method that guarantees the elimination of Legionella. Cleaning, descaling and disinfection are separate operations.

Does a water filter protect against Legionnaires' disease?

A whole-house filtration system does not guarantee the prevention of Legionnaires' disease, as bacteria can grow in the pipes and equipment located downstream. Prevention relies mainly on temperature, water circulation and maintenance.

Should you have the water from your water heater tested?

A specific analysis may be relevant when a problem is suspected or as part of a health investigation. It is not systematically necessary for every individual home. If in doubt, ask a qualified professional or the regional health agency (ARS) for advice.

French sources

• Santé publique France – The disease: legionellosis. Definition, routes of infection, symptoms, risk factors and French epidemiological data.
• Santé publique France / ARS Auvergne-Rhône-Alpes – Preventing legionellosis at home, Public Health Bulletin, March 2023, page 22. Practical recommendations on temperatures, equipment maintenance and water circulation.
• CSTB / ARS Normandie – Controlling the risk of Legionella growth in domestic hot water networks. Technical document on temperatures, networks and means of prevention.
• ARS Normandie – Preventing Legionella growth and the risk of scalding, 2024. Technical guidelines for centralised domestic hot water systems in buildings open to the public; these obligations do not automatically apply to detached houses.
• Santé publique France – Poitou-Charentes health surveillance bulletin. Information on the factors that encourage the proliferation of Legionella and its transmission.