Legionella and Domestic Hot Water: Why Traditional Storage Tanks Are at Risk and How to Prevent It with I-TES Batteries

In recent months, health authorities have detected the presence of legionella in some domestic hot water systems in the Milan area.
Incidents like this are not attributable to the water supply network, but to the internal management of systems: tanks, boilers, rarely used pipe sections, and areas with stagnant water.

This news story is a useful opportunity to reflect on a point often overlooked: not all hot water production or storage systems carry the same level of risk.
In particular, traditional storage tanks are much more vulnerable compared to modern solutions like i-TES thermal batteries, which by their nature do not allow the proliferation of the bacteria.

Why Legionella Proliferates in Domestic Hot Water Systems

Legionella does not originate from drinking water, but from the conditions created inside certain systems. The bacteria proliferate in three situations:

1. Stagnant or poorly circulated water
2. Lukewarm temperatures, generally between 25 °C and 45 °C
3. Presence of biofilm in pipes or tanks

These factors are not inevitable: they are a direct consequence of how a DHW system is designed and managed. That is precisely why traditional storage tanks are more critical.

Storage Tanks and Boilers: Why They Are Among the Most At-Risk Components

Systems with boilers or domestic hot water tanks must store large volumes of water to ensure immediate availability. However, this automatically creates the ideal environment for the proliferation of legionella.

Here’s why.

1. Water Remains Still for Many Hours

A 300 or 500-liter tank does not circulate continuously.
Part of the water remains there, unmoving, especially during low-demand hours. Where there is stagnant water, there is risk.

2. The Temperature Is Not Uniform

Even when the boiler is set to 60 °C:

• The water at the top can be very hot,
• While the bottom often stays lukewarm, exactly within the range that promotes bacterial growth.

The so-called thermal stratification is one of the most difficult problems to manage.

3. Recirculation Networks Are Complex

In apartment buildings, hotels, gyms, healthcare facilities, the return loop of hot water may include:

• Branching pipelines,
• Rarely used outlets,
• Areas with low flow.

These are the elements where legionella finds its ideal habitat.

4. Anti-Legionella Treatments Don’t Solve the Root Cause

Thermal shocks, disinfections, microbiological tests:
treatments are useful, but they do not eliminate the root of the problem, which is the very existence of a large volume of stagnant water. As long as that volume exists, the risk cannot be completely eliminated.

Why i-TES Thermal Batteries Are Not at Risk of Legionella

This is the fundamental difference. i-TES thermal batteries do not store water, but thermal energy.
Inside, there is no tank where water waits to be used. The water flows through the battery, exchanges heat with the PCM, and immediately continues in the circuit.

What This Means in Practice

• No volumes of stagnant water
  No internal areas where water can stop and become lukewarm.
• No thermal stratification
  No water storage, hence no temperature layers.
• No need for specific anti-legionella cycles for the thermal battery
  Because the bacteria has no suitable habitat to develop.
• Greater safety in complex systems
  Apartment buildings, hotels, sports facilities, healthcare centers: in all these cases, reducing stagnant water volumes is the most important factor in prevention.

In other words:

The i-TES thermal battery does not eliminate legionella: it eliminates the possibility for it to proliferate inside.

And that is exactly what a designer aims to achieve when considering the microbiological safety of a DHW system.

How to Design Safer DHW Systems

Regardless of the technology used, there are some basic principles:

1. Minimize volumes of stagnant water
2. Avoid oversized tanks
3. Ensure adequate temperatures
4. Limit unused pipe sections
5. Prefer systems that store energy, not water

Thermal batteries fall exactly into this last category, offering a crucial design advantage: no water container where legionella can proliferate. Discover how PCM thermal storage works and why it allows immediate availability of hot water without storing large volumes in this article on heat pumps and thermal batteries.

Do you want to make your DHW system safer and reduce the risk of legionella? Our team can support you in evaluating alternatives to traditional boilers and integrating thermal batteries into new systems or retrofit projects. Contact us for a dedicated consultation.