21 April 2026
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Dr. Peter Meža, Senior Product Manager at Hisense Europe, examines the link between low‑temperature hot water systems and Legionella risk and outlines engineering solutions that ensure hygienic safety while maintaining high efficiency. It discusses design principles, control strategies, and smart thermal‑disinfection methods, with particular focus on systems integrated with photovoltaic power.
Domestic hot water production accounts for a substantial share of energy consumption in residential and hospitality buildings. In recent years, heat pumps have increasingly replaced conventional gas boilers as part of broader decarbonization strategies in the European building sector. Their high efficiency, reduced greenhouse gas emissions, and compatibility with renewable electricity sources make them a key technology for sustainable building operation.
One of the advantages of heat pump systems is their ability to integrate with photovoltaic electricity generation. When PV production exceeds the building’s instantaneous electricity demand, heat pumps can convert surplus electricity into thermal energy stored in domestic hot water tanks. This approach increases the self-consumption of renewable energy and reduces dependence on grid electricity.
Despite these advantages, the operating characteristics of heat pumps differ from those of conventional water heating systems. Heat pumps achieve their highest efficiency at moderate temperatures, which often leads to lower storage temperatures in DHW tanks. While this improves system performance, it may also create conditions that allow the growth of Legionella pneumophila, the bacterium responsible for Legionnaires’ disease.
For this reason, hygienic design and appropriate operational control are essential for ensuring the safe operation of heat pump domestic hot water systems.
Legionella growth and temperature conditions
Legionella pneumophila is a naturally occurring bacterium found in freshwater environments. In artificial water systems such as building plumbing networks, it can multiply if environmental conditions are favorable. The primary mode of infection is inhalation of contaminated aerosols, for example from showers, faucets, or cooling systems.
Temperature plays a critical role in Legionella growth. The bacterium multiplies most rapidly in water temperatures between approximately 25 °C and 45 °C. Growth slows significantly above 50 °C, while temperatures above 60 °C led to rapid inactivation.
In many heat pump DHW systems, storage temperatures are typically maintained between 45 °C and 55 °C to maximize the coefficient of performance (COP). Although this range improves energy efficiency, it may overlap with conditions that allow bacterial survival.
Several additional system characteristics can increase the risk of Legionella proliferation:
• large storage volumes with extended water residence times,
• temperature stratification within storage tanks,
• insufficiently balanced circulation loops,
• sections of piping with stagnant or low flow water,
• irregular heating patterns resulting from PV-driven operation.
These factors illustrate that Legionella risk is primarily related to hydraulic design and operational control, rather than to heat pump technology itself.
Hygienic design of domestic hot water installations
Proper hydraulic design is the foundation of hygienic domestic hot water installations. Systems should be designed to ensure continuous water movement and avoid stagnation zones where bacteria may multiply.
One important aspect is the elimination of dead legs, which are pipe sections with little or no water circulation. Such sections can create stagnant water volumes that support microbial growth. In practice, design guidelines recommend limiting stagnant volumes to less than approximately three liters.
Hydraulic balancing of circulation loops is equally important. Balanced systems maintain consistent temperatures throughout the distribution network, ensuring that hot water reaches all outlets at hygienically safe temperatures. Circulation pumps must therefore be correctly dimensioned and equipped with suitable control valves. Material selection also contributes to maintaining hygienic conditions.
Materials used in drinking water installations must comply with European and international standards and should not promote microbial growth. Commonly used materials include
copper, stainless steel, and approved polymer materials such as polypropylene.
Thermal insulation of pipes is another critical design element. Proper insulation prevents excessive heat losses and helps maintain stable temperatures throughout the system. Hot water pipes should also be separated from cold water pipes to prevent unwanted heat transfer.
Finally, systems must be designed to allow effective maintenance and inspection. Access to key components such as storage tanks, circulation pumps, and temperature sensors is necessary to ensure long-term hygienic operation.
Thermal disinfection in heat pump systems
Even with well-designed hydraulic systems, periodic thermal disinfection remains one of the most reliable methods for controlling Legionella in domestic hot water installations.
Thermal disinfection involves raising the water temperature to levels that inactivate bacteria for a specified period. The required exposure time depends on the temperature achieved. For example, water maintained at 60 °C requires only a short exposure time for effective disinfection, while lower temperatures require longer treatment periods.
Many modern heat pump systems include a dedicated anti-Legionella or pasteurization cycle. During this cycle, the storage tank temperature is increased to 60–65 °C or higher for a defined duration.
For thermal disinfection to be effective, several conditions must be satisfied:
- the entire storage tank volume must reach the target temperature,
- circulation loops must also be heated to disinfecting temperatures,
- the exposure time must be sufficient to ensure bacterial inactivation,
- temperature profiles should be monitored and recorded.
In some systems, auxiliary electric heaters are used to support high-temperature cycles. Although this temporarily reduces system efficiency, such cycles are typically performed only periodically and therefore have a limited impact on overall energy consumption.
Digital monitoring and smart control
Advances in digital control technologies have significantly improved the management of domestic hot water systems. Modern controllers can monitor temperature conditions throughout the installation and automatically verify whether disinfection cycles have been successfully completed.
Data logging allows facility managers to analyze temperature trends and identify potential operational problems. Monitoring platforms can track parameters such as minimum and maximum loop temperatures, disinfection cycle frequency, and flow irregularities.
Smart monitoring systems also enable predictive maintenance. By detecting unusual patterns in temperature or circulation performance, the system can identify issues such as pump malfunction, sensor drift, or incomplete disinfection cycles before they develop into serious hygiene risks.
Integration with building management systems (BMS) further enhances operational transparency. In larger buildings such as hotels or apartment complexes, centralized monitoring allows operators to supervise multiple systems and ensure compliance with hygiene requirements.
Interaction between PV Systems and DHW Operation
The integration of heat pumps with photovoltaic systems introduces new opportunities for improving energy efficiency. During periods of high solar electricity production, heat pumps can convert surplus electricity into thermal energy stored in domestic hot water tanks. However, PV-driven operation may also introduce irregular heating patterns. If hot water production is concentrated primarily during midday hours, storage tanks may remain at moderate temperatures for longer periods. This brings energy savings, but can also introduce several disadvantages, which can be mitigated by the actions shown below.
Disadvantages of PV coupled heat pumps and solutions
Disadvantages
- Longer periods of low temperature storage
- Irregular heating patterns
- Reduced nighttime circulation temperatures
Solutions
- PV optimized disinfection cycles
- Dynamic setpoints based on solar forecasts
- Hybrid systems combining heat pumps with electric immersion heaters
- Thermal storage strategies that maintain hygiene without without sacrificing efficiency
Without proper control strategies, this operating mode could increase the time during which water remains within the bacterial growth range. Several technical measures can mitigate this risk:
- scheduling thermal disinfection cycles during periods of solar surplus,
- dynamically adjusting temperature setpoints based on solar production forecasts,
- combining heat pumps with auxiliary electric heaters for high-temperature cycles,
- implementing control algorithms that balance energy efficiency with hygienic safety. Through these strategies, PV integration can improve overall system performance while maintaining hygienic operating conditions.
Conclusion
Heat pumps are becoming a central technology for sustainable domestic hot water production in modern buildings.
Their high efficiency and compatibility with renewable electricity make them an essential component of low-carbon energy systems. At the same time, the safe operation of heat pump DHW systems requires careful attention to water hygiene. Legionella growth is primarily influenced by hydraulic design, temperature management, and operational control rather than by the heating technology itself.
By combining proper system design, elimination of stagnation zones, appropriate material selection, and periodic thermal disinfection, it is possible to maintain hygienically safe domestic hot water systems without compromising energy efficiency.
The integration of digital monitoring technologies and photovoltaic electricity generation further enhances the ability to manage both energy performance and hygienic safety in modern buildings.
Source
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