Heat pumps are quickly becoming a standard solution for hot water (DHW) production across Europe. Driven by decarbonization targets, electrification policies and the rapid growth of rooftop solar photovoltaics (PV), many buildings are replacing conventional gas boilers with efficient electric systems.
The technology offers clear advantages. Heat pumps require much less electricity than direct electric heating and significantly reduce CO2 emissions2 emissions and can be easily integrated with modern building management systems. In combination with PV systems, they can shift hot water production to periods of solar energy surplus, increasing self-consumption and reducing demand on the electricity grid.
However, the increasing adoption of hot water heat pumps also brings with it a significant technical challenge: maintaining water hygiene while operating at lower temperatures.
The temperature dilemma
Heat pumps achieve their high efficiency by operating at relatively low temperatures. Domestic hot water tanks in heat pump systems typically operate between 45°C and 55°C at source – a range that maximizes performance and minimizes electricity consumption.
Unfortunately, this temperature band overlaps the growth range of Legionella pneumophila, the bacterium responsible for Legionnaires’ disease. Legionella bacteria can thrive in water between about 25 C and 45 C and can survive up to about 50 C. Reliable inactivation usually requires temperatures above 60-65 C that are maintained for a sufficient period of time.
Image: College of Industrial Technology
Under certain conditions, domestic hot water systems can inadvertently create an environment where bacteria can multiply. Large storage volumes, long water retention times or temperature stratification in tanks can contribute to this risk. Circulation loops that are too large, poor pipe insulation or stagnation zones in pipe networks can further increase the risk of bacterial growth.
The integration of PV systems can introduce another variable. When heat pumps are used primarily during midday solar production, heating cycles can become less frequent, potentially prolonging periods of water stagnation if the system is not properly designed.
Importantly, these risks are not inherent to the heat pump technology itself. They are mainly related to system design and operational strategies.
Several system characteristics can increase risk:
- Large storage volumes with long retention times
- Thermal stratification, which leaves cooler layers at the bottom of tanks
- Circulation loops that are too large or poorly insulated
- PV powered intermittent heating, allowing stagnation periods to be extended
- Irregular or incomplete thermal disinfection cycles
- Dead legs in pipework, especially during renovations
- Presence of biofilm and limescale
These factors are not inherent defects of heat pumps; they are design and control challenges that can be addressed with the right technology. As heat pumps become more common in multi-family buildings, hotels and renovations, these risks become more relevant.
Designing hygienic hot water systems
Good engineering can significantly reduce the legionella risk in hot water systems based on heat pumps. A well-designed installation ensures stable temperatures, sufficient water circulation and the ability to perform effective thermal disinfection.
Hydraulic design plays a central role. Stagnation zones in pipes should be avoided where possible and so-called ‘dead legs’ – parts of pipes where water remains unused – should be kept to a minimum. Adequate flow rates in the circulation loops help maintain consistent temperatures throughout the system.
Material choice is also important. Components used in drinking water systems must comply with relevant standards and must not promote microbial growth. Copper, stainless steel and approved plastics are commonly used in modern installations.
Equally important is good pipe insulation. Heat losses in poorly insulated distribution networks can quickly lower water temperatures into the bacterial growth range.
Finally, circulation pumps must be properly sized and regularly maintained to ensure long-term system performance.
Thermal disinfection remains essential
The most reliable method for combating Legionella in sanitary hot water systems remains thermal disinfection. This process involves periodically raising the water temperature to a level that is lethal to the bacteria.
Most modern heat pumps have special anti-legionella or pasteurization modes that temporarily increase the tank temperature to approximately 60–65 C. During these cycles it is important that the entire storage volume reaches the required temperature and that the circulation loops are also sufficiently heated. Although thermal disinfection requires additional energy, it represents a practical compromise between efficiency and safety.
Digital monitoring and control systems make this process easier to manage. Modern controllers can track temperatures, record disinfection cycles and automatically trigger heating events when necessary.
Smart control meets solar energy
The combination of heat pumps and PV systems is becoming increasingly common in homes and commercial buildings. By operating heat pumps when solar energy is available, buildings can significantly increase self-consumption.
Smart controllers can also use PV forecasts or electricity price signals to plan thermal disinfection cycles during periods of solar surplus or low rates. With this approach, hygiene requirements can be met without increasing total energy costs.
At the same time, digital monitoring platforms can track key parameters such as loop temperatures, power conditions and the frequency of disinfection cycles. This enables predictive maintenance and early detection of potential hygiene risks.
European hygiene regulations
The hygiene of sanitary hot water is regulated in Europe through various directives and technical standards. The European Drinking Water Directive (EU) 2020/2184 sets quality requirements for water intended for human consumption and emphasizes the importance of preventing microbiological contamination in building distribution systems.
International guidelines and technical standards generally recommend maintaining storage temperatures of at least 60 C and distribution temperatures above 50 C, combined with regular monitoring and disinfection procedures.
Professional organizations such as REHVA emphasize that energy efficient systems – including heat pump installations – must combine efficiency measures with hygienic hydraulic design and reliable temperature control.
Efficiency and safety can coexist
Heat pumps play a key role in the decarbonization of buildings and the integration of renewable electricity. Their ability to work efficiently with photovoltaic systems makes them particularly attractive for modern energy systems.
But energy efficiency should never come at the expense of water hygiene.
With good hydraulic design, elimination of stagnation zones, adequate temperature management and regular thermal disinfection, the risks associated with Legionella can be effectively minimized. Smart digital controls and PV integration further ensure that systems can maintain both high efficiency and safe operating conditions.
As heat pumps become the new standard for domestic hot water production, balancing energy performance and hygiene will remain a central task for engineers, installers and building operators.
Conclusion
Heat pumps are becoming a central technology for hot water production in modern buildings, especially in combination with photovoltaic systems. Their efficiency and flexibility make them an important part of the transition to low-carbon energy systems.
At the same time, maintaining good water hygiene remains essential. Legionella risks are primarily related to system design, temperature management and operational practices, rather than the heat pump technology itself.
With careful hydraulic design, elimination of stagnation zones, sufficient circulation and regular thermal disinfection, these risks can be effectively controlled. When supported by smart controls and digital monitoring, heat pump DHW systems can deliver both high energy efficiency and safe, hygienic operation.
Author: Peter Meža
Peter Meža is affiliated with the Slovenian College of Industrial Engineering and contributes to academic and technical work in engineering education. His work focuses on industrial engineering topics and supports the development of practical skills and research in modern engineering fields.
The views and opinions expressed in this article are those of the author and do not necessarily reflect those of the author pv magazine.
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