Domestic hot water systems in well-insulated residential buildings: A comparative simulation study on efficiency and hygiene challenges

Domestic hot water (DHW) is essential for daily life, yet the production can be energy intensive. Advances in building insulation reduced space heating demand, while DHW energy demand remained constant or even increased. The need for a higher proportion of renewable heat amplifies the conflict in DHW systems between energy efficiency, hygiene, and comfort, since high temperatures are required for hygienic purposes. Thus, developing DHW systems efficiently utilising renewable heat without excessive temperature requirements is essential. This paper reviews efficiency and hygiene challenges in DHW systems and assesses proposed solutions through comparative simulations in well-insulated residential buildings. Results show higher efficiencies in decentralised than centralised DHW systems. However, even in decentralised systems the necessity for circulation is a significant limitation. Lowering DHW temperature or transitioning to decentralised systems significantly reduces final energy demand, improves heat pump performance and increases the renewable heat share. Reducing DHW temperature from 60 to 50 °C increases stagnation periods in lower temperature intervals (22–34 °C) in warm water pipes. The study indicates significant potential to increase system efficiency and reduce final energy demand in decentralised or low-temperature DHW systems and introduces a novel method to compare conditions with regard to hygiene of DHW system simulations.