Domestic hot water in existing residential buildings: Comparative simulation study of efficiency and hygiene challenges

Refurbishing existing buildings is essential for meeting emission targets. Domestic hot water (DHW) systems significantly impact energy use but receive less focus than space heating during refurbishment, limiting efficiency potential and renewable energy integration. This study examines DHW system configurations across building standards regarding efficiency and hygiene challenges, focusing on final energy demand, renewable energy use, and stagnation temperatures. Results demonstrate that decentralised DHW systems are 20-25 percentage points more efficient than centralised systems. Lowering DHW temperatures or switching to decentralised configurations reduces energy demand, improves heat pump performance, and increases renewable heat utilisation. For gas boilers and district heating, decentralised instantaneous systems with supplemental heating achieve optimal performance, reducing energy demand by 4-17% through eliminating circulation losses. For heat pumps, centralised instantaneous systems with two-zone buffer storage prove optimal, achieving savings of 16-35%. Improving DHW system design significantly influences district heating supply and return temperatures in well-insulated buildings, enabling renewable source integration. Thermal separation of cold and warm water pipes reduces stagnation temperatures below 25 °C. System design and operational strategies prove crucial in refurbishment, reducing hygiene risks while improving efficiency. As space heating demand decreases, optimising DHW systems becomes essential for minimising energy demand and supporting renewable energy integration.