Development and test: Future-proof substation designs for the low-temperature operation of domestic hot water systems with a circulation loop

On the transition toward low-temperature district heating (DH), generation sectors, distribution networks, and building consumers should all be adapted to low-temperature operation conditions. However, a bottleneck in lowering DH return temperatures is the domestic hot water (DHW) system with a circulation loop in multifamily buildings. Existing systems with a single heat exchanger often led to elevated return temperatures because of the reheating of the circulation loop. This study developed several innovative designs for future-proof DHW substations that decouple the heating of cold water and circulation flows, ensuring lower DH return temperatures in large multifamily buildings. First, a theoretical analysis was performed for benchmarking the return temperature for various proposed design configurations under low-temperature operation conditions; then, the proposed configurations were tested for a Danish multifamily building connected to a medium-low-temperature DH network. In the field tests, compared to a typical DHW substation with a single heat exchanger, the proposed configuration with the circulation loss booster reduced the average DH return temperature from 46.4 °C to 34.1 °C and 27.9 °C for parallel or serial connections, respectively. Economic analysis confirms the viability of the proposed solution, with a payback period ranging from 3.4 to 7.9 years.