Efficiency and Hygiene in DHW Systems for MFH: Evaluating Heat Exchanger Arrangements and Charging Strategies with R290 Heat Pumps
This study examines domestic hot water (DHW) preparation in multi-family dwellings using an R290 brine-water heat pump under realistic dynamic conditions, focusing on energy efficiency and hygiene compliance. Three system concepts were tested: Potable water storage tank with external heat exchanger (two charging strategies), buffer water tank with instantaneous flow water heater (IFWH) (two variants) and buffer water tank with internal heat exchanger for discharging. Charging strategy strongly influences efficiency. Multi-pass charging achieved the lowest heat pump temperature and highest performance factor, while single-pass charging required higher temperatures and reduced efficiency. Systems using buffer tanks with instantaneous flow heaters or internal heat exchangers for discharging showed lower performance due to higher storage temperatures in the lower part of the tanks and DHW circulation effects. Swiss hygiene standards (=55 °C in circulation return, =50 °C at taps) were met in all systems. Histogram analysis confirmed that potable water storage tanks offer strong Legionella protection because most water remains hot and incoming cold water remains below the critical range between 25 and 40 °C due to good stratification, while storage tanks with internal heat exchangers for discharging may have localized zones near this range. Buffer systems do not store potable water, so risk is confined to the circulation loop. In conclusion, multi-pass external heat exchanger charging offers the highest efficiency, while temperature analysis does not show long periods of time with temperatures within the hygienically critical range and temperatures in the upper part are always sufficiently high. Optimized control strategies, inlet velocities lower than 0.1 m/s and DHW circulation management are essential to enhance stratification and to minimize electricity use of the heat pump. For systems with ISWH, it has been shown that a separate heat exchanger for circulation may improve performance and safety, though economic feasibility must be considered.