Influence of mixing valve dynamics and recirculation loop connection to solar tank on large hot water system performances

•Experimental validation of dynamic models for transient recirculation flow rate.•Cold plug flow generation lowers energy savings and promotes legionella development.•Increased hot water set-point underperforms looking at energy, health or durability.•Self-limitation phenomenon of solar tank cooling by recirculation loop. A centralized solar domestic hot water system for a multi-family building is analyzed numerically. The solar preheated water in the solar tank is flowing into an auxiliary tank, connected to a gas boiler. At its output, a mixing device is used to ensure hot water at steady temperature before going through the recirculation loop. This reference system is sized as a typical installation under mild climate in France. The load profile is generated statistically with a one-minute time step. Small time scale phenomena have consequences on the global system behavior: recirculation flow rate variations due to pressure drop change is modeled and validated experimentally. This work’s goal is to analyze these effects on two sub-parts of the plant: mixing valve and recirculation loop connection to solar storage. In each case, yearly simulations with 6 s time step provide quantitative performance indicators for different sizing and control options. Results show that mixing valve slow response, due to ageing or bad design, leads to a loss of 3.5% energy savings compared to using a fast component. Furthermore, health risks increase as the minimal temperature in the recirculation loop is often under 50 °C, which promotes legionella development due to cold water plug flows generated in the loop by the mixing valve dynamic response. The recirculation loop, linked to a solar tank, leads to performance improvement with 6–15% increase in energy savings following solar oversizing and reduced overheating risks.