## 3 Discussion
This measure not only curtails unnecessary bypass flows but also confers operational flexibility and a more dynamic response to fluctuating heat loads. During charging, the heat storage tank draws a flow rate from the supply line analogous to a bypass, yet it does so more purposefully: rather than merely circulating the medium back into the return, it stores thermal energy for subsequent utilization. This setup offers a more energy-efficient alternative, as it moderates return temperatures and significantly lessens avoidable heat losses. Figure 6 provides a concise comparative illustration of how bypass configuration and substation design influence average return temperatures. Substations that rely on a dedicated domestic hot water storage unit, rather than an instantaneous heat exchanger, show a distinct advantage in driving down return temperatures. This outcome highlights that the thermal buffering effect of the storage tank not only meets hot water demand but also helps temper the flow returning to the DH network. In parallel, the data suggest that internal bypass approaches wherein a modest flow is maintained at the substation side, particularly when supply temperatures decrease below 50 \({}^{\circ}\)C further minimize return temperatures compared to branch-end bypass loops. In contrast, branch-end (external) bypasses configured with a set temperature of 35 \({}^{\circ}\)C typically produce higher return temperatures. Nevertheless, systems designed around a storage tank that uses a built-in heat-exchanger coil necessitate water setpoints exceeding 55 \({}^{\circ}\)C to meet hygienic requirements, specifically for preventing legionella growth. These higher setpoint temperatures curtail some of the energy efficiency benefits from operating at lower return temperatures, since the need to maintain elevated tank temperatures unavoidably leads to an overall increase in return temperature. Hydraulic bypass valves are categorized into minimum flow bypasses and flow control bypasses. Minimum flow bypass valves tend to increase return temperatures throughout the heating season, with a more significant rise during low-load periods. To address the high return temperature issue caused by these valves, variable-speed pumps are recommended. These pumps can maintain the necessary DH flow while being capable of shutting off when the network is not under load. Similarly, flow control bypass units benefit from the use of variable-speed pumps. Admixing bypasses are problematic as they lower the supply temperature just before it enters the radiator unit, leading to decreased performance and efficiency. This reduction in supply temperature results in a higher return temperature, reduces efficiency at the heat source, and increases network heat loss. To mitigate these issues, supply temperature adjustments should be made at the heat source rather than at the end-user site. The temperature management scheme, with a focus on return temperature effects, positively influences the functionality and capacity of heat storage units. However, research in this area remains limited. Cascading applications allow multiple subbranches within a DH network to utilize heat sequentially. Each unit or group of similar units can use the thermal energy from the return medium of preceding units when available. By further employing the residual thermal energy from a previous unit in another unit operating at a lower temperature, the overall heat potential is maximized, and the total DH return temperature is reduced. A cascaded network is feasible when the return line from a high-temperature sub-network can supply a lower-temperature sub-network, especially if the latter benefits from building renovations. For instance, low-energy buildings near high-temperature DH systems, whether newly constructed or renovated, can be efficiently supplied through network cascading. This approach enhances heat utilization and significantly lowers the return temperature in the existing DH system.