## 3 Discussion  
To enhance the efficiency of DH systems and support the integration of variable renewable energy sources, maintaining low return temperatures is increasingly critical. Return temperature, influenced by factors such as heat load, supply temperature, flow rate, and system design, cannot be directly adjusted but is a key indicator of system performance. This review aims to consolidate current knowledge on return temperature reduction techniques, focusing on the most cited methods and evidence from the literature. Hydraulic imbalance is a significant cause of elevated return and supply temperatures in DH systems (see Figure 3). This imbalance arises from uneven hydraulic pressure distribution, leading to high flow rates near the pump station and low flow rates farther away. Low differential pressure can cause thermal discomfort and necessitate higher supply temperatures, while high differential pressure can lead to overheating and disrupt control equipment. Bypass flows, which redirect the heat carrier medium from the supply line to the return line without transferring its thermal energy, also contribute to elevated return temperatures. While various types of bypasses are employed in DH networks, not all are redundant, particularly when considering the current technological advancements in DH systems. To maintain thermal comfort for household hot water usage and reduce wait times, thermostatic bypass units are used to prevent excessive cooling of the supply medium during off-peak periods and summer. As illustrated in Figure 4, rising bypass flow rates correlate with increasing return temperatures during low demand periods. The figure also highlights that during periods of lower heat demand (e.g., at 3281 h and 0.26 kW\({}_{\text{th}}\) load duration), return temperatures increase steadily. This corresponds to the situation that return temperature tends to increase under partial-load conditions, primarily due to low flow rates reducing the efficiency of heat extraction at substations. Figure 5 shows various traditional and modern thermal bypass applications, each affecting return temperature reduction differently. Historically and traditionally, bypass applications have served to sustain a minimal flow rate through the network, most notably to prevent the supply line from cooling excessively during low-demand periods. In contemporary DH systems aiming for optimal efficiency, several alternative strategies can obviate or significantly reduce the need for such bypass flows without undermining system operation or user comfort. One of these approaches is the comfort bathroom concept. Rather than employing continuous bypass flow to keep the supply line at a favorable temperature, a minimal but consistent supply to the bathroom heating unit is maintained during low-demand times (including summer in colder regions). By ensuring that sufficient thermal energy remains in the supply line, the medium does not have the opportunity to cool down extensively, thus eliminating the demand for a dedicated bypass circuit. This concept effectively repurposes what might otherwise be wasted bypass flow into a beneficial partial load for maintaining user comfort. A further, albeit distinct, solution is the application of recirculation strategies that recycle surplus heat within the supply side instead of re-introducing it into the return line. These strategies help to reduce the incremental rise in return temperature by ensuring that any underutilized heat is redirected for further use or staging rather than contributing to a higher return temperature profile in the main DH loop. Consequently, the network benefits from more consistent and lower return temperatures. An additional method to replace or minimize bypass flow involves integrating heat storage solutions into the system design. Instead of counting on continuous bypass operations to retain stable supply temperatures, storage units can absorb surplus heat and release it on demand.