## 2 Thematic Review and Analysis
### The Distribution Network
#### 2.1.2. Bypass Applications  
Bypass flows (also known as'recirculation'), when not properly managed or due to faulty operations, can significantly elevate return temperatures by transferring heat carrier water from the supply line to the return line without utilizing its thermal energy. Various types of bypasses in DH networks include [23; 51; 52] (with additional types and their purposes outlined in [52; 53]):  
1. 'Thermostatic bypasses', which redirect supply water to the return line during low-heat demand to prevent excessive cooling. 2. 'Minimum flow bypasses', which ensure a minimum flow rate when there is no demand. 3. 'Flow control bypasses', which are equipped at constant-speed pump outlets to return excess flow to the return line and adjust the DH flow rate. 4. 'Admixing bypasses', which are installed at building substations to lower the supply temperature and prevent scaling. [PERSON] and [PERSON] [52] estimate that bypass flows constitute 10% to 20% of the total annual flow. Evidence of their impact on return temperature is detailed in [44; 54], where the deliberate opening of flush bypasses in a new housing complex of 200 apartments increased the flow rate from 10-20 m\({}^{3}\)/h to 60 m\({}^{3}\)/h, causing the supply-return temperature difference to drop from 25-30 \({}^{\circ}\)C to less than 5 \({}^{\circ}\)C. Subsequently, closing these bypasses restored the temperature difference to 30-35 \({}^{\circ}\)C despite lower heat demand during the test period [44]. Figure 3: Variation in operation temperatures as observed for various buildings located in the same region of the DH network, the first three used hydraulic balancing (HB) while the others were without hydraulic balancing (NB: Not Balanced); the top of each bar is the supply temperature while the bottom is the return temperature—reproduced partially from the source data shown in [50]. * Thermostatic Bypasses  
Thermostatic bypasses, installed at substations or DH network branch ends, prevent supply water from cooling excessively during summer and low-load periods. Without them, supply quality may fall short of thermal comfort needs and cause long wait times for domestic hot water. The bypass redirects supply water to the return line to maintain warmth when the temperature drops below a set threshold (a process known as'standby' or 'idling' for substations [52; 55]), helping avoid inadequate supply quality and prolonged hot water wait times [56; 57; 58; 59]. For instance, in the Lystrup low-temperature DH system (55/25/20 \({}^{\circ}\)C), thermostatic bypasses at consumer substations are set to 40 \({}^{\circ}\)C, while intermediate substations are set to 35 \({}^{\circ}\)C and the flow rates are adjusted to 3 L/min with a \(\pm\)2.5 \({}^{\circ}\)C dead-band [58; 60]. [PERSON] et al. [61] demonstrate the variation in thermostatic bypass flow (installed only at branch ends) and its impact on DH return temperature across heat load periods (Figure 4). They also note that, according to some DH practices, the bypass set temperature should be 10 \({}^{\circ}\)C lower than the supply temperature. Figure 5 shows various alternatives to bypass use, with (a) and (b) representing current bypass practices and (c)-(e) depicting alternative solutions. Substations equipped with storage tanks for domestic hot water (e) typically require minimal or no bypass flow. Continuous charging of these tanks during off-demand periods maintains network activity, preventing supply water cooling and stagnation [57; 58; 59; 61; 62]. [PERSON] et al. [63] show that storage options, achieving a return temperature of approximately 27.6 \({}^{\circ}\)C, are superior to heat exchanger equipment with thermostatic bypass, which results in a return temperature of approximately 38.4 \({}^{\circ}\)C (measured in May). An alternative layout with an additional booster heater (e.g., electric heater and micro-heat pump) near the hot water tap can quickly meet hygiene requirements.