## 2 Thematic Review and Analysis
### The Distribution Network
#### 2.1.2. Bypass Applications
In a subsequent experiment conducted in July, buildings with only heat exchangers achieved a return temperature of approximately 22.3 \({}^{\circ}\)C, a significant improvement from the 38.4 \({}^{\circ}\)C observed in May. Figure 4: The change in the bypass flow rate and the DH flow rate as obtained through the low-heat demand periods together with the DH return temperature simulated (peak heat demand is given as 2.29 kW\({}_{\text{th}}\) for each low-energy house while the linear heat density of the DH system as 177 kWh\({}_{\text{th}}\)/year); the top of each bar in the figure indicates the overall DH flow rate—reproduced partially from the source data given at [61]. [PERSON] et al. [64] explore using booster heaters with micro-storage tanks as an alternative to thermostatic bypasses. A micro-storage tank helps avoid excessive electricity use by the booster heater when the hot water temperature is low at the start of tapping. The local booster heater maintains the water at a comfortable temperature until the hot DH water arrives. The study finds that with a micro-storage tank, the return temperature can be as low as 16 \({}^{\circ}\)C with an ultra-low-temperature supply of 35 \({}^{\circ}\)C. In comparison: (i) a return temperature of 22 \({}^{\circ}\)C is achieved with a low-temperature supply of 50 \({}^{\circ}\)C and a heat exchanger alone together with the comfort bathroom concept; (ii) a return temperature of 27 \({}^{\circ}\)C is noted with a heat exchanger and thermostatic bypass at a supply temperature of 65 \({}^{\circ}\)C; and (iii) a return temperature of 25 \({}^{\circ}\)C is observed with a storage substation at a supply temperature of 65 \({}^{\circ}\)C [64, 65]. [PERSON] et al. [59] performed a simulation-based analysis comparing various substation designs and bypass solutions under two bypass application strategies. The cases analyzed include (i) a substation with an external bypass line and instantaneous heat exchanger (Figure 5a), (ii) the same configuration with an internal bypass line (Figure 5b), (iii) a substation with a storage tank and instantaneous heat exchanger without bypass (Figure 5e), and (iv) a substation with only a storage tank with built-in coils circulating DH water. For the instantaneous heat exchanger, the average return temperature during on-demand operation is 23 \({}^{\circ}\)C, ranging from 21 \({}^{\circ}\)C to 31 \({}^{\circ}\)C, while in the charging mode, the average return temperature is 32 \({}^{\circ}\)C.  
Figure 6 presents the simulation results, showing that the conventional strategy using branch-end bypasses with a set temperature of 35 \({}^{\circ}\)C (supply temperature at 60 \({}^{\circ}\)C) achieves the lowest return temperature when storage is used. Another strategy, involving bypass lines to all DH network substations and maintaining a modest flow when the supply temperature falls below 50 \({}^{\circ}\)C, is also considered. This internal bypass approach, as depicted in Figure 5b, further reduces return temperatures compared to external bypasses. However, the return temperature for domestic hot water storage tanks with built-in coils is considered excessive due to hygiene requirements that necessitate maintaining storage temperatures above 55 \({}^{\circ}\)C to prevent legionella bacteria growth [59]. [PERSON] et al. [57] propose the 'comfort bathroom' concept (Figure 5c) as an alternative to thermostatic bypasses, using bathroom floor heating instead. This approach reduces the average DH return temperature from 27.7 \({}^{\circ}\)C to 23.8 \({}^{\circ}\)C, decreasing heat loss from the return line by 35% and total DH heat loss by 13%. [PERSON] et al. [66] note, however, that due to flow restrictions, bathroom floor heating cannot fully replace thermostatic bypasses, limiting the extent of the return temperature reduction.