## 2 Thematic Review and Analysis
### The Distribution Network
#### 2.1.2. Bypass Applications
Figure 5: Bypass applications as (**a**) external and (**b**) internal types; and alternatives with a (**c**) comfort bathroom solution, (**d**) summer re-circulation line, and (**e**) equipping of storage tank; the concepts are based on descriptions given in [57, 59, 60, 61, 62]. Another alternative is the \"summer recirculation line,\" a three-pipe system that redirects cooled supply water to a dedicated recirculation line before returning it to the heat source (Figure 5d) [52; 61]. Simulations by [PERSON] et al. [61] show that with flue-gas condensing units, the return line temperature is 22 \({}^{\circ}\)C, while the recirculation line can reach 44 \({}^{\circ}\)C or 53.5 \({}^{\circ}\)C depending on whether an instantaneous heat exchanger or storage equipment is used. In a two-pipe network with bypass units, return temperatures are 35.5 \({}^{\circ}\)C or 45.6 \({}^{\circ}\)C, respectively [61]. In constant-flow DH systems utilizing constant-speed pumps, thermostatic bypasses function similarly to thermostatic control valves in variable-flow systems by regulating indoor temperatures. When the desired indoor temperature is achieved, the heating flow is redirected through 3- or 4-port valves directly to the return pipeline to prevent overheating, while the total DH flow remains constant due to the constant-speed pump [23; 67]. * Hydraulic bypassesFlow control bypass units use a recirculation circuit between the pump outlet and inlet to manage excess flow in systems with constant-speed pumps. These bypasses adjust the overall DH flow by redirecting surplus flow [23; 51; 68]. An alternative to this method is installing a control valve at the pump outlet, which adjusts the system curve and pump operation. However, while this method can alter the flow, it often results in high return temperatures at the heat source [23; 51; 68]. To mitigate excessive return temperatures and hydraulic issues, variable-speed pumps are recommended [23]. Certain heat generation units, such as conventional boilers, require a minimum return temperature to prevent condensation at the exhaust gas, which can damage the heat recovery exchanger and boiler shell. Typically, flow control bypasses are used to raise the return temperature above the condensation point by mixing it with the supply medium before entering the boiler [53; 70; 71]. Newer boilers address this issue by using corrosion-resistant materials [70; 71]. In building applications, DH systems with constant-speed pumps often use a bypass with a 3-way control valve installed between the inlet and outlet of the interior heating units for flow adjustment. This setup, unlike flow control at the pump outlet, maintains indoor temperatures similarly to thermostatic radiator valves. By bypassing the flow when the indoor temperature is at the set level, this method increases the return temperature to match the supply medium's temperature [23; 72; 73]. * Admixing Bypasses  
An admixing bypass line reroutes return medium to the supply medium in direct substation setups to lower the inlet temperature to the interior heating system. This approach prevents excessively high temperatures that could scalp users and helps maintain the operational condition for the thermostatic radiator valves. It is rewarding to note that increasing the supply temperature leads to return temperature reduction for a given radiator size. However, this mixing reduces the thermal potential of the supply medium at the building inlet, leading to exergy loss. Additionally, high supply temperatures contribute to exergy loss during heat generation and increased heat loss from the DH network [1; 28; 74]. [PERSON] and [PERSON] [74] demonstrate that exergy loss in DH systems occurs due to the mixing of supply and return mediums, as well as heat transfer losses at the substation heat exchanger and radiator. Specifically, there is a 50% loss over the fuel exergy content during heat production, a 12% loss due to heat loss over the DH supply, and a 66% loss from the delivered heat to the building.