## 2 Thematic Review and Analysis
### Control Strategies
#### 2.3.1 Flow Control  
This section examines flow control techniques for reducing return temperatures in DH systems, specifically variable-flow and constant-flow strategies. As illustrated in Figure 7, the variable-flow strategy is more effective in achieving lower return temperatures compared to the constant-flow strategy, a finding supported by various studies [23; 47; 75].  
* The Constant-Flow Strategy  
Constant-flow operation has been prevalent due to its simplicity in maintaining hydraulic balance and the cost benefits of using a constant-speed pump. This pump is typically set to handle peak heat loads at its most efficient speed, providing a consistent flow rate year-round. Heat output is generally controlled by adjusting the supply temperature according to the outdoor temperature, using a weather compensation curve. Bypasses that divert  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Cases**} & \multirow{2}{*}{**Substation Type**} & \multicolumn{3}{c}{**Control**} \\ \cline{3-5}  & & **Control Type** & **Control at Indoor** & **Control at Substation** & **Radiator Inlet** \\  & & & **Heating System** & **Temperature** \\ \hline \multirow{4}{*}{Case D} & \multirow{4}{*}{Indirect} & \multirow{4}{*}{Constant Flow} & \multirow{4}{*}{-} & Supply temperature adjustment by flow control at DH side of heat exchanger (no flow adjustment at the indoor heating system) & Set to weather compensation \\  & & & & & \\ \cline{1-1}  & & & & \\ \  
supply water to the return pipeline when the desired indoor temperature is reached are also used to regulate heat output. However, constant-flow systems are criticized for high return temperatures, poor heat production control, and excessive heat loss [47; 48; 49; 73; 75; 88; 89].  
Applications to modify DH flow with constant-speed pumps, not considered in variable-flow methods, include adjusting the network system curve via a throttling valve at the pump outlet, and installing a bypass line between the pump inlet and outlet to redirect excess flow to the return pipeline [23; 51; 68].  
* The Variable-Flow Strategy  
The variable-flow strategy, an alternative to the constant-flow method, employs a variable-speed pump and decentralized self-regulating flow control valves at consumer sites (e.g., thermostatic radiator valves). This approach enhances flexibility and responsiveness to fluctuations in heat demand, thereby reducing overconsumption and improving thermal comfort. This allows for the optimization of return temperatures according to load conditions, achieving the lowest feasible return temperature [17; 23; 48; 73; 90].  
Thermostatic radiator valves can be used to maintain variable flow in DH systems with constant-speed pumps, though this requires minimum flow bypasses, which can increase return temperatures [73; 75]. Additionally, central flow control, where pump speed is adjusted based on outside temperature, is another proposed control option [75].  
[PERSON] [49] highlights the benefits of transitioning from a constant-speed to a variable-speed indoor heating pump. The variable-speed pump adjusts its speed to handle low-flow conditions, whereas the constant-speed pump operates at up to 30% of its nominal flow, with flow regulation achieved through throttling at the control valve. During low-load periods, the variable-speed pump results in lower DH return temperatures compared to the constant-speed pump, with reductions of up to 2.8 \({}^{\circ}\)C and averages of 1.1 \({}^{\circ}\)C observed for the 80/60 \({}^{\circ}\)C temperature scheme, and reductions of up to 1.9 \({}^{\circ}\)C and averages of 0.9 \({}^{\circ}\)C for the 55/45 \({}^{\circ}\)C scheme.