## 3 Discussion
Notably, changes in DH return temperature lead to a substantial increase in DH pump load with only minor variations in efficiency (see Figure 9). The temperature scheme of DH operation significantly influences electricity loss in an extraction-condensing cogeneration unit relative to DH heat load (see Figure 10). Furthermore, the supply temperature has a more pronounced effect on the electricity-to-heat ratio compared to the return temperature, depending on the cogeneration cycle configuration. For heat-only boilers, regardless of their condensing capacity, the return temperature significantly impacts boiler efficiency, as illustrated in Figure 13. The same figure also highlights that incorporating condensing technology in the boiler unit markedly enhances overall efficiency. Heat pump units are gaining popularity, with evidence indicating that lower return temperatures improve seasonal performance factors. However, it is noted that the impact of DH supply temperature on heat pump efficiency is more pronounced than that of the return temperature. Solar collector performance improves with decreasing return temperatures. For prosumers, the effect of return temperature variations depends on the chosen substation layout. In the recovery of industrial excess heat, where an industrial process medium is cooled while a DH medium is heated, lowering the DH return temperature enhances the industrial cycle's heat recovery capacity. The heat supplier determines the supply temperature, while the characteristic return temperature of the DH system is influenced by the cooling capacity of consumer units at the specified supply temperature and the control considerations for all system components. This underscores the necessity for site-specific temperature optimization, which must account for all factors affecting lifecycle costs and environmental impacts, alongside an annual review. The performance of the system is significantly influenced by its components, and changes in return temperature markedly affect the system's characteristics, each with distinct implications (see Figure 14). Optimization problems vary widely and can be formulated with different objectives and constraints. The effectiveness of a particular layout may differ based on the specific boundary conditions of the site. When addressing the load on the peak plant, the set supply temperature is a critical factor. An optimization problem can be formulated to determine the appropriate radiator dimensions to reduce the return temperature, as both supply temperature and radiator dimensions have distinct impacts on return temperature. Additionally, another optimization task involves evaluating whether enhancements to the DH operational temperature achieved by instantaneous heaters can be offset by improvements to the DH network and heat supply. Although radiator add-on fans significantly improve return temperature, an optimization issue arises regarding whether their electricity consumption is justified by the additional electricity generated from the heat source operation due to the reduced return temperature. To effectively reduce return temperature in DH systems, it is essential to consider both technical and non-technical factors. Given that heat demand drives DH systems, increasing consumer awareness is crucial, as it significantly influences demand levels. Implementing a pricing structure that incentivizes return temperature reduction can encourage performance improvements at end-user sites (see Figure 15). A common approach involves establishing a motivation tariff with bonuses for excessive cooling and/or penalties for insufficient cooling. Successful examples from Denmark and Sweden are detailed in Section 2.6, outlining the steps taken and results achieved. Another strategy includes charging consumers based on the internal energy of the supplied medium or its volume, which gives end users control over their use of thermal capacity. The ownership of end-user substations is also important, as these require regular maintenance.