tures in networks can secure the phase-out of fossil-fuel-based heat generation, minimize heat losses in the distribution net- works, and reduce the energy use and overall cost of heating. Under the assumption of a future European DH sales of 950 TWh and a temperature reduction of 30 °C, it was estimated that low-temperature district heating (LTDH) could secure a potential yearly cost reduction of 14 billion euros. The DH operators can control the supply temperatures and the pressure in the networks according to the seasonal variation of the heat demand. In contrast, the DH return temperature depends exclusively on the operations of the heating systems. Several studies found that existing buildings may be comfort- ably heated with supply temperatures below 55 °C for most of the heating season, but this is rarely the case. In fact, the inefficient control of space heating and domestic hot water systems limits low-temperature operations, particularly for large existing multi-storey buildings. Another common issue is related to the non-uniform heat distribution among different flats leading the entire building to be operated with unneces- sary high supply temperatures. Hence, the buildings are seen as one of the major bottlenecks for the green transition of the DH industry. Role of the digitalization of demand-side The new European Energy Efficiency Directive 2018/2002 is binding member states to have all energy meters, including submeters in multi-storey buildings with central heating or connected to DH networks, remotely readable by 1 January 2027. While the primary intention of the policy is to secure bill- ing transparency for the end-users, the impulse to digitalize the demand side is opening new opportunities to monitor the heating systems, improve the operation, and ensure the ex- pected comfort in existing buildings with lower temperatures.

ture sensors were developed to estimate the minimum supply temperature necessary to secure the indoor comfort based on the apartment with the greatest heat demand in multi-storey buildings. A strategy to gradually reduce the supply tempera- ture in existing multifamily buildings was investigated in col- laboration with industrial partners and the local DH operator in Viborg (Denmark). Experience from a local Danish district heating network The experiment was carried out in five existing multifamily buildings connected to the local DH network. None of them went through a deep energy renovation, representing a sam- ple of ordinary Danish residential buildings. The results doc- umented that it was possible to ensure the heating comfort in the apartments with supply temperatures below 55 °C at outdoor temperatures of 0 °C in all buildings. This was in line with the general low-temperature DH require- ments, as illustrated in Figure 1. The new minimum supply temperature curve for the space heating system, presented in the figure, was estimated for the apartment with the highest heat demand in one of the buildings. This was below the cor- responding DH supply temperature control curve, highlight- ing that low-temperature heating can be introduced in typical residential properties by simply adjusting the central weather compensation controller settings. The figure also reported the real energy distribution for 2021 according to the different outdoor temperatures. Over 70% of the total heat consumption was used at outdoor temperatures above 0 °C. This suggests that for the most significant part of the heating season, the DH networks can be operated with a supply temperature of 55 °C. Furthermore, outdoor tempera- tures below -9 °C were never recorded in 2021, showing that the design conditions rarely happen in actual operations and that the heating systems are generally oversized and therefore suitable for being operated with lower temperatures.

Innovative integration of data from heat cost allocators (mounted on each radiator), DH energy meters, and tempera-

Figure 1: Minimized operating temperatures in a multifamily building and DH low-supply temperature control curve.

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