Temperature reductions in existing apartment building in Viborg, Denmark
The temperatures in the district heating network in Viborg, Denmark are gradually reduced these years, as the district heating operators plan to make use of large heat pumps in the heat supply. Current annual average district heating supply and return temperatures are therefore as low as 68°C/40°C. In order to reduce the district heating temperatures, a large effort has been put into reducing supply and return temperatures in the buildings in the network. One example of this is a test that was carried out in an existing apartment building from the 1970s containing 33 apartments. The building is seen in Figure 88.
The radiator system in the building is connected to the district heating network by a central heat exchanger located in the basement of the building. Inspection of the apartment building showed that the radiator system was not equipped with proper equipment for hydronic balancing, and that the behaviour of occupants in the building had a negative impact on the heating system temperatures (Benakopoulos et al., 2019). As suggested in Chapter 3, the supply temperature of the heating system was therefore optimized according to obtain a low supply temperature and a high flow in the heating system. This strategy minimizes the negative impacts and malfunctions in the control of individual heating elements. The result was that the annual average supply and return temperatures were reduced from 46°C/38°C to 40°C/37°C.
Domestic hot water is prepared in a central heat exchanger in the basement of the building and circulated to the hot water taps in all apartments. To reduce the risk of Legionella bacteria, a disinfection system based on electrolysis of saltwater was installed in the building (Danish Clean Water, 2020). The installation is seen in Figure 88. The disinfection system reduces the risk of Legionella growth in the domestic hot water system, and hence it may be possible to reduce supply and return temperatures in the circulation system from 55°C/51°C to approximately 47°C/44°C. If this is possible, the heat loss from the domestic hot water system can be reduced by approximately 38% and it may also be possible to reduce district heating supply and return temperatures (Benakopoulos et al., 2021). An ongoing research project will test the thesis that the disinfection system can deliver Legionella safe domestic hot water even if circulation temperatures are lowered, which would be a prerequisite to apply the strategy in the future.