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Best-Practice Buildings for Low District Heating Return Temperatures


Dear reader,

low return temperatures are a key objective to enable an efficient operation of district heating networks and especially for the transition towards 4th Generation District Heating (4GDH). On the one hand, it seems clear enough that when it comes to return temperatures, lower is better. And on the other hand, we know that in practice, many buildings operate at higher return temperatures than planned and desired. Therefore, we are happy to present a recent research paper that sets out to investigate which return temperatures are currently achievable with the best building heating systems already in operation today and whether these return temperatures are low enough to satisfy the requirements of 4GDH. Furthermore, the authors evaluate whether they can identify shared features across these best-practice buildings with a positive effect on the heating system operation.

The paper addressing these research questions is titled What does a well-functioning heating system look like? Investigation of ten Danish buildings that utilize district heating efficiently and has been published by Dorte Skaarup Østergaard et al. from the Technical University of Denmark. As a reference, the authors define the range of desired return temperatures for 4GDH between 25 and 30 °C. In order to identify well-performing buildings, the authors collaborated with district heating companies in the Copenhagen area and identified the buildings with the lowest return temperatures based on energy meter measurement data. This search for the best-performing real buildings showed that while some single family buildings were able to reach the desired reference range of 25 - 30 °C, the best-performing apartment buildings reached return temperatures of 30 - 40 °C.

From a detailed analysis of these best-performing buildings including site visits and schematic drawings of the building heating systems, the authors analyzed the common features and differences of the systems. Because of the high required effort, this analysis focused on 10 buildings. Therefore, it is not possible to take this sample as representative for the entire building stock and generalize the findings. Nevertheless it is interesting to note that all of the best-performing buildings worked without a night set-back, and all but one of the 7 single family buildings had a direct network connection and none had domestic hot water (DHW) circulation while the 3 apartment buildings did have a DHW storage and DHW circulation in place. Furthermore, most buildings relied on a two-pipe radiator system to transfer the heat. And in conclusion, the authors show that the best performance with regard to return temperatures does not depend on very low heat demands and intricate knowledge of the heating system operation, but rather that low return temperatures can be achieved with several differences in the building heating systems if the system is designed and operated well.

In the following sections, we go into more detail regarding the methods used and results described in the paper:

Finding buildings with lowest return temperatures

For the scope of the presented paper, the best building performance was defined by a low return temperature. This indicates that the building heating system is able to extract heat from the district heating network efficiently, but does not necessarily mean that the building itself has a low heat demand, too. To find the best-performing buildings according to this requirement, the authors evaluated measurement data from district heating companies in the Copenhagen area and found around 30 buildings with similar very low return temperatures. Out of this selection based on measurement data, 10 district heating customers agreed to a detailed survey of their building heating system. These 10 buildings provide the basis for the investigations presented in the paper and consist of 7 single family houses (1 detached, 6 row houses) and 3 multi-family houses/apartment buildings.

For each of these 10 buildings, the authors set out to analyze the available measurement data in detail as well as evaluate the building characteristics, occupant habits, and heating installations. This included site visits and drawing schematics of the district heating substation as well of the building heating systems. This includes the systems for space heating as well as for DHW. Furthermore, the authors characterized the occupants' habits and their engagement with the heating system operation. Based on this extensive data collection, the authors were able to characterize and compare the different buildings to investigate whether certain factors and designs tend to correlate with the good performance in utilizing district heating for low return temperatures.

Common features and differences between best-performing buildings

From the authors' analysis of the collected data, a first interesting finding is that most of the investigated buildings do not show a significant seasonal variation of the return temperatures, indicating that also the DHW installations can provide very low return temperatures during summer. This is the case both for the single family houses, of which only one building has a DHW storage tank and none of which use DHW circulation, and for the multi-family houses that all use storage tanks and circulation.

Regarding the control strategies, the paper finds that none of these best-performing buildings used a night set-back and most buildings used thermostatic radiator valves. For all but one of the single family houses, these thermostats were set up without pre-settings, while all the multi-family buildings included such pre-settings. Similarly, weather compensation is only active in one of the single-family buildings, while all of the multi-family buildings make use of this control strategy.

Another interesting aspect of the study is that the buildings themselves include buildings being built in the 1920s, the 1970s as well as a recent building from around 2005. The specific heat consumptions range from around 40 kWh/m²a up to around 130 kWh/m²a. This illustrates that not only the most efficient new constructions, but also the building stock with higher heat demands is capable of reaching best-practice return temperatures beneficial for future 4GDH systems. In addition, the occupant engagement level was characterized as "very high" for some buildings, but also as "low" - "medium" for others. This shows that very high engagement and knowledge of the system operation is not a requirement for well-functioning buildings.

Insgesamt wurde das theoretische Potenzial von sieben unterschiedlichen Wärmequellen untersucht. Die folgende Übersicht beschreibt die von Su et al. getroffene Charakterisierung der Wärmequellen. Dabei wurde neben der jährlich verfügbaren Energie auch das Temperaturniveau beschrieben.

In summary, while the sample size of 10 buildings is not enough to draw direct conclusions about the best design options for all buildings in district heating networks, we think this studies provides a very valuable starting point to improve return temperatures in district heating. To this end, the paper establishes a good reference with the fact that return temperatures can go as low as 25 - 30 °C for single-family houses and 30 - 40 °C for multi-family houses. In addition, the investigation shows that these temperatures can be achieved with a variety of different setups. We hope that this can inspire further studies on success factors for low temperatures and practical efforts to improve the efficient operation of district heating networks in the future. In past projects at heatbeat, we have also ranked connected buildings based on their return temperatures and helped operators to find buildings with too high return temperatures. For the future it will be interesting to also look deeper into the lower end of well-functioning buildings to learn from these best-practice examples.

Further information on buildings for low return temperatures

For more information, we highly recommend the original research paper in full, which can be found at https://doi.org/10.1016/j.energy.2021.120250 Further information on the investigated buildings and many interesting research findings beyond can be found in the final report of the IEA DHC Annex XII, which is available with a free registration at https://www.iea-dhc.org/index.php?id=529

And to conclude this 12th issue of our newsletter we want to thank you for your continued interest in district heating research. This issue completes the first first year of our newsletter and we hope to continue with many more interesting research findings in the upcoming issues.

Best regards,
your heatbeat team