• Energy Research

Abstract Proper heating system operation in buildings is a vital area for the realization of low-temperature district heating with supply and return temperatures of 55 °C and 25 °C respectively. But the operating area of the district heating companies usually only extends to the substations at the entry of the buildings, and very little is known about the actual use and distribution of heat inside the buildings. This paper describes one possible method for district heating companies or building technicians to monitor heating system operations, and thereby identify heating system malfunctions, inside apartment buildings. The method uses data from existing electronic heat cost allocators to locate radiators with high return temperatures. The method was tested by comparing data from heat cost allocators with detailed measurements of radiator return temperatures in an apartment building in Frederiksberg, Denmark. The investigations indicate that data from the heat cost allocators can be used to identify radiators with continuously high return temperatures, and thereby locate severe problems with e.g. hydraulic balancing in heating systems. However, the method needs further development and tests to ensure the accurate identification of the various heating system malfunctions and their effect on overall heating system efficiency.

Low-temperature district heating is a promising technology for providing homes with energy-efficient heating in the future. However, it is of great importance to maintain thermal comfort in existing buildings when district heating temperatures are lowered. This case study evaluated the actual radiator sizes and heating demands in 4 existing Danish single-family houses from the 1930s. A year-long dynamic simulation was performed for each of the houses to evaluate the potential to lower the heating system temperatures. The results indicate that there is a large potential to use low-temperature district heating in existing single-family houses. In order to obtain the full potential of low-temperature district heating, critical radiators must be replaced. Based on a novel method, a total of nine radiators were identified to be critical to ensure thermal comfort and low return temperatures in two of the case-houses. If these radiators were replaced it would be possible to lower the average heating system temperatures to 50 °C/27 °C in all four houses.

District heating networks increasingly rely on heat pumps, condensing biomass boilers, and excess heat in the transition to sustainable energy systems. Accordingly, district heating operators seek to reduce their networks’ supply and return temperatures to maximise production efficiencies, minimise heat losses from distribution pipes and allow greater utilisation of renewable heat sources and excess heat. Experts have predicted that investing in solutions that reduce heating temperatures in buildings will yield a return on investment of 300% for district heating operators. Therefore, expecting incentives, building operators should identify methods to reduce supply and return temperatures to enable a rapid, widespread transition to low-temperature district heating. Ample research has investigated and documented the feasibility of low-temperature heating in buildings, and this paper presents the first comprehensive review. It synthesises available literature and adds new perspectives to help guide future implementation, research and development of low-temperature heating. The energy and temperature demands of various heating systems provides a background, leading to a review of typical malfunctions and their impacts. The article subsequently reviews the obtainable supply and return temperatures before and after renovating the building envelope and heating systems. It further identifies and summarises vital measures for decreasing heating system temperatures. Ultimately, the authors recommend minimising heating system temperatures using automatic balancing of space heating and ventilation systems, novel solutions for safe domestic hot water supply, and digitally-enabled performance monitoring and optimal control.

Abstract As existing buildings are renovated and energy-efficiency measures are implemented to meet requirements for reduced energy consumption, it becomes easier to heat our homes with low-temperature heating. This study set out to investigate how much the heating system supply temperature can be reduced in typical Danish single-family houses constructed in the 1900s. The study provides a simplified theoretical overview of typical building constructions and standards for the calculation of design heat loss and design heating power in Denmark in the 1900s. The heating power and heating demand in six typical Danish single-family houses constructed in the 1900s were estimated based on simple steady-state calculations. We found that the radiators in existing single-family houses should not necessarily be expected to be over-dimensioned compared to current design heat loss. However, there is considerable potential for using low-temperature space heating in existing single-family houses in typical operation conditions. Older houses were not always found to require higher heating system temperatures than newer houses. We found that when these houses have gone through reasonable energy renovations, most of them can be heated with a supply temperature below 50 °C for more than 97% of the year.

Abstract This study analyses the heat load in a total of 11,584 rooms in 1645 Danish houses and the heat output of the radiators in them to evaluate whether typical radiators are over-dimensioned. The aim was to find out whether radiators in existing houses are suited for low-temperature district heating. We found that new houses are generally more likely to have over-dimensioned radiators, though the heat output of the radiators installed varies a great deal. We also found that many old houses can be equally fit for low-temperature district heating, especially if they have been through some energy renovation. Our results show that approximately 80% of heating systems are over-dimensioned relative to their current design heat load. This share will rise to about 92% as expected energy renovations are carried out towards 2050. Houses with currently over-dimensioned heating systems can be heated with supply temperatures below 60 °C for most of the year. Due to extreme design conditions, even under-dimensioned heating systems can be operated with low temperatures for much of the year, although slightly higher supply and/or return temperatures would have to be accepted.

Abstract Low-temperature heating provides an efficient way of heating our buildings. To obtain a high efficiency it is important that the heating systems in the buildings are operated with both low supply and return temperatures. This study set out to investigate how typical assumptions in the modelling of heat emissions from existing hydraulic radiators affects the heating system return temperatures calculated in a building simulation model. An existing single family house with hydraulic radiators was modelled in the simulation program IDA-ICE. Simulations were performed with various levels of detail and the calculated indoor temperatures and radiator return temperatures were compared to temperatures measured in the case house. The results showed that the detail of the simulation model has a large influence on the results obtained. The estimated return temperatures from the radiators varied by up to 16 °C depending on the assumptions made in the simulation model. The results indicated that a detailed building simulation model can provide a good estimate of the actual heating system operation, provided that actual radiators and realistic indoor temperatures are taken into account in the model.

This study presents a method to adapt existing hydronic systems in buildings to take advantage of low temperature district heating (LTDH). Plate radiators connected to double string heating circuits were considered in an optimization procedure, based on supply and return temperatures, to obtain the required logarithmic mean temperature difference (LMTD) for a low temperature heating system. The results of the analysis are presented as the average reduction of LMTD over the heating season compared to the base case design conditions. Two scenarios were investigated based on the assumption of a likely cost reduction in the end users' energy bills of 1% for each 1 °C reduction of return and average supply and return temperatures. The results showed possible discounts of 14% and 16% respectively, due to more efficient operation of the radiators. These were achieved without any intervention in the thermal envelope or to the heating systems, through simply adjusting the temperatures according to demand and properly controlling the plate radiators with thermostatic radiator valves (TRVs).

Abstract In order to reach targeted 4th generation district heating temperatures around 55 °C supply and 25 °C return, it is necessary to ensure that heating installations inside buildings are designed and operated properly. In this study we investigated the best-case of current design and operation of building installations with the aim of identifying whether there is a gap between current best-case examples and future temperature targets. The study included 7 single-family dwellings and 3 apartment buildings, that were selected based on their low district heating return temperature. Data from the building substations showed that single-family dwellings obtained return temperatures in the range from 25 to 30 °C while the apartment buildings had return temperatures in the range of 30–40 °C. This indicates that there is a gap between the best functioning heating installations in apartment buildings today, and the targeted district heating return temperatures of 25–30 °C in future 4th generation district heating networks. District heating return temperatures in the range of 30–40 °C could however be the initial ambition for the existing buildings all around Europe that are expected to be connected to new district heating systems in the near future.