• Energy Research
• 2021-2025close

The operation of typical domestic hot water (DHW) systems with a storage tank and circulation loop, according to the regulations for hygiene and comfort, results in a significant heat demand at high operating temperatures that leads to high return temperatures to the district heating system. This article presents the potential for the low-temperature operation of new DHW solutions based on energy balance calculations and some tests in real buildings. The main results are three recommended solutions depending on combinations of the following three criteria: district heating supply temperature, relative circulation heat loss due to the use of hot water, and the existence of a low-temperature space heating system. The first solution, based on a heating power limitation in DHW tanks, with a safety functionality, may secure the required DHW temperature at all times, resulting in the limited heating power of the tank, extended reheating periods, and a DH return temperature of below 30 °C. The second solution, based on the redirection of the return flow from the DHW system to the low-temperature space heating system, can cool the return temperature to the level of the space heating system return temperature below 35 °C. The third solution, based on the use of a micro-booster heat pump system, can deliver circulation heat loss and result in a low return temperature below 35 °C. These solutions can help in the transition to low-temperature district heating.

Automated hydronic balancing in space heating systems is crucial for the fourth-generation district heating transition. The current manual balancing requires labor- and time-consuming activities. This article presents the field results of an innovative electronic radiator thermostat tested on two Danish multi-family buildings. The prototypes had an additional return temperature sensor on each radiator and an algorithm was used to accurately control valve opening to ensure automated hydronic balancing. The results highlighted that the new thermostat performed as expected and helped secure the cooling of district heating temperatures —defined as the difference between supply and return temperature—4–12 °C higher during the test compared to results obtained in 2020, when the prototypes were replaced with state-of-the-art thermostats in the first building. The measurements from the other building illustrated how only two uncontrolled radiators out of 175 could contaminate the overall return temperature. The remote connection of the thermostats helped pinpoint the faults in the heating system, although the end-users were not experiencing any discomfort, and secure, after fixing the problems, a return temperature of 35 °C. Future designs may consider integrating a safety functionality to close the valve or limit the flow in case of damage or malfunction to avoid a few radiators compromising the low-temperature operation of an entire building before the cause of the problem has been identified.

Abstract The low-temperature operation of existing radiator systems connected to district heating is hindered by various factors, including non-ideal local control by users. The use of only a few of the available radiators to provide the necessary thermal comfort in an apartment is a common example. This requires a higher supply temperature and results in a high return temperature. This study investigated the potential of minimising the supply temperature of the radiator system to stimulate the use of all the available radiators in each apartment. Data from existing electronic heat cost allocators were used to detect the number of radiators not being used at any given time. A thermal/hydraulic model of the radiator system of a building was created to calculate the minimum supply temperature required according to the maximum pump operation. Energy-weighted average supply and return temperatures of 44 °C and 30 °C, respectively, could be achieved when all the radiators were used. The investigation showed that under the minimum supply temperature and a sufficient hydraulic balance, the required heat could only be delivered if all the available radiators were used. A good hydraulic balance could be secured by appropriately setting the balancing valves in each riser.