Influence of operation of the domestic hot water system on the return temperature in the district heating network

An excessively high district heating return temperature of heating medium of the primary side may result in inefficient operation of the district heating system due to a reduction in the difference between the supply and return temperature of the district heating network medium. The operational return temperature of the district heating network is influenced by various factors, occurring both at the thermal substations and on the end-user side. However, the problem of high return temperature of heating medium in district heating in summer is not well documented in scientific literature. The aim of this study was to assess the real magnitude of the temperature difference between the supply and return of the district network medium and the degree of utilization of the heat supplied to prepare domestic hot water in existing district heating substations on the basis of field study. The research was carried out for 481 buildings supplied by the district heating network in Poland. The characteristic parameters of thermal substations on the network side were measured at hourly resolution for a period of one month during summer. It was noticed that only in the case of 2 % of all analyzed buildings, the actual decrease of temperature of district heating network medium reached and exceeded the level of designed 30 K. Besides, it was found that the significant (44 %–65 % in the case of multifamily buildings and 23 %–78 % in the case of educational buildings) oversizing of the hot water exchangers affects the excessively high temperature of the water returning to the DH network from the thermal substations. Finally, possible solutions of this problem in existing DH network and new, lower unit indicators of the daily demand for domestic hot water were proposed for residential and educational buildings at the level of 55 dm3/(d·person) and 6 dm3/(d·person), respectively.