Thermal and sanitary performance of domestic hot water cylinders: Conflicting requirements

In order to understand the sanitary implications around the demand side management of domestic hot water cylinders, microbial samples were taken from the bottom of 10 UK domestic electric hot water tanks whose heating elements are connected to a controlled off-peak supply. The results indicated high concentrations of bacteria in the water and biofilm. Microbial concentrations remained high in spite of the application of seven hours of heating during off-peak hours. Further numerical and experimental work shows that this problem arises due to the differing modes of heat transfer that prevail above and below the immersion element. The results from thermal and bacterial growth models suggest that it is impossible to achieve sanitary conditions throughout standard domestic hot water tanks without significantly increasing the heating element temperature or lowering the heating element from its current position. Raising the immersion thermostat temperature results in additional heat losses whilst lowering the immersion position compromises thermal stratification leading to uneconomical operation. Guidelines around storing hot water at temperatures that are sufficient for the purposes of sterilizing human pathogens such as Legionella, fail to take account of the conflict between thermal and sanitary performance. By better understanding the distribution of temperatures and bacteria within hot water tanks along with the associated risks, improved design and control strategies may be adopted to facilitate effective demand side management of hot water systems whilst meeting sanitary requirements.