Temperature Distributions and Bacterial Growth Implications in a Hot Water Storage Tank under Scheduled Draw-off and Heating Cycles

In the context of sustainable energy and smart grid technologies, the sanitary water-heating load represents an important target for load control interventions aimed at energy conservation and implementing demand response. These interventions typically include measures such as, lowering hot water set point temperatures and implementing hot water drawoff and heating cycle scheduling. Utilities can, furthermore, implement demand response measures by automated control of hot water loads. These scheduling interventions generally give rise to lower average water temperatures inside the storage vessels, which can encourage the growth of pathogens such as Legionnaire bacteria. This paper presents the results of an experimental investigation to determine the internal temperature distributions of a domestic water heater for predetermined hot water draw-off and heating cycle scheduling. The development of a dedicated system for measuring the internal water temperature distribution of a cylindrical water heater is discussed. Measured temperature distribution results are presented for horizontal and vertical distributions, using daily draw-off and heating cycle patterns aimed at reducing standing losses. The results show that the scheduling methodology can give rise to increased risk of the bacterial growth, especially for horizontally orientated cylinders.