Domestic hot water storage: Balancing thermal and sanitary performance

Thermal stratification within hot water tanks maximises the availability of stored energy and facilitates optimal use of both conventional and renewable energy sources. However, stratified tanks are also associated with the proliferation of pathogenic bacteria, such as Legionella, due to the hospitable temperatures that arise during operation. Sanitary measures, aimed at homogenising the temperature distribution throughout the tank, have been proposed; such measures reduce the effective energy storage capability that is otherwise available. Here we quantify the conflict that arises between thermodynamic performance and bacterial sterilisation within 10 real world systems. Whilst perfect stratification enhances the recovery of hot water and reduces heat losses, water samples revealed significant bacterial growth attributable to stratification (P<0.01). Temperature measurements indicated that users were exposed to potentially unsanitary water as a result. De-stratifying a system to sterilise bacteria led to a 19% reduction in effective hot water storage capability. Increasing the tank size to compensate for this loss would lead to an 11% increase in energy consumed through standing heat losses. Policymakers, seeking to utilise hot water tanks as demand response assets, should consider monitoring and control systems that prevent exposures to unsanitary hot water.