Water Heater Type, Temperature Setting, Operational Conditions, and Insulation Affect Ecological Niches for <i>Legionella</i> Growth

Residential water heating represents an important nexus of energy/water conservation, waterborne disease, hygiene, and consumer preference. Here, we examine attributes of two off-the-shelf 151-L tank water heaters, one with hot water recirculation (recirculating) and another without recirculation (standard), compared to a tankless on-demand heater (on-demand). Energy efficiency decreased in the order on-demand > standard > continuous recirculation. However, the electric on-demand water heater repeatedly malfunctioned and could not consistently achieve target temperatures >48 °C. At a temperature setting of 48 °C, the volume of water in the pipe and tank within a temperature range at very high risk for <i>Legionella</i> growth (38-47 °C) decreased from recirculating (150 L) > standard (40 L) > on-demand (∼0.47 L). However, at a temperature setting of 66 °C, the standard tank was stratified, and the bottom 13 L fell within the very high-risk temperature range, whereas the recirculating tank system maintained 100% of its volume >55 °C, which is not suitable for <i>Legionella</i> growth. Addition of insulation was found to markedly increase the temperature throughout the tank. In the standard tank set at 66 °C with insulation, no volume was maintained within the very high-risk range. Insulation can holistically increase energy efficiency and reduce health risks at a sufficiently elevated temperature setting.