Water Heater\nType, Temperature Setting, Operational\nConditions, and Insulation Affect Ecological Niches for Legionella Growth

Residential water heating represents an important nexus\nof energy/water\nconservation, waterborne disease, hygiene, and consumer preference.\nHere, we examine attributes of two off-the-shelf 151-L tank water\nheaters, one with hot water recirculation (recirculating) and another\nwithout recirculation (standard), compared to a tankless on-demand\nheater (on-demand). Energy efficiency decreased in the order on-demand\n> standard > continuous recirculation. However, the electric\non-demand\nwater heater repeatedly malfunctioned and could not consistently achieve\ntarget temperatures >48 °C. At a temperature setting of 48\n°C,\nthe volume of water in the pipe and tank within a temperature range\nat very high risk for Legionella growth\n(38–47 °C) decreased from recirculating (150 L) > standard\n(40 L) > on-demand (∼0.47 L). However, at a temperature\nsetting\nof 66 °C, the standard tank was stratified, and the bottom 13\nL fell within the very high-risk temperature range, whereas the recirculating\ntank system maintained 100% of its volume >55 °C, which is\nnot\nsuitable for Legionella growth. Addition\nof insulation was found to markedly increase the temperature throughout\nthe tank. In the standard tank set at 66 °C with insulation,\nno volume was maintained within the very high-risk range. Insulation\ncan holistically increase energy efficiency and reduce health risks\nat a sufficiently elevated temperature setting.