Legionella considerations

One should exercise caution to avoid the risk of the growth of the Legionella Pneumophila bacterium. These bacteria are naturally found in water, but at low concentrations. They can multiply quickly when the water temperature is between 86 and 113 °F (30 and 45 °C).† Their numbers start to diminish at temperatures above 122 °F (50 °C), and almost instantly die above 150 °F (66 °C). The greatest health danger is not in drinking contaminated water, but in inhaling contaminated dispersed water droplets, which can happen when taking a shower. Storing large quantities of water at temperatures in the range of 86 to 113 °F (30 to 45 °C)should be avoided when it is intended to be used (from a tap) later. Local legal requirements may apply to this issue and should be taken into account.

The typical approach in the United States to avoiding the Legionella bacteria issues is to store the hot water at 140 °F (60 °C). Other design options are to limit the quantity of usable hot water by means of a heat exchanger or tank-in-tank construction, where the temperature of the delivered water is 140 °F (60 °C). In Europe, other options include periodically reheating the water above 150 °F (66 °C) (setting the controller for the auxiliary heater to do this when solar heat alone cannot achieve this), or instantly heating the water just before use to above 185 °F (85 °C).