In order to determine whether it is possible to reduce energy use for domestic hot water (DHW) production and distribution, without increasing the risk of Legionella pn. development in sanitary installations, a full-scale test facility was built, consisting of a 200 liters water tank, a circulation system of nearly 40 meters long and 2 draw-off pipes. On a daily basis, a consumption profile corresponding to the DHW use of a single family (4 persons) was applied using two draw-off pipes, one corresponding to a kitchen and the other to a bathroom. Legionella pn. was cultivated in a separate water tank and then introduced into the test facility. In this test facility, applying different types of thermal shocks at 60°C (up to 2h) on a contaminated installation with a DHW production temperature of 45°C was not sufficient to keep Legionella pn. concentrations beneath 1000 cfu/l, in both the water tank and the circulation loop. Even daily shocks at 60°C were insufficient. After the thermal shocks the Legionella pn. concentrations reached 105 - 106 cfu/l within a couple of days. The expansion vessel, installed on the cold water inlet of the DHW production, proved to be an important source of recontamination of the installation after a thermal shock. Applying a weekly thermal shock of 24h at 65°C, in combination with regular draw-off during this shock on both draw-off pipes (of minimum 150s in this test setup), lead to stable Legionella concentrations <1000 cfu/l.
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