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Legionella development in sanitary heat exchangers.

Legionella development in sanitary installations is often thought to be a storage problem. In order to determine whether specific measures are necessary to prevent Legionella development in instantaneous domestic hot water (DHW) production at low DHW production temperatures, a test setup with a small plate heat exchanger was built. Legionella spp. was cultivated in a separate water tank and used for the initial contamination of the heat exchanger. Once the biofilm was formed, the test setup was connected to uncontaminated drinking water and the DHW production temperature in the heat exchanger was set at 45°C, whilst the exchanger was continuously maintained at this temperature. Flushes at different flow rates and thermal shocks were implemented in order to reach Legionella spp. concentrations beneath 1000 cfu/l. Flushing proved to be ineffective at all flow rates tested. Shocks at 60°C, and even weekly shocks of 10 and 30 minutes at 65°C didn't suffice. Although increasing the DHW production temperature to 60°C reduced the Legionella spp. concentrations significantly, values above 1000 cfu/l were still obtained.