Legionella in hot water systems

When designing hot water systems the water temperature, water retention time, pipe material and making way for regular system maintenance is of great importance in order to prevent microbacterial growth.

Legionella is a mesophile bacteria which lives in water both in nature and in drinking water systems. All hot water systems are at risk getting infected but there is an increased risk of growth in systems where:

•         Warm water remains more or less stagnant due to low consumption

•         Biofilm inside tanks and pipes have been allowed to build up.

•         Water temperature is between 25 °C and 46 °C which is ”ideal” for legionella

•         There are ”dead ends” without flow

•         There are sediment, rust, scale and sludge which provide good food sources for the bacteria.

•         Pipe- and tank insulation is missing or in poor condition.

•         The system is not maintained properly    

All hot water system should be provided with effective water disinfection system which is able to remove both biofilm and kill free bacteria and other microorganisms without affecting taste and smell of the water.   

However water temperatures in hot water tanks should always be kept at 60 °C. Temperatures at the taps pipes should be no less than 55 °C. If the water temperature in exceed 60 °C, undesirable scaling will occur in tanks and pipes.

National building codes, legislation and other national guidelines concerning hot water systems have to be observed as well.      

When designing at hot water system it should always be considered if a hot water exchanger can be used instead of a hot water tank. A hot water exchanger is often a plate exchanger and is characterized by having no water volume where bacteria can grow.