The Health and Safety Executive’s Approved Code of Practice (ACoP) and Guidance document titled “Legionnaires’ disease: The control of legionella bacteria in water systems” (L8) gives practical advice on how to comply with UK health and safety law with respect to the control of Legionella bacteria.
HSG274 Part 2: The control of legionella bacteria in hot and cold-water systems
2.8 The cleanliness of the system must be maintained, as legionella bacteria are more likely to grow in a system fouled by deposits. In hard water areas, softening of the cold water supply to the hot water distribution system should be considered to reduce the risk of scale being deposited at the base of the calorifier and heating coils, and to reduce the potential for scale build-up within the system pipework and components.
2.72 Light scale formation on the inner surfaces of pipes can be protective against the leaching of metals such as lead or copper, but heavier deposits are likely in hard water areas. These deposits increase the surface area and therefore the potential for microbial colonisation (biofilm formation) and can provide protection from the effects of biocides.
In hard water areas, softening of the cold-water supply to the hot water system should be considered. This is to reduce the risk of scale being deposited at the base of the calorifier and heating coils, especially at temperatures greater than 60°C, and the potential for scale build-up within system pipework and components (e.g. TMVs) which may significantly reduce flow and adversely affect the efficiency of the system.