The Technical Document HSG274 Part 2: The control of legionella bacteria in hot and cold water systems states that:-

“The risk from exposure to legionella should be controlled by keeping the water system and water in it clean and free from nutrients, including those arising from contamination and corrosion; and maintaining its cleanliness. Hardness scale may also trap nutrients, encouraging biofilm formation and so form a barrier to disinfectants.”

“Where necessary, hot and cold water services should be cleaned, flushed and disinfected in the following situations, as specified in BS 8558:”

“On completion of a new water installation or refurbishment of a hot and cold water system.”

“On installation of new components, especially those which have been pressure tested using water by the manufacturer (see the manufacturer’s instructions).”


“Where the hot and cold water is not used for a prolonged period and has not been flushed as recommended or the control measures have not been effective for a prolonged period. For example, this could be as little as two or three weeks, but will depend on the ambient temperature, condition of the water system, potential for exposure to aerosols and the susceptibility of users considered in a specific risk assessment.

On routine inspection of the water storage tanks, where there is evidence of significant contamination or stagnation.”


“If the system or part of it has been substantially altered or entered for maintenance purposes that may introduce contamination.”

“Following water sampling results that indicate evidence of microbial contamination of the water system during, or following an outbreak or suspected outbreak of legionellosis linked to the system.”

“Or where indicated by the risk assessment.”