Hot and Cold Water Services

Guidance

Hot water systems present the greatest risk in environments which allow the the proliferation of legionella, for example:
* at the base of calorifiers where the incoming cold water merges with the existing hot water. This water collects sedimented organic and mineral deposits which support bacterial growth, including legionella. This can then be distributed throughout the system to colonise its periphery, especially where optimum temperatures and stagnation occur, e.g. in infrequently used outlets.
* water held in pipes between a recirculating hot water supply and an outlet (e.g. tap or shower), particularly when not in use, as they may not be exposed to biocides and high temperatures.
Water systems may occasionally be contaminated with legionella (usually in small numbers) which enter cold water storage systems from the mains supply. This presents little risk under normal circumstances. Legionella will only grow in cold water systems and the distribution pipework when there are increased temperatures (e.g. due to heat gain), appropriate nutrients and stagnation.
Cases of legionellosis have been reported in hospitals where water systems have been colonised by legionella. In addition, there have also been reports of infection when tap water was used to fill personal humidifiers and to wash jet nebulisers and other respiratory equipment. This, together with the presence of susceptible individuals, means that there may be an increased risk in health care premises and additional precautions may be needed.

Substitution

Some of the features of gravity hot water systems which influence the risk of exposure to legionella, such as having open tanks and relatively large storage volumes, can be eliminated by moving to mains pressure systems. This requires confidence in the reliability and continuity of the mains supply and may not be acceptable in all cases. Other problems, such as the maintenance of water temperatures throughout the distribution system and changes in demand, can be simplified by changing to point-of-use water heaters with minimal or no storage. Guidance on the general principles and limitations of instantaneous water heaters is given in BS6700:1998.

< Back - Next >