Legionella bacteria live in the natural environment and water supplies, normally in low concentrations and little can be done to prevent this bacterium entering potable water systems. Legionella will grow in cold water systems and distribution pipework when there are increased temperatures, appropriate nutrients and stagnation.


Unfortunately, these conditions often exist where poorly insulated water storage tanks are located on rooftops and in poorly insulated roof spaces. Potential risks are further increased with poorly designed and maintained distribution pipework systems.

During the design stage it is important to establish whether cold water storage is required and how much. Many premises rely on the continuous supply of hot and cold water. Storage systems are often over-sized to actual usage because of uncertainties at the design stage. If the design needs to allow for fluctuating or increasing occupancies the system should be organized in a modular fashion – always ensure unequal flow rates and stagnation are avoided.

In the first instance cold water systems should always be designed to aid safe operation by preventing or controlling conditions which permit the growth of legionella. In particular the following should be considered:

• The volume of cold water stored should be kept to a minimum and not normally greater than one day’s water use.
• Cold water tanks should be sited in a cool place and well insulated. Distribution pipework should be kept away from heat sources such as hot water piping, ductwork etc.
• Water storage tanks should be fitted with covers and insect screens fitted to open pipes. Water storage tanks fitted in non-insulated roof spaces should be avoided where high ambient temperatures occur.
• Avoid materials such as natural rubber, hemp, linseed jointing pastes and fibre washers. It has been shown that approved materials resist microbiological growth when new, however, with age this resistance may deteriorate.
• Avoid flexible hoses connected to showers whenever possible, when in use the movement of hoses has been shown to dislodge biofilm. Biofilms found on the internal aspects of pipes and hoses have been shown to provide a suitable habitat for legionella colonization.
• Low-corrosion materials such as copper and stainless steel should be used where possible. Zinc coated steel pipework is often subject to localized pitting from corrosion, which can provide a firm footing for biofilm growth.
• Reduce the number of outlets whenever possible to avoid under use, avoid installing conventional showers that are not used weekly or consider fitting self-purging showers. Avoid thermostatic mixer valves serving multiple showers.
• As cold water is not normally recirculated in pipe work around premises, lower-use outlets should be installed upstream of higher use outlets.
• The Water Supply (Water Quality) Regulations permit water utilities to supply water to premises at temperatures up to 25°C. Normally the water temperature is 5-10°C in winter and up to 20°C in summer. However, beware during hot summers and check water temperatures – the risk assessment should reflect the appropriate action taken if necessary.

Good design and maintenance of water systems is always the minimum safety requirement.

Article by:
Legionella Control Journal