How to manage legionella risk

Stagnant or standing water can cause conditions that increase the risk of growth and spread of legionella and other biofilm-associated bacteria, and care must be taken to encourage the safety of building water systems and devices once the current lockdown is lifted.

The CIPHE Water Safety Group recently published an insight guide called Legionella risk assessing in domestic properties which is aimed at advising the installer on how to carry out a legionella risk assessment in a domestic property, with additional guidance on how to install safe hot water systems.

Some of the key documents that should be consulted for practical advice on how to reduce the risk of legionella in residential buildings are: HSG 274 parts 1 to 3, HSG 282, Part G of the building regulations and the water regulations. However, if an installer carries out work in a healthcare setting such as a hospital, then they must further consult documents such as the Approved Code of Practice (ACOP) L8 and Health Technical Memorandum HTM 04 parts A to C, which are specifically applicable to the health sector.

What is legionella?

There are many types of water-borne diseases, however the most well-known type is the condition caused by legionella pneumophila, commonly referred to as Legionnaires’ disease. The disease is contracted by the inhalation of small water droplets containing the bacterium legionella pneumophila, which can also cause the less fatal disease Pontiac fever. The bacterium becomes active and thrives by multiplying in warm water and can be contracted through the inhalation of fine aerosols such as from a shower.

Legionnaires’ disease produces severe pneumonia-type symptoms and can result in death so it is important that every plumbing and heating installer possesses the knowledge and competence to protect the public.

How to prevent legionella growth in a domestic hot water system

When installing a domestic hot water system it is always important to understand some very basic principles; keep it hot, keep it cold, keep it moving, keep it clean (figure 1) which will control and mitigate legionella pneumophila bacteria and pseudomonas aeruginosa biofilm bacteria.

Keep it moving

One of the most important installation practices to follow is to avoid installing pipework “dead legs” and “blind ends”. A dead leg could simply be a pipe that is supplying a rarely used outlet, or a circuit which has been capped off after an outlet has been removed. If a dead leg is to be removed then you must cap the pipe off at the t-piece (figure 3) or, ideally, remove the branch and t-piece.

As the heading above suggests (keep it moving), it has long been known that if water can stagnate for a period of time, it will allow bio slime and bacteria to thrive. This principle also applies to cold water storage cisterns where it is paramount that the pipework is installed to avoid stagnation within the cistern (figure 4). The cistern needs to be installed in a way which ensures that the whole water volume within the cistern can be replenished with fresh water when in use. Excessive storage capacity can be a contributing factor. The inlet and outlet should be installed on opposite sides of the cold water storage cistern to encourage water flow and avoid low flow areas.

One of the issues when installing a plumbing system that runs at higher water temperatures is the increased risk of scalding, therefore it is important to install a thermostatic mixing valve (TMV) as close as practically possible to the outlet. However, the TMV must not be installed in a position outside of these requirements, such as by the hot water cylinder, as it can pose a significant risk.

Keep it clean

One of the highest risk areas within a plumbing system for the growth and infection of legionella is from an outlet such as a shower head. As discussed earlier, legionella can be inhaled through fine water vapour droplets such as the fine mist created from the use of a shower. Therefore, it is important to ensure shower head outlets are inspected and appropriately cleaned on a monthly basis following a legionella risk assessment that has highlighted a risk. In addition to keeping outlets clean, it is important to prevent the over-sizing of hot and cold pipework, as an over-sized pipe can lead to laminar flow regions at the pipe wall creating a vulnerability to the formation of biofilm. BS 806 parts 1 to 5, Specification for Installations Inside Buildings Conveying Water for Human Consumption, should be consulted when designing hot and cold water services for residential dwellings.

How can I progress my knowledge in legionella risk assessment?

The CIPHE insight guide Legionella risk assessment in domestic properties is a simple but effective practical guide that explains how to carry out a risk assessment in a domestic environment, which includes a step-by-step process for producing a professional risk assessment. In addition to this, CIPHE members will be able to take the online assessment to become a CIPHE approved legionella risk assessor from this summer.

Please contact the CIPHE technical department via technical@ciphe.org.uk or 01708 463 117.


This article first appeared in the May/Jun 2020 issue of P&H Engineering, the magazine for members of the CIPHE. Find out how to join here.

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