PIPES, VALVES & FITTINGS


Legionella risks minimised through good system design


Aside from cisterns as discussed on pg 23, bacterial contamination such as legionella in water systems, can also pose a real threat to building occupants. Here, James Griffiths senior marketing manager (North West Europe) at Uponor explains how the correct design of hot and cold water systems, combined with the right materials, can help to significantly reduce the risk


W


hile legionella control has always been an important consideration, recent high-profile cases in places as diverse as government owned


buildings, schools and public swimming pools has meant an increased focus on this issue.


Legionella bacteria occurs in natural water


sources and is often found in small numbers in mains water supplies. This means that it is potentially present in the water system of any building. However, the bacteria are generally only a hazard when allowed to multiply. Bacteria, including legionella, thrive in water at temperatures between 20°C and 45°C and can propagate quickly. Exposure to water droplets containing legionella bacteria can cause the potentially fatal form of pneumonia known as Legionnaires Disease as well as the similar but less dangerous Pontiac Fever and Lochgoilhead Fever. There are several ways that those responsible for the design and maintenance of water systems within a building can help prevent the growth of legionella and other bacteria.


Temperature control


Keeping water temperatures out of the range where bacteria grows quickest is an important consideration. It is recommended that hot water is delivered to the outlet at above 50°C and cold water at below 20°C. However, this is not always simple. The fact is that these temperatures may be achieved directly from the boiler, where the water enters the building or where it is stored, but any heat loss or gain as it is distributed throughout the building must be avoided. These changes in temperatures can be minimised through good design by


routing cold water pipework away from sources of heat. Effective insulation of both the hot and cold water pipework should also be a part of any system design and installation. However, it is worth noting that if hot water is


provided to the outlet at 50ºC or higher there may be additional safety measures needed to prevent the risk of scalding. This is especially true for environments used by vulnerable people, such as the elderly, children and those with disabilities. It may be as simple as installing a thermostatic mixing valve (TMV) at the outlets.


Avoiding stagnation


One of the biggest issues with regard to legionella is water retained in the pipework for extended periods of time. This can be for a number of reasons. There may be a section of pipework that water is not frequently drawn from and therefore not replenished with fresh water. This could be caused by having a rarely used outlet at the end of a long pipe run. In general pipe runs should be kept as short as possible, however there are design strategies that can be employed to optimise the frequency of pipework flushing and reduce the volume of water held in these ‘dead legs’. The first is a serial approach, where the system is designed so that the most-used outlet is installed at the end of the pipe run, meaning when it is used the water will be drawn past all the other outlets. For example, placing a WC cistern at end of the pipe run to replenish the water in the pipework leading to the basins every time the WC is flushed. An alternative to this is a loop system, which is similar to the serial design, but with a return pipe from the most remote point back to the supply. With this layout, water is drawn through the system every time a water outlet is used, minimising the potential for stagnation. Another potential issue is oversized pipework.


Larger pipe sizes increase the volume of water in the system and if this is not matched by the demand from outlets, water may remain in the system for longer than the three to five days that is recommended. To help ensure proper sizing, some leading plumbing system manufacturers provide software tools to help installers and specifiers calculate the correct sizing for pipework based on the specific products selected.


For some buildings a level of management is needed, regardless of the system design.


24 BUILDING SERVICES & ENVIRONMENTAL ENGINEER DECEMBER 2023


The pattern of use of the building may mean that water remains in the pipework for longer. For example, school, college and university buildings may have limited use during the summer months while some sports facilities and hotels may see less use in the winter. In these cases, a plan needs to be developed to flush the system periodically to prevent stagnation.


Choosing the right products


From a legionella prevention perspective, it is important to select products that will minimise areas within the pipework system where bacteria can thrive. Products that provide smooth, frictionless surfaces and minimal joints are always recommended. Plastic pipe systems, such as cross-linked Polyethylene (PEX), generally have an advantage over alternatives such as copper. This is because the material is not vulnerable to corrosion and is more resistant to limescale build up, both of which provide places where bacteria can grow. In particular, multilayer composite pipe (MLCP), which consists of an aluminium core with internal and external layers of polyethylene, is ideal for many applications. This construction gives the pipework strength and durability as well as flexibility and the resistance to scale and corrosion. In terms of reducing the number of connections, MLCP has two clear advantages. Firstly, unlike copper it is not restricted to relatively short lengths that must be connected.


For example, Uponor Uni-Pipe PLUS MLCP is available in coils of up to 500 metres. Secondly, the flexibility means a bend can be created by hand or by using handheld tools, rather than two sections which are connected using an elbow joint.


In addition, selecting pre-insulated pipe can provide a simple way of protecting hot and cold water pipework from changes in temperature. Pre-insulated MLCP, available in coil lengths of up to 75m, can be quickly installed and provides continuous, unbroken insulation of up to 13mm. Legionella is a serious concern for all types of building. However, with best practice system design, correct product choice and careful building management, the risk can be minimised. Engaging with a trusted supplier of plumbing system products can help ensure the most effective solutions are selected.


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