In the first of a two-part series, Colum Ó Bric looks at guidance documents that inform the design, installation and management of building water systems and considers how engineering can prevent waterborne bacteria such as Legionella


The complexity of building water systems has changed significantly over recent decades with modern design and installation standards, new materials and installation methods, and an ever-increasing range of equipment installed as part of building construction. Water systems are now used for drinking water, heating, cooling, hygiene and welfare, cleaning and leisure.

The occupant or user is exposed to the building water system and, therefore, the safe operation of building water systems is now a significant part of the overall remit of any building manager or owner. The potential for bacterial growth within water systems and the risk that arises as a result must be considered by those responsible for the safe operation of the buildings.

The water entering our buildings is generally treated mains water, which achieves the standards outlined in SI 122 of 2014 European Union (Drinking Water) Regulations 2014. This does not mean that the water is sterile, merely that the bacteria and microorganisms that would be directly harmful such as Escherichia coli and Cryptosporidium are removed. The water still has a level of bacteria and microorganisms present but these are in small numbers which are generally not harmful.

System contamination can occur when this water with low bacterial numbers is taken into a building and the levels of pathogenic (i.e. bacteria that will cause illness) bacteria, are allowed to grow as a result of ideal conditions for growth being present within the building water systems. Factors which will increase the potential for pathogenic bacteria like Legionella to grow and multiply in building water systems include:

  • Temperatures in the 25°C to 45°C range;
  • Stagnation in water systems;
  • Use of unapproved materials which promote bacterial growth; and
  • Dirty water systems.

A number of bacteria are potentially harmful where these conditions occur and these include species of Legionella, Pseudomonas, Acinetobacter, Burkholderia and Stenotrophomonas bacteria, to name but a few. Exposure of the building users through ingestion or inhalation completes the chain of infection and an outbreak of illness can occur.

Engineering a solution to Legionnaires’ disease

One of the bacterial species mentioned above provides numerous examples of the difficulty in achieving a balance between design, engineering and functionality in a building’s water systems. Legionella can cause Legionnaires’ disease in susceptible individuals. Legionnaires’ disease is a potentially fatal form of pneumonia that is acquired by inhaling contaminated water droplets (aerosol). Aerosol sources in buildings include equipment and fittings such as sinks, showers, baths, cooling towers, decorative fountains and spa pools.

A number of guidance documents are published that inform the design, installation and management of building water systems, with the aim of reducing the risk presented by Legionella bacteria. The most recent Irish guidance document is the Health Protection Surveillance Centre (HPSC) National Guidelines for the Control of Legionellosis in Ireland, 2009. The UK document HSG 274 Legionnaires’ Disease Technical Guidance Parts 1, 2 & 3 was published in 2014 and is an updating of the well-known ‘ACoP L8’ document.

Hot and cold water systems installation standards are frequently based on BS 8588: 2011- Guide to the design, installation testing and maintenance of services supplying water for domestic use within building and their curtilages. Most healthcare facilities give consideration to the UK Health Technical Memorandum 04-01: The control of Legionella, hygiene, ‘safe’ hot water, cold water and drinking water systems. Part A: Design, installation and testing. Part B: Operational management.

Building services design and installation and the selection of components and equipment must be cognisant of the potential for bacterial growth and its impact on a building over the medium and long term. A proliferation of bacteria in building water systems can have significant cost implications in terms of time, management, repair, remediation and monitoring to say nothing of the potential health risks.

Many components of water systems have an impact on the potential for bacterial growth, but in this article the author would like to highlight one element of water systems that is often not given due consideration in design, construction and commissioning and is consistently a cause of the proliferation of bacterial proliferation within building water systems.

The common standard for hot water systems for washing in buildings is to store water at >60°C and deliver the water throughout the system at >50°C at all times. Temperatures in the 25°C to 45°C range must be avoided as bacterial growth will occur most significantly within this range. In large buildings such as offices, industrial and healthcare facilities, the standard for our convenience and to meet our expectations is to have hot water at our sinks and showers almost instantaneously.

To achieve this, we install the hot water systems with a constant circulation loop or secondary hot-water return system (DHWR). Typically, hot water is heated in calorifiers and flows out to the building by expansion of the hot water and drawn by the use of the system. The hot water is looped back close to the outlets and piped in a return circuit back to the calorifier. The hot water is pulled back through this pipework distribution circuit by the secondary hot water return pump. This pump is in continuous operation ensuring circulation of hot water on a constant basis regardless of outlet usage and when a tap or shower is operated hot water is then available almost instantaneously.

All larger buildings have circulating hot water systems and where these systems are not circulating in a balanced manner, there will be areas within the distribution system where the required control temperatures are not achieved. If there is poor circulation to certain areas of this circulatory system, then there will be limited flow and poor temperatures – both of which will contribute to the growth and proliferation of potentially pathogenic bacteria such as Legionella.

In many of these cases, the monitoring and testing that is being undertaken by the building manager or responsible person (on a monthly basis as required by the HPSC guidelines) will indicate that the cold water, hot water storage and flow and even the central return temperatures are satisfactory but water sampling will return positive bacterial results as a consequence of these areas within the systems allowing bacterial growth to occur.

Chemical disinfection and chlorine

To further confound the situation, one of the remedial measures in the case of positive results is to chemically disinfect a water system using an appropriate agent such as chlorine. Chemical disinfection agents such as chlorine are often corrosive and cannot be left in systems for long durations as this restricts the use of the water system. In the case of a poorly circulating DHWR the poor flow to the contaminated area of the DHWR will mean that the chlorine or chemical disinfectant is also unlikely to penetrate through to the area required.

After the disinfection, the contamination remains in the system. It will re-seed other areas of the system very quickly and a system can return positive bacterial results immediately.

A poorly balanced DHWR is likely to be as a result of flaws in the design, installation or commissioning. In some cases, systems are fitted with small pumps and in some instances the installation of a larger pump will increase the draw through the return system and the flow through areas with poor flow and poor temperatures. In these cases, the design does not seem to take into account the complexity of the system distribution throughout the length of the building.

Poor installations include where modifications are made during the build or additional bends, elbows, and branches added to the system which were not in the original design. This will add to the overall system distribution and the pump may not have the capacity to meet the demand of the system. Another example of poor design or installation is a lack of balancing valves in the system or the installation of inappropriate valves which will not enable measured balancing to be completed.

The double regulating valve (DRV) should be installed throughout the system to enable accurate system balancing at the commissioning stage. The use of a DRV enables tracking and recording of the system settings utilised to achieve balancing in the system which is not possible if other types of valves are used.

A lack of validated system commissioning is arguably the principal cause of poor DHWR operation. Heating and chilled water systems used for air conditioning by contrast generally undergo rigorous commissioning where flow rates throughout the system are validated and balancing of the system is supervised and confirmed by commissioning engineers. The commissioning process is fully documented and a report is developed by the engineer and presented as part of the system handover. The DRVs are set and locked in position once the system is balanced ensuring the settings are not altered inadvertently which could upset the balance of the entire system.

In contrast, there is generally little or no commissioning of hot water systems beyond some temperature checks on the calorifier storage, flow and return temperatures and checks that ensure outlets achieve the required temperatures. These are very often done by the mechanical engineering company responsible for the pipework installation. This kind of brief overview of the system operation does not assess the operation or balance of the network of secondary hot water return pipework or identify areas within the distribution system where there is poor circulation.

HPSC guidelines for Legionellosis control

The HPSC’s National Guidelines for the Control of Legionellosis in Ireland, 2009 states in Section 5.1.6: “Water systems should be designed, installed and commissioned to ensure risks from Legionella bacteria are eliminated wherever possible, or reduced as far as is reasonably practicable. Designs should also ensure that adequate provisions are made to facilitate safe system operation and maintenance since a poorly designed system can be both difficult and expensive to operate and maintain.”

Where a system has been installed and has areas of little or no flow and poor temperatures, it is most likely that there are no balancing valves fitted. An engineering solution would be to fit DRVs and to balance the system but to undertake this on a live system is expensive and highly disruptive.

UK HSE document HSG 274 – Legionnaires’ Disease, The control of Legionella bacteria in hot and cold water systems, in Section 2.37, states: “The general principles of design aim to avoid temperatures within the system that encourage the growth of Legionella. Consideration should be given to the following… to overcome localised failures in the distribution system, circulating pump design and the correct commissioning of balancing valves are key issues to ensure flow through all parts of the hot-water system, particularly the hot-water return leg. Balancing the hot-water flow and return circuits is critical to avoid long lengths of stagnant pipework that is likely to be at a lower temperature.”

The guidance documentation outlined above relates to specifically to Legionella bacteria but the same system failures will promote the growth of other pathogenic bacteria mentioned previously and therefore these standards should be followed to ensure control of potential waterborne pathogenic bacteria that may grow within the distribution system.

The DHWR is a convenience that provides us with near instantaneous hot water at our outlets. As outlined, a DHWR that is poorly circulating can harbour potentially pathogenic bacteria and contaminate the rest of a well-functioning hot water network, despite the best efforts of a building manager or owner. In this situation, this convenience becomes a potential hazard to the health of the building user. Temperature is an efficient means of controlling waterborne bacterial growth in our water systems, but a DHWR that is poorly balanced cannot achieve the required temperatures to maintain control.

As is outlined in the guidelines, consideration must be given to the potential for the proliferation of waterborne bacteria at the design stage. The specification for new buildings must include the balancing of the secondary return hot-water system and this should be verified by an independent competent person as part of a commissioning process.

There is a legal obligation on the duty holder and responsible person in charge of any building to ensure they are operated in a safe manner while not creating a risk to employees, visitors, residents and members of the public. Considering the certainty that bacteria will enter water systems, those building owners and managers must operate their premises in accordance with HPSC guidelines to maintain safe water systems. In the case of a DHWR that has poor flow and low temperatures, the building owner or manager may not be able to achieve this standard due to the legacy of poor design and installation.

It is incumbent on all involved to ensure the design, installation and commissioning of new water systems and equipment is at a standard that will ensure that bacteria will not proliferate to high numbers.

Colum Ó Bric is the technical manager with Environmental Services Ireland. He is responsible for water quality and Legionella awareness training, Legionella risk assessment, consultation on new builds prior to handover and the creation of water management policy documents for healthcare facilities. Ó Bric also participates in healthcare environmental monitoring committees as an external consultant, and he interprets and advises with regard to bacterial sampling results. He also offers assessment and risk management for other non-domestic water systems. Ó Bric has lectured onWater Quality in the Built Environment’ for students of UCC’s Environmental Science & Social Policy course and for students of Facilities in Portobello College. Ó Bric holds a BSc Hons in Microbiology and a Diploma in Environmental Science & Social Policy (both from UCC). O'RiordanChembacteria,European Union,water
  The complexity of building water systems has changed significantly over recent decades with modern design and installation standards, new materials and installation methods, and an ever-increasing range of equipment installed as part of building construction. Water systems are now used for drinking water, heating, cooling, hygiene and welfare, cleaning...