CPD PROGRAMME Professional Development Hot water provision in commercial applications minimising legionella risk, maximising efficiency This module looks at reducing the risk of legionella in commercial hot water systems and maximising system efficiency with continuous flow water heaters The CIBSE Journal CPD Programme Members of the Chartered Institution of Building Services Engineers (CIBSE) and other professional bodies are required to maintain their professional competence throughout their careers. Continuing professional development (CPD) means the systematic maintenance, improvement and broadening of your knowledge and skills, and is therefore a longterm commitment to enhancing your competence. CPD is a requirement of both CIBSE and the Register of the Engineering Council (UK). CIBSE Journal is pleased to offer this module in its CPD programme. The programme is free and can be used by any reader. This module will help you to meet CIBSEs requirement for CPD. It will equally assist members of other institutions, who should record CPD activities in accordance with their institutions guidance. Simply study the module and complete the questionnaire on the final page, following the instructions for its submission. Modules will be available online at www.cibsejournal.com/cpd while the information they contain remains current. You can also complete the questionnaire online, and receive your results by return email. cpd QUESTIonnAIrE To take part in this months cpd, simply read through the module here, then follow the link at the bottom of this section. alternatively, visit www.cibsejournal.com/cpd you will receive notification by email of successful completion, which can then be used to validate your cpd records. over half were considered to have been due to exposure in the community and healthcare facilities, with the remainder being associated with travel abroad. This apparent figure of under 200 annual cases derived from a UK source is likely to be an underestimate5 of the true incidence of legionellosis, as symptoms are often similar to the influenza virus and other types of bacterial pneumonia, and routine laboratory tests will not readily identify the legionella bacteria. The group at greatest risk from contracting legionellosis is older males with underlying health problems such as heart conditions and diabetes. The susceptibility of this group is thought to be associated with industrial occupations and lifestyle-related risk factors, such as smoking.2 The historic England and Wales fatality rate is just over 10% for community-acquired legionellosis and around 25% for nosocomial (healthcare) related. However, infections in the UK 100 Temperature (oC) 80 60 The control of the legionella bacteria and avoiding the risk of incidents of Legionnaires disease (and Pontiac fever) are justifiably regarded as essential for both the products and the systems designed for the servicing of buildings. The impact of poorly designed or operated hot water distribution systems poses a significant risk. More than 50 species of legionella have been identified1, and half of these can cause infection that may be either asymptomatic or result in one of the forms of legionellosis. However, practically all of the cases of infection reported throughout Europe are caused by legionella pneumophila (Lp).1 Lp is present in the environment in soil and water and survives and thrives in many potable hot water systems. Legionella infections only occur through direct exposure to aerosols/droplets from an environmental source colonised by the legionella bacteria there are no reported or documented cases of Legionnaires disease associated with person-to-person transmission. Legionella grows in warm, stagnant water in natural and artificial water systems in particular, cooling towers, evaporative condensers, hot and cold water systems and spa pools. These environments as well as being ideal for growth may also provide the means by which aerosols/droplets are generated and the organism dispersed into the atmosphere.2 So, a significant potential danger is at the point of hot water delivery, and there are numerous reports of Lp being present in 10% to 50% of sampled taps and showers in Europe and the United States.3 To provide an indication of the impact of UK exposure to legionella in recent years, the number of cases of legionellosis in the UK has remained reasonably constant, at between 300 and 400 per year, as shown in Figure 1 (with a slight, unexplained dip in 2011). For the most recently available data collected in 2012 of the confirmed cases of Legionnaires disease, just are erratic, and in many cases a specific source of infection is difficult to trace. As identified in Figure 2, the legionella bacteria will multiply abundantly between the temperatures of 20C and 46C a range that coincides with temperatures that are acceptable for supplying potable hot water. It is notable that cold water services can often be in environments that allow them to rise to a high-risk temperature. The most favourable temperature for the multiplication of legionella is approximately 37C. Above 70C, the bacteria will be killed almost instantly, and at a temperature of 60C, 90% of the legionella pneumophila have been shown to be killed in two minutes (compared with several hours at around 50C). Below 20C, legionella becomes dormant but ready to multiply when the opportunity presents itself. Figure 3 shows the interesting effect that the thermal resistance of pipe insulation has in prolonging opportunities for non-flowing hot water temperatures to drop to the most critical temperatures. For the higher resistance, better insulation (shown as Type 1) keeps the water hotter for a longer period; but, as it cools, it will also mean that the water temperature will remain for a greater time in the range where legionella will multiply. This problem in water distribution systems is exacerbated where there is accumulation of sludge, rust, scale and particulate deposits that act both as a haven for bacteria and, as biofilms develop, provide nutrients. 450 400 394 374 376 369 Reported cases 350 300 243 250 200 150 100 50 0 2008 2009 2010 Year 2011 2012 Figure 1: Number of cases of legionellosis reported in UK, 2008-2012 (Data source: Annual epidemiological report 2014. Respiratory tract infections4) prevention of opportunities for legionella proliferation during design HSG274 part 2 provides comprehensive guidance for the design, commissioning and operation phases of a commercial hot water system. The recommended design aspects are abstracted below more comprehensive details are available in HSG274 part 2. There should be an adequate supply of hot water, particularly at periods of peak demand, while avoiding excessive storage and ensuring a supply temperature of at least 60C from the heat source and/ or storage vessel (calorifier). In buildings where stored water is not essential, consideration should be given to direct mains systems this could be provided through continuous flow hot water systems or point of use heaters. Modular high output continuous flow hot water heaters are very capable of providing thousands of litres of hot water per hour, without the need for storage. If a calorifier is employed, it should meet the normal daily fluctuations in hot water use, without any significant drop in target supply temperature. Also, the temperature in the base of the vessel should be monitored and must be drainable to remove accumulated sludge Steam humidification No viable legionella LTHw heating system Hot water service storage HwS tap outlets Legionella will not multiply and will die in time Spas, showers 40 Legionella will multiply Cooling towers Cold water services Fire sprinklers 20 0 Spray humidifier Mains cold water, air cooling coil condensate and chilled water systems Legionella will remain dormant Increasing risk of multiplication of legionella Figure 2: Typical system operating temperatures and the risk of legionella proliferation (Source: CIBSE AM13:20136) removed the technical guidance (which was Part 2 in previous editions), and published this separately as HSG274.8 HSG274 outlines measures that, together, meet the relevant statutory safety requirements, including: identifying and assessing sources of risk; preparing a scheme to prevent or control risk; implementing, managing and monitoring precautions; keeping records of precautions; and appointing a manager responsible for others. This guidance is split into three parts that consider specific applications for the control of legionella bacteria: part 1 evaporative cooling systems; part 2 hot and cold water systems; and part 3 other risk systems. These are all freely downloadable from the internet and provide extensive, essential, very practical, and well-illustrated guidance. 70 Type 1 insulation Type 2 insulation Non-insulated 50 Optimal growth temperature for Lp Temperature (oC) 60 40 30 45 mins 20 86 mins 10 0 131 mins 0 50 100 150 200 Elapsed time (minutes) Figure 3: Heat loss during stagnation of hot water in 1.25cm diameter copper pipes, with and without insulation at room temperature (Source: Bedard3) and particulate matter. The distribution system should be thoughtfully designed to avoid water stagnation by ensuring flow through all parts of the system, and particular care should be taken to prevent risk temperatures in system components that support microbial growth. Low-use outlets should, for example, be installed upstream of frequently-used outlets, to maintain frequent flow. Hot water temperatures at an outlet The design should ensure reasonable access to all pipework sections and active system components for inspection and maintenance purposes, with valves located sensibly for practically useful isolation. Any energy efficiency or water conservation measures should be assessed carefully at the design stage, to ensure the control of legionella is not compromised. Instantaneous water heaters By using instantaneous water heaters, as shown in Figure 4, the potential legionella hazards inherent in stored water systems are avoided as by design they do not store any volume of hot water but are still able to supply large commercial demands. The incoming cold water is instantaneously heated to 60C, beyond the temperature at which the bacteria can multiply, and that heated water is not resident in the hot water generating equipment for prolonged periods. Without the need for calorifiers or other storage vessels, the volume of hot water in the system is reduced and, as these systems do not include a standing store of water, they do not require a daily pasteurisation cycle using shunt pumps. However, regardless of whether the hot water is provided by a continuous flow services can be carried out by raising the temperature of the whole contents of the circulating water to 60C (or above) for at least an hour. To meet the requirements of HSG274 thermal disinfection, every hot water outlet throughout the system must then be flushed and allowed to flow for five minutes at full temperature. To be effective, the temperature at the hot water generator should be maintained to ensure that the temperature at the outlets does not fall below 60C. In the case of a storage calorifier, this could necessitate the whole volume of stored water being raised to a high temperature beyond normal safe temperatures that prevent scalding at outlets to undertake disinfection, then that stored water temperature would need to be reduced (or the water drawn off and discarded) before normal building occupation. However, with a continuous flow system, such as in Figure 6, the electronic control is such that the temperature can be instantly raised potentially, to beyond 75C and within a tolerance of +/- 1K to undertake disinfection. The temperature is controlled based on the incoming water to the continuous flow water heater and so, as with normal operation, it will modulate (typically down to around 50 watts), only consuming the energy should reach 50C (55C in healthcare premises) within one minute of turning on the tap. This will require a recirculation loop in most commercial systems that should be designed to give a returning temperature of at least 50C (55C in healthcare premises) there should be temperature measurement points so that this may be monitored. Circulating pump design and the correct commissioning of balancing valves are Cold supply Approved code of practice L8 Legionnaires disease. The control of legionella bacteria in water systems In the UK, HSE L87 gives advice on reducing the risk from exposure to legionella bacteria. The L8 revision in 2013 key issues to ensure flow throughout all parts of the hot water system particularly the hot water return legs to avoid long lengths of stagnant pipework that are likely to be at a lower temperature. Where a traditional indirectly heated hot water calorifier is employed, cold water will typically enter at the base of the calorifier, creating an area below the coil where the initial blended water temperature may support microbial growth and provide potential breeding ground for legionella. This may be mitigated with a shunt pump to mix the water in the calorifier to ensure a temperature of at least 60C is achieved throughout the vessel for at least one continuous hour a day. This will, unfortunately, reduce the energy effectiveness of the system. All system materials chosen should reduce corrosion, prevent excessive scale formation and not readily support microbial growth. In hard water areas, softening of the cold water supply to the hot water distribution system should be considered this can reduce the risk of scale being deposited, the risk of scale accumulation within the system pipework and components and, if used, in the base of the calorifier. Gas Hot out Figure 4: Continuous flow hot water system has no storage of hot (or warm) water, substantially reducing the opportunity for any Lp growth (Source: Rinnai UK) Gas Pump Figure 5: Continuous flow with recirculation (Source: Rinnai UK) water heater, in larger systems where there is a need for recirculation pipework (as illustrated in Figure 5) there is inherently increased system water volume and opportunities for dead-legs. When taps or shower-heads stop running, after use, the final branch pipework will be full of cooling water capable of breeding legionella bacteria, so appropriate safeguards must be put in place such as self-draining shower heads. Thermal disinfection The hot water system particularly rarely used shower heads or terminal fittings will still require regular checking and disinfection. When the building is not in occupation for example, weekends or at night thermal disinfection of hot water Figure 6: A pair of continuous flow water heaters installed to produce in excess of 1,400 litres per hour of hot water at 60C (Source: Rinnai UK) required to meet the instantaneous flow requirements. Following the disinfection procedure, the flow temperature can then, just as swiftly, be returned to 60C ready for safe building operation. Tim Dwyer, 2015. systems, Water Research, 71, 2015. 4 ECDC Surveillance Report, Annual epidemiological report. Respiratory tract infections, 2014. 5 whiley, H, et al, Uncertainties associated with assessing the public REfEREncEs: health risk from legionella, Frontiers 1 Currie, SL, et al, Legionella spp. in Microbiology, September 2014. in UK composts a potential public health issue?, Clinical Microbiology and Infection, Volume 20, Issue 4, 6 CIBSE TM13 Minimising the risk of Legionnaires disease, CIBSE, 2013. April 2014. 7 HSE L8 Legionnaires disease 2 Legionnaires disease in England The control of legionella bacteria in and Wales 2012, Public Health water systems, UK Health and Safety England, 2014. 3 Bedard, E, et al, Temperature diagnostic to identify high risk areas Executive, 2013. 8 HSE HSG274 Legionnaires disease and optimize legionella pneumophila Technical guidance, UK Health and surveillance in hot water distribution Safety Executive, 2013. Module 75 April 2015 Fill in this months questionnaire online. You will receive notification by email of successful completion, which can then be used to validate your CPD records in accordance with your institutions guidance. Click Here To Fill In The Questionnaire