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Life is about more than solar shading controls

Killing the legionella bug

By David Pepper, Managing Director of Lochinvar M

any health experts are alarmed by the increased risk of Legionnaires disease

transmitted from stored hot water systems because more of that hot water is generated by renewables. As well as the health risk, this is also becoming a hindrance to the growth of the solar thermal market, in particular. The problem is that temperature at which

the water from a solar system is held in the storage vessel can often be right in the middle of the ‘danger zone’ of 40 to 45°C which is perfect for microbiological growth. Even more worrying, new strains of the legionella bacteria are emerging which can survive at temperatures above 60°C and have developed resistance to some of the most popular chemical water treatment solutions.

According to specialists, at least 9,000 people catch Legionnaires disease every year in the UK. This figure is higher than official statistics because many sufferers are misdiagnosed as the symptoms can be very similar to flu. They also believe it is fatal in 12% of all cases, and in 50% of cases among the elderly and infirm. The healthcare industry is particularly

worried about the way in which water systems in care homes, hotels, schools and a host of other establishments with vulnerable residents are monitored and treated.

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Often engineering attempts to tackle the risk lead to overly complex and expensive solutions that rely on regular interventions from an end user or maintenance team during the system’s operating lifetime. As with many other aspects of building maintenance, water treatment processes that act against the threat of legionella build up, are suffering from budget cuts. There is also growing evidence that many building and system logbooks, designed to identify the dangers in water systems and temperature breeding zones within internal pipework, are not being filled in properly. FMs and building managers are far more

aware of the cost of energy these days and are looking to economise. This can result in water heating systems being shut down for periods, or hot water temperatures lowered, to save energy – which is to be commended. However, this can create perfect breeding conditions for legionella bacteria. These can then be spread to potential victims as aerosol water droplets via showers and taps.

Many indirect hot water systems and

renewable pre-heat cylinders need regular pasteurisation as a precautionary measure to prevent the build-up of legionella. This is a highly energy intensive process so, ironically, in trying to reduce the energy burden of a building by deploying renewables, a building operator can end up having to use more energy to purify the water system. Also, pasteurisation is regarded by many as an imperfect solution. The NHS in Scotland, for example, will no longer accept it as a suitable method for tackling the legionella threat. The alternative is to simply run the whole system at a higher temperature, by increasing the amount of hot water generated by conventional, fossil fuel fired systems – again driving up energy costs and also creating other maintenance issues, such as increased scale build-up.

Tackling the threat

So what is the answer? Well, how about completely reversing the conventional solar water heating process and taking out the problem created by stored water? We have been wrestling with this problem

for some time. In the end, if you want to totally eliminate the threat you have to remove the risk by making sure that the building occupant is not drawing water from a bulk storage vessel. So we have developed a system that still

allows the design engineer to integrate up to three separate heat sources, which can be renewable or fossil-fuel based, but has no stored hot water within the pre-heat vessel. We have reversed the conventional method by having the main body of water within our storage vessel transferring heat to the stainless steel indirect coil, which is then used as feed water for traditional water heater(s).

This means building occupants do not

receive water from taps, showerheads etc. that has been sitting in a storage vessel and so could be held at a temperature that encourages bacterial growth. By reversing the conventional approach,

we take away the responsibility on the building operator to intervene at regular intervals, either to treat the stored water or adjust the temperature to minimise the risk. In an era of reduced staff and budgets, and higher workloads, there is more room for human error when trying to monitor hot water systems – the non-storage approach manages the risk by making it disappear.


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