Chris Goggin on Rinnai looks at how smart design and control options can design out legionella proliferation.

Legionella control and risk avoidance thereof are top of the list when designing heating and hot water systems.

The whole supply chain – manufacturers and distributors through contractors to end-users – dreads the thought of having a Legionella problem tracked back to them. It is one area where avoidance and prevention are the only sensible action to take to avoid problems further down the road.

Our modern lifestyle means that we are calling for more and more hot water on tap at any time, day or night. So how can installers and designers ensure there is no fear of Legionella breaking out – and give peace of mind to their customers and end-users alike?

Quite simply, don’t use a traditional stored hot water system.

A glance through the Health & Safety Executive ‘Approved Code of Practice’ highlights the vast amount of detailed maintenance needed to keep a stored hot water system Legionella free; the level of work involved is time consuming, expensive and must be adhered to religiously and rigorously.

According to the HSE stored hot water systems present an enhanced risk for the proliferation of Legionella, especially at the base of calorifiers where the incoming water merges with the existing hot water.

Legionella bacteria multiply where temperatures are between 20-45°C. Water at the base of the calorifier is liable to collect sedimented organic and mineral deposits, which support bacterial growth, including Legionella, which is then distributed throughout the system.

To ensure total safety, therefore, the whole water contents of the calorifier, including at the base, must be heated to a temperature of 60˚C for one hour each day.

A lot of these problems are solved by choosing to install a continuous flow hot water heating system. With a gas-fired continuous flow system there is no cylinder to cause problems because the cold water enters the water heater directly from the mains and is immediately heated to advisory levels.

With Rinnai continuous flow units water temperature for distribution is accurate to ±1˚C. The fact that there is no stratification as there is no storage ensures permanent even temperature. There is no requirement to heat a volume of water just to ensure the elimination of Legionella bacteria as the minimum temperature that a continuous flow unit achieves is in excess of advisory levels.

However, when considering design parameters that incorporate secondary return systems pipework and dead legs can act as water storage. Engineers and installers can argue that ‘tankless’ (continuous flow units) systems and point of use have a much reduced risk of legionella proliferation. They are correct – until it comes to secondary return systems.

When the taps or shower heads stop running, after use, the pipework will be full of cooling water capable of breeding Legionella bacteria; therefore you do have stored water and appropriate safeguards must be put in place with that type of system.

For example, a large leisure centre of usual size and public usage, where there may be no tanks but the pipework could hold up to 10,000 litres of static water when not in use.

So what is a smart design consideration to eliminate any risk of Legionella bacteria multiplying in a hot water system without keeping water circulating constantly at 60 degrees? The technology is available in the form of smart controls linked the water heating system, inside which is a timer channel programme and a temperature controller.

The timer will over-ride the temperature control for a short period and then revert back to safe levels. This combination allows for safe running of water at 42°C core temperature during the day and thermal shock of the system at 60°C intermittently at times when the building is not in use. By the time the building opens again, core temperature is back down to 42°C for safe use.

It is a very simple technology that takes advantage of innate high tech controls within the water heater, it eliminates risk from Legionella bacteria and is ideal for use in schools, office blocks and sports clubs – any large or small scale site. In fact, it could even have uses in domestic environments as the current house building trend is to add more bathrooms. If a property is to be used as commercial premises, including landlord’s rented accommodation, then there is a duty of care to protect employees, tenants, and the public from the possible dangers of hot water systems including scalding and Legionella.

Some parts of the hot water system, such as rarely used shower heads or terminal fittings, will require regular maintenance and frequent checks should be carried out to ensure the hot water system is operating correctly.