In buildings demanding a steady supply of hot water, recirculating systems (or ring mains) are incredibly popular – not least as they are also a great facility for deterring legionella growth. Mark Dowdeswell  – Plumbing Applications Manager for Uponor UK – discusses why it’s imperative for installers to get to grips with the British Standards.

First and foremost, it’s important to understand the basics of a recirculating system. In most UK domestic properties, there is one main hot water line, with a series of branches running off it to the water outlets. This leads to potential dead-legs and impacts the water temperature profiles and stagnation which can lead to bacterial growth and water contamination.

Furthermore, it’s not good for water economy as a lot of water is wasted before the desired hot water temperatures are achieved at the outlet.

The benefit for the end-user of a recirculating system is a constant and almost instant supply of hot water at each and every outlet. As water is used from the system, more clean hot water enters and replaces the old.

Crucially, there are a number of key elements that a recirculating system must do. Firstly, hot water temperatures must be maintained at 60°C in the hot water storage tank with the difference in temperature between flow and return on the water heater must be equal or less than 5°C (EN 806-2).

Secondly, if there is more than one recirculation line, it is mandatory to balance the system.
Thirdly, where the hot and cold water systems are positioned closer together, they must be insulated to maintain the system temperature and to prevent warming the cold water lines. Finally, in systems with a total water content of less than three litres, such as sanitary rooms, it is not necessary to recirculate water.

While much of the residential market operates closed-loop systems, there remains a requirement for recirculating solutions in commercial premises where there is a high demand for clean, hot water at all times – such as the health sector. One of the great benefits of a recirculating system is that it discourages legionella growth – a major concern in a healthcare or commercial environment.

Prevention is better than cure

Legionella pneumophila is a bacteria that is responsible for 90% of cases of legionnaire’s disease. The bacteria is found in water systems and can be deadly to humans if water droplets are inhaled. The potential for the legionella virus to grow is at its most acute when the temperature of the water is between 25°C and 50°C (optimum 30–45°C). Outside of these ranges, the bacteria either remains dormant or is killed off.

As the elderly and those with existing medical problems are most at risk, prevention is paramount – but it is important to remember that even the healthy are not immune to serious illness when the bug strikes.

It’s imperative, then, in any application, to act to minimise the potential for legionella growth in the hot water system during the specification, design and installation phases. One such solution is to make use of alternative plumbing methods that have proved to be highly successful in continental Europe, but have not yet penetrated the UK market – serial and loop systems.

The other key recommendation, as a means of starving the bacteria, is focused on temperature – keeping cold water below 25°C and hot water above 55°C. For
re-circulating systems, however, the temperature increases up to 70°C for sterilisation purposes. Any pipework installed, therefore, must be designed to cope with these temperatures as a means of thermal disinfection.

Moving beyond a simple system design

As with most products in the UK marketplace, pipework is governed by the British Standards and products are designed upon the principle of standardisation across the market place. For many installers, manufacturers have already done the job for them in the form of packaged solutions or – at the very least – provided recommendations for suitable products to work in tandem.

What is particularly interesting about the British Standards in relation to re-circulating systems is that a number of products have now been classified as unsuitable. The changes form part of the latest amendments to the standards within the UK building regulations, which came into effect in 2012 following the publication of the BS EN 806-5 and BS 8558.

It is important to understand these amendments and where the changes have been applied. The standards under BS 7291 remain the same and, as such, pipes manufactured to this standard cannot be used for continuously-operated re-circulating systems. However, pipe systems that comply with BS ISO 15875 can be used.

Crucially, the standards still state that pipework in recirculating systems should be maintained at water temperatures of 60°C and must feature a high limit cut-off facility to ensure safety.

Furthermore, the BS EN 21003 standards for multi-layer piping systems also demand the same standards for re-circulating systems, with a high limit cut-off facility, and pipework capable of handling hot water temperatures of 65oC and above.

For installers and specifiers alike, the changes are crucial if a development is to pass through planning, but more importantly to comply with safety requirements. Needless to say, the standards are designed and formed following industry consultation and feedback – with changes implemented to ensure safety standards are maintained throughout the marketplace.

What is now important is that the exacting requirements of the changes are adhered to and that installers and specifiers understand the benefits a re-circulating system can bring.