PREHEATED DOMESTIC HOT WATER

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STORAGE OF PREHEATED domestic hot water AND POSSIBLE GROWTH OF LEGIONELLA BACTERIA

1          INTRODUCTION

The search for alternative energy sources which minimise carbon dioxide emissions is attracting manufacturers to seek WRAS Approval for heating systems for domestic premises which use heat pumps drawing energy from the ground or air, or solar panels, to pre-heat water to feed the domestic hot water system.  The preheated water is held in a storage cylinder before being passed to the water heater and on to outlets.  Where the water heater is a combination boiler, some systems are designed so that if the pre-heated water is hot enough (generally >60oC), the combi-boiler is bypassed.  If pre-heated water is hotter than is required for domestic purposes, blending valves mix cold water to bring the temperature to the desired range. (see Figure 1).

2          CONCERN REGARDING LEGIONELLA

If the pre-heated water is stored at temperatures between 20oC and 45oC, which favours the growth of the Legionella bacteria, and large numbers of the bacteria develop, measures may be required to prevent these bacteria causing harm to health.  The HSE Approved Code of Practice (L8) “The control of Legionella bacteria in water systems”, applicable to the management of hot water systems in non-domestic premises, recommends for hot water storage cylinders that the whole contents should be heated to 60oC for one hour each day to prevent growth of Legionella bacteria in the cooler water at the bottom of the cylinder.  Briefly raising the water temperature to 60oC is not effective.

If pre-heated water between 20oC and 45oC containing Legionella is subsequently passed through a combination boiler, its temperature does not usually exceed 60oC and it is held at that temperature only for seconds before being distributed and used.  If hot water from taps or showers contained Legionella present in large numbers, it might create an aerosol of droplets which could be harmful to the health of those breathing in the droplets. 

During the warm-up of a combination boiler up to 20 litres of water could be passed to outlets before the boiler achieves its pre-set water temperature (which in any case may be less than 60oC.)

3          ASSESSMENT OF RISK FROM PRE-HEAT SYSTEMS

The WRAS Technical Committee members were concerned whether, under some circumstances, a health risk might occur in these pre-heat systems.  A report  was commissioned from an expert medical microbiologist to provide information about the possibility of Legionella growth in these systems. The Technical Committee agreed to publish the report below to provide information which had not been easy to find elsewhere.

 

Preheated water report Sep 09  

 

4          WRAS APPROVAL

When considering WRAS Approval of the preheat systems, the Technical Committee concluded that relevant individual components of pre-heat systems, such as twin-coil storage cylinders, could quite properly continue to be approved where they met the test requirements under the Water Fittings Regulations, because as isolated components, they do not incur a contamination risk.  However, because these components could be combined into a pre-heated stored water system, approval of the individual components should be qualified by a warning about the possible risk arising from prolonged storage of tepid water.  Where WRAS-Approved, these components will carry the following note in their entries in the Water Fittings and Materials Directory:

“Compliance with the relevant requirements of the Water Supply (Water Fittings) Regulations 1999 has been assessed for this product only as a component of a heating system.  If it is to be installed as part of a system using solar energy or ground- or air- source heat pumps for pre-heating water which is to be used as domestic hot water, the Water Fittings Regulations place a legal duty on the installer and user to ensure that the installation and operation of the complete system prevents contamination of domestic hot water by Legionella bacteria, which can grow in water stored at temperatures between 20o and 45oC. 

Where disinfection by heating is relied on to meet this obligation, information on minimum conditions for thermal disinfection of Legionella bacteria can be found on the WRAS website - www.wras.co.uk.”

WRAS Approval will still be granted for complete systems for pre-heating water where they have the means to maintain adequate thermal disinfection of the pre-heated water, if they comply with the Regulations in other respects.  Where adequate thermal disinfection is not included, and in the absence of other means to prevent potential Legionella contamination, WRAS Approval will be withheld pending the outcome of the risk assessment.

 

WRAS, Oakdale, Last updated –  September  2009