Solar Legionella Control

Legionnaire’s Disease is a type of pneumonia which in many cases proves fatal. The disease is caused by a bacterium called ‘Legionella pneumophila’ and is contracted by inhaling tiny droplets of water containing the bacterium. The four major methods for diagnosis are: determination of antibody level, demonstration of the bacterium in tissues or body fluids by using immuno-fluorescent microscopy, actual isolation of the organism on culture media, and detection of antigenuria. If left untreated, the disease has a 5-80% mortality rate.

Air-conditioning cooling towers, whirlpool spas, are known sources for L. pneumophila, however domestic water supplies can also be problematic.

The main difficulty with Legionella and the assessment of the risk that it poses is the assertion that perhaps 95% of cases are never diagnosed properly. In the classic case of people being taken ill on returning from holiday abroad, it has always been assumed that the infection was caught abroad and not after having used legionella contaminated water for a shower on their return home (where the hot and cold waters systems may have been left in an uncontrolled stagnant state for a fortnight).

Legionella will grow in water at temperatures between 30°C and 50°C. Control measures are aimed at keeping cold water storage and delivery below 25°C and hot water storage and delivery above 60°C. There is great variation between countries on the assessment of the risk posed by legionella, with the trigger levels for action varying by a factor of about 50 from country to country.

In addition there are about 30 different strains of Legionella with differing resistance to hot water. Some strains will exhibit a 90% mortality after 2 mins in 60°C water, while others will require 20 minutes at 60 °C to reach the 90% figure.

In the UK the relevant Code of Practice is L8. However its defined scope is restricted to public and commercial buildings and domestic dwellings that are rented, due to the requirement for a nominated responsible person in charge of the implementation of control measures.

The main implications of L8 are

• cold water storage at temperatures less than 25°C
• hot water storage should be brought above 60°C for at least one hour every day
• hot water distribution should have no dead legs
• hot water should emerge at greater than 50°C from any delivery point within one minute of turning on the tap
• cold water should emerge at a temperature below 20°C after 2 minutes of turning on the tap
• thermostatic mixing valves should be fitted at point of use
• no serial connection of hot water storage (no pre-heat cylinders)

Note the tension that exists between wishing to generate and use hot water efficiently from solar systems at lower temperatures, (many countries allow 45°C) to reduce the use of fossil fuels and the requirements of L8 to heat the storage above 60°C for one hour each day and to distribute hot water at least at 55°C (and to blend below scalding temperature at the relevant outlets).