Legionella is present in municipal (chlorinated) water supplies in very low numbers. In non-municipal water supplies i.e. non-chlorinated, such as dam and lake water the numbers can be higher. A recent death of a man who contracted Legionnaires' Disease from a domestic hot water system in Brisbane has highlighted the potential dangers of turning down the water temperature of hot water tanks to reduce power costs and/or to reduce the risk of scalding. Domestic and commercial hot water systems are generally set by the manufacturer to hold the water temperature above 60 oC.

Legionella bacteria must make their way to the lower part of the lungs before they can cause disease. There are two mechanisms for this process. Firstly by breathing fine droplets of water containing the bacteria such as would occur from a shower rose. Secondly the bacteria can enter the lung by aspiration. Some scientists believe that aspiration is the most common way that Legionella bacteria can enter the lungs. Aspiration means choking such that secretions in the mouth get past the choking reflexes and instead of going into the esophageus and stomach, mistakenly, enter the wind pipe and enter the lungs. The protective mechanism against bacteria and particles moving from the mouth to the lungs is the fine hair (cilia) that carry foreign material up from the lungs. People who smoke or have lung disease can have defective cilia and this allows legionella bacteria present in the mouth (from water contaminated with Legionella) to enter the lungs.

At 60 oC Legionella bacteria will not survive. Legionella will die in time at temperatures over 50 oC and will multiply between 20 oC and about 50 oC. At temperatures below 20 oC Legionella will remain alive but will not multiply in numbers. When testing a hot water system temperature it is essential that the water temperature be measured from an outlet tap at the furthest distance from the storage tank. There is always a loss of heat due to radiation while the water travels along pipe work. If there is a mixing value attached at the outlet this should first be set to 100% hot water and after the tap has run for 30 seconds to a minute the temperature should be measured. If the temperature is less than 60 oC the storage tank temperature needs to be raised. Be aware that if this is necessary then each mixing valve will need to be adjusted to ensure that scalding does not occur for those outlets closer to the storage tank.

Turning off hot water systems in non-peak periods allow bacteria to multiply as the water temperature decreases. But as the generation time for Legionella is about 24 hours this should not represent a major risk so long as temperature is set to 60 oC and sufficient "on time" occurs for the water to reach 60 oC before use.

If scalding is of concern, then the storage tank temperature should still be set at 60 oC and a mixing valve should be installed at the tap outlet.

Hot water storage tanks should be emptied, all sludge removed and the system disinfected at least yearly. All new hot (or cold) water systems or those that have been out of use for extended periods should be cleaned and disinfected before being brought into use. Superchlorination followed by thorough rinsing is a suitable procedure. People who go on holidays and turn off the hot water system should flush the system out before use on their return.

Electric hot water systems with the heating element about one-third the way up the tank encourage heat stratification of the water and can enable Legionella growth below the heating element. Gas tank hot water systems are better as the heating element is at the base of the unit - i.e. less chance of heat stratification. Flow through gas hot water systems greatly remove the potential for legionella overgrowth as there is no storage tank.

After major plumbing repairs the system should be purged and disinfected as the plumbing process will often disturb biofilm on the pipes that can harbour high levels of Legionella bacteria.

Domestic hot water systems need not be tested for Legionella so long as the guidelines described are followed. Hospital, commercial and aged care facilities need to have their water tested regularly.