Could My Hot Water System Give Me Legionnaires Disease?


The recent scare of Legionnaires Disease at the Wesley Hospital in Brisbane has lead to the questioning of tempering water on storage hot water units. Hot water units in QLD require a tempering valve to control the water temperature to 50 degrees Celsius. The question being asked is ‘Could my hot water system give me Legionnaires Disease?’

Legionnaires disease is transmitted by inhaling water that is contaminated with the Legionella bacteria. The bacteria can be found in natural water sources including rivers and lakes as well as man-made water sources like water tanks, hot water units, air conditioning cooling towers and spas. The optimum temperature for the bacteria to develop and thrive in is between 25 and 45 degrees Celsius.

The reason tempering valves were introduced in 1995 as a mandatory installation on hot water units was to avoid a person scalding themselves by turning on the hot tap. Prior to 1995, hot water was delivered at 70 degrees Celsius. The tempering valve reduces the temperature to 50 degree Celsius.

I have had clients complain that the ruling is ridiculous and request the valve to be removed so they can have the choice of hotter water throughout the home.  Being licensed plumbers, we are unable to do so because if someone did scald themselves, we the plumbers would be liable.

In my opinion, so long as the tempering valve is set correctly and working to produce tempered water to 50 degrees Celsius, the chances of growing the legionnaire bacteria is slim. But the probability can’t be ruled out.

If however, the temperature of the hot water unit has been turned down electrically by a licensed electrician, there is a greater risk of Legionella bacteria forming. There have been cases where the Legionella bacteria have been found in hot water systems because of this practice. You can read the article here.

Instantaneous gas hot water units are the least likely to grow the harmful bacteria because they don’t store hot water, they produce the hot water instantaneously which is used by the consumer immediately when they turn a hot tap on.

The QLD Health Minister has requested all hospitals to check their hot water systems and test for the Legionella bacteria.

Householders shouldn’t worry about the state of their hot water systems. If concerned, use a thermometer to test the temperature of the hot water. If the temperature is on the cool side below 48 degrees Celsius, you will need to call your local plumber to adjust the tempering valve. In some cases the tempering valve may need to be replaced. Tempering valves must be checked and tested by a licensed plumber every 12 months as per standards AS 4032.3 – 2004 and AS 3500.5-2012. If the hot water unit has been turned down electrically, an electrician will need to be called to raise the temperature.

It’s interesting when a law comes in place to prevent one outcome, it can consequently create a different problem. It will be interesting to see how the State Government deals with this situation and whether the tempering laws will be looked into.