Legionella bacteria can thrive in hot water systems even when cylinders are set at the recommended temperature. A deadly disease which cost a woman both her feet could be lurking in our hot water systems, despite official requirements designed to prevent it.

A Hawke's Bay woman was one of two separate cases to have contracted legionellosis, also known as legionnaires' disease, which is a potentially fatal type of pneumonia. Tests on the water in both victims' homes found the presence of legionella bacteria, which can cause legionellosis, when it is inhaled in the form of vapour or steam containing the bacteria.

Legionella bacteria can grow in water temperatures of 20-45 degrees Celsius. They thrive at temperatures of 32C-44C, but can't live at 60C or higher. People are advised to keep their cylinders set at 60C for this reason.

Hawke's Bay District Health Board initially thought the woman may have contracted the disease because the temperature on her hot water cylinder had been set to somewhere between 50-55C.

But the temperature was actually set somewhere between 60-65C. Subsequent investigations revealed there was a tempering valve between the hot water cylinder and the tap, and that the bacteria had grown in this part of the system.

Institute of Environmental Science and Research microbiologist David Hart said while water should be stored at at least 60C, it did present a scalding risk at that temperature. The Building Act recommended that hot water coming out of the tap should be no hotter than 55C.

This was often done through the use of a tempering valve, which mixed cold water with hot water from the cylinder. If done correctly this would mean the hot water at the tap was at 55C (or 45C for elderly or young) but if too much cold water was mixed in it could bring the temperature down to a level at which the bacteria could flourish.

"The best way to lower the temperature at the tap is by installing a tempering valve as close as possible to the outlet," Hart said.

Many hot water cylinders have an adjustable temperature knob so they can be set at 60C, but some models may need a plumber or electrician to adjust them.

 Source: Stuff.co.nz