Safe temperature of water heaters

Our household has just received a leaflet on energy savings tips – a practical guide to energy efficiency in the home issued by the Malta Intelligent Energy Management Agency. This leaflet is a very interesting and useful document but it contains one fundamental piece of wrong advice that can have dangerous, even deadly, repercussions.

It suggests that people should “adjust the thermostat on (their) water heater to a comfortable temperature such that no cold water needs to be mixed in”. This is totally incorrect and should never be practised.

The reason is a microbe called legionella that is normally found in tap water and even more commonly in well and bowser water. Drinking this bacterium is harmless; however if it is allowed to multiply and then sprayed as an aerosol, it can be inhaled into the lungs where it causes the chest infection known as legionnaire’s disease. This is a very severe form of pneumonia which, even with antibiotic treatment, can have a mortality rate of 30 per cent in the elderly, whom it affects mostly.

The main problem with legionella is that, unlike most bacteria, it survives and even grows in hot water; indeed it multiplies most readily at temperatures between 30 to 40 degrees Celcius, which is exactly the temperature that water in a geyser would be if “the thermostat is adjusted to a temperature such that no cold water needs to be mixed in”.

If the agency’s advice is followed, the risk is therefore that any legionella in tap water would be allowed to proliferate from its normally harmless levels to high concentrations; taking a shower (which the same leaflet correctly recommends, rather than taking a bath, for energy saving reasons) would then be the ideal way in which the contaminated water would be aerosolised and inhaled.

Hot water heaters must be set at a minimum of 60°C which is the temperature at which legionella is killed. If anyone wants to save money on the electricity bill, it is preferable – not to mention safer – to incorporate a timer that switches on the water heater some three hours before it is needed. This would ensure that the water would not only have reached a safe temperature but also that a minimal two-hour thermal shock dosing period would have been in place to eliminate any bacteria that may have started to multiply during the time when the hot water heater was switched off.