The temperature of a hot shower is around 40°C, heating and storing water significantly hotter than this and then lowering the temperature with cold water is simply wasting energy (hotter water will lose more heat no matter how good the insulation). It is therefore surprising to find that the regulations in New Zealand state:
Building Code G12 - 6.14.3
Irrespective of whether a mixing device is installed, the storage water heater control thermostat shall be capable of being set at a temperature of not less than 60°C to prevent the growth of Legionella bacteria.
And then compare with this letter:
Safe hot tap water and the risk of scalds and legionella infection
Senior Data Analyst, Queensland Injury Surveillance Unit, Mater Hospital, South Brisbane QLD 4101, Australia; email@example.com The recent paper in Injury Prevention by Jaye et al on the barriers to safe hot tap highlights the problem I raised last year regarding the perception of plumbers of the risk of legionella infection from hot water systems compared with that of scalds. The paper reports that half the respondents thought it more important to control for legionella than to prevent hot tap water burns. Much of the blame for this perception can be attributed to the Australian and New Zealand Standard (AS/NZS 3500.4.2:1997) which sets a minimum temperature of 60°C for hot water storage systems to protect against legionella.
In 1994 the standard was amended to require that the delivery temperature be a maximum of 55°C in personal hygiene areas in domestic installations through the installation of tempering devices. Several years ago the NSW Heath Department tried, as part of its "Burns like fire" campaign, to have this minimum revised down to 50°C and presented compelling evidence supporting the view that there would be no increased risk from legionella at this temperature.
This view is also reinforced by the situation in the US where the maximum storage temperature is set at 50°C, a measure that has resulted in a reduction in scalds without any discernible increase in legionella infections. Unfortunately the Joint Technical Committee for Plumbing Standards, which not unexpectedly is dominated by plumbers, dismissed this evidence and we are left with a confusing, contradictory, and expensive standard when a simple solution exists. Setting the storage temperature at 50°C, which is also easily implemented in most existing homes, would also protect children and more particularly elderly persons from hot water burns in the kitchen and laundry (a not insignificant problem).
I would also contend that in a warm country like Australia, where during the summer months cold water is often delivered at temperatures exceeding 30°C, it is likely that cold water is a more significant source of legionella infection (the ideal temperature for the growth of legionella is 20-43°C).
The other issue that was touched on in the paper was the reliability of tempering valves, something which plumbers I have spoken to have alluded to and needs further investigation.
Both scalds and legionella are important public health problems, however, very little is known about the contribution of a domestic hot water supply to legionella. No one would deny that hot water systems are a source of infection, with up to 30% of systems testing positive to the organism, but the one study published examining domestically acquired legionella failed to show a relationship between hot water heater temperature and the disease. On the other hand the evidence of the association between hot water temperature and scalds is compelling.
I hope other countries can learn from Australia's experience.
What is your view?
posted on 30-01-07
The Approved Code of Practice in the UK requires storage to be at 60c minimum and temp at the point of use no less than 50c.
In high risk areas it is necessary to use mixing values at the point of use to reduce the temp to prevent scalding.
In the UK these requirements apply to all buildings including residential ones that are let out. So if anyone has a buy to let or similar then they must do a risk assessment.
Personally I think its getting ridiculous and all they do is create a rule and dont give a damn what the knock on effects are. ie the additional heat loss from storing water at 60c.
posted on 30-01-07
What does this mean?:
"Irrespective of whether a mixing device is installed, the storage water heater control thermostat shall be capable of being set at a temperature of not less than 60°C to prevent the growth of Legionella bacteria"
Sounds like it means that you have to be able to set the temperature above 60°C, but presumably they intend that you may not be able to set it below 60°C?
In Canada, consumers are advised to reduce the set point of their (older) water heaters down to 50C, precisely to avoid the problem of scalding. Most thermostatic shower valves here have a preset detent at 38C. I notice that when the GSHP is running, the hot water temperature is above the tank's normal setpoint of 50C - I think the desuperheater output temperature is 55C. I recall reading somewhere the time it takes to get a scald at 60C is mere seconds and rapidly diminishes to several minutes at 50C.
posted on 31-01-07
It's all a symptom of society's attitude to risk: we exaggerate low risk activities and play down real risks. Thus we take out maintenance contracts at exorbitant rates for freezers, despite these being among the most reliable appliances with relatively low replacement costs or boycott the mmr jab because of a poorly researched and spurious link to autism whereas the risks associated with the diseases the jab guards against are infinitely greater. So I wonder what are the relative risks of contracting legionella versus being scalded?
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