Does water cylinder temperature need to be 60°C?
What the Science and Regulations Say:
Legionella thrives at temperatures between 20°C and 45°C and multiplies most effectively around 35°C.
Temperatures of 60°C or above are known to kill Legionella bacteria effectively, which is why regulatory guidelines often recommend maintaining hot water systems at this temperature.
In New Zealand, Clause G12 of the Building Code states that water in storage systems (e.g., hot water cylinders) should be heated to at least 60°C to control bacterial growth, particularly in systems that don't continuously circulate water.
An Australian standard which can be applied in New Zealand, a water cylinder thermostat can be set to 55 degrees.
Misconception Element:
The misconception lies in the idea that the water must always be 60°C. The Building Code allows for variations, such as heating to 60°C at least an hour daily, provided that these measures effectively control bacterial risks.
Additionally, some standards, like the Australian/New Zealand standards for heat pumps, allow for thermostats set at 55°C under certain circumstances. This is because water coming into the cylinder from a heat pump is often hotter, and the cumulative effect of temperature and duration of exposure can still inhibit Legionella.
To comply with New Zealand regulations, the cylinders specified come with an in-built electrical element, plumbers typically set a cylinder thermostat to 60°C or higher, and then fit a tempering valve – as water at personal hygiene outlets must not be more than 55. (Schools, hospitals and aged care facilities have lower maximum temperatures).
It is important to note that when a heat pump is used to heat a cylinder to 55, the water from the heat pump has to be somewhat higher - i.e when the cylinder is 54 degrees, the water from the heat pump going into the cylinder will more along the lines of 59 or 60.
What does this mean?
If a hot water heat pump is fitted to a cylinder, with its thermostat set at 55, a timeclock can be introduced to control the electric element to provide the booster affect when you need more hot water and bring the cylinder to 60 degree or above.
The big advantage of using heat pump as main hot water heating source is reduced running costs, with no safety compromises.
Hot water heat pumps that use R410a or R32 are quite happy to take water to 55, with the additional advantage that they also operate quite happily at sub-zero temperatures. Newly released R290 Heat Pumps has the ability to heat the water to 70 - 75 degree. Read more…
A hot water heat pump can save $200 of electricity costs…per person, per year.
Therefore, a family of five will save around $1000 a year.
The result?
Every household’s hot water demand is different, switching on the electrical element if needed. Fit a hot water heat pump, and you can still enjoy the savings on hot water heating!
Additional reading: Heat Pump Hot Water Options on Building Performance