It is a very common misconception that water in a cylinder must be at 60 degrees to kill legionella bacteria. Legionella is a naturally occurring organism present in all water. It thrives and multiplies around 35 degrees but is killed by higher temperatures.
To comply with regulations, plumbers will set a cylinder thermostat to 60 (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).
Clause G12 of the building code referencing solar hot water, states the cylinder should be taken to 60 once a day, or 60 for an hour once a week. However, following an Australian standard which can be applied in New Zealand, a water cylinder thermostat can be set to 55 degrees.
It is important to realize that when a heat pump is used to heat a cylinder, in order to get to 55 – the water from the heat pump has to be somewhat higher, ie when the cylinder is 54 degrees – the water from the heat pump going into the cylinder will be say 59 or 60. So a heat pump probably indirectly complies with the 60 degree standard – it certainly complies with 55 degrees regardless of circumstances.
What does all this mean?
If a hot water heat pump is fitted to a cylinder, with its thermostat set at 55, there is no need to use the element at all. No need for 60 degrees once a day or once a week. A tempering valve will still be required (which, even if set at 55, will reduce the temperature a few degrees), as a fault or switching the element on could potentially take the water to much higher temperatures.
The big advantage with all this 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.
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.
Fit a hot water heat pump, set the thermostat to 55, turn the element off, enjoy the savings.