We heat our water by an I Solar boost and since May the gas heating water has been turned off, apart from a few days when no solar heating was possible with the crap weather.
However we noticed that the boiler had been randomly switching on to heat the water.
After much head scratching by us and our heating engineer it was found that my Nest thermostat has a anti legionnaires setting that was causing it to switch on. This setting is not visible in the app only on the device buried in the Equipment setting.
The water needs to be heated above 60c to kill the bacteria and my solar heats it to about 58c.
So if you are just heating by solar ,something to be aware of.
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Yes, that’s a good point. I dont have batteries, so I tend to heat mine to 65plus anyway (via a solar diverter), so that feels like poor-mans batteries 
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My immersion heater thermostat only goes to about 58max
I’m no plumber, but that doesn’t sound right. It may be possible to change just the thermostat, but worth checking with the supplier first. Is it a modern fancy cylinder, or an old basic copper cylinder?
It’s quite modern. The immersion heater temp probe just sits in a tube in the middle of the element and has a dial on the top, and it’s at max.
This could just be an inaccurate thermometer or the heating element may need to be replaced.
I think the chances of you getting listeria are low, but the official recommendation is that you want 60°C minimum.
Anti legionalla measures are determined by risk factors. As Chris said, domestic cylinders will be low risk.
Things to consider.
Tank thermostats are not that accurate.
If the water is 60c at the top of the cylinder, it will be a lot lower than 60c bottom of the cylinder. (To do the job properly, the cylinder needs to be fitted with a destatification pump).
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