This project will show you how to convert your old solar water heating to be smart using HA and low cost Sonoff device. This solar heater are common in Israel.
see wiki more details
Just fyi: the minimum temperature to prevent legionella bacteria forming in storage hot water services is 60°C. Though if this water is used for hydronic heating rather than showering there is probably little risk.
Hi @tom_l
The sensor temperature is the TH16 temperature (located at the bottom) not the water temperature. The water is much hotter, especially at the top of the tank. this is how those devices works.
BTW I found that turning it to minimum save a lot of water. The reason is that my son consumes all the hot water he can. So tuning it to 100 degree means that he will have a lot of water to consume . Keeping it at the minimum I can indirectly tune the maximum time of the shower 
I wish I could have a mechical water switch that could switch the hot water with icy cold water.
Every parent of teenagers wishes for that lol.
I wish I could have a mechical water switch that could switch the hot water with icy cold water.
Haha maybe something like this could be added:
https://www.smarthome.com/smarthome-select-electronic-water-shutoff-valve-3-4-inch-12v-dc.html
Even so this is a nice Idea of using HA to control water temp, you can even set alerts if temperature goes too high or low.
You probably mean 3 ways solenoid switch. Switching the water will keep the water pressure on pipes.
