Water heater automation progress (EU/UK)

I'm writing this to record important discoveries I got whilst researching the goal of automating electrical heater for roof-top water tank.

Normally those tanks are heated by Sun (I'm on Cyprus), and in cases of bad weather electricity is used (as little as possible). I want to see the temperature in the tank (in few places), and to control electricity to the tank, with 'local first' approach (e.g. switch on the wall overrides robots), and no clouds.

Few important discoveries:

1. Cyprus and UK requires using of dual pole switches for water heaters (so called 'cooker switch' or 'heater switch'). That means, both live and neutral must be disconnected in OFF state. This disqualifies about 99% of all zigbee relays I found. I'm in the process of finding zigbee-controlled relays for double poles (and 20A relays...).
2. Water temperature is a more complicated problem then I anticipated. A simple DS18B230 is okay, but the water tank is pressurized with a pump, well insulated (a think layer of insulator is around it, with tight metal cover). Having inputs only from outlet is unreliable, because outlet is cooling down when not used. I'm still searching for the way to get to the tank temperature.
3. Even without #2, transmission from the roof is a problem too: zigbee do not penetrate, and all wifi devices I saw for that (e.g. Sonoff THR320D) are cloud bounded.
4. Any solution should be weather-proof (I can shield from direct rain on the roof, but not from moisture).

This is 'progress report', not a success story. If you can correct me or know a (partial) solutions to the subproblems, please, advise.

Submitted by amarao_san

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