Electric boiler - what brand for integration with openHAB?

I’m struggling to find a brand of electric boiler that allows wired external controls. There are dumb devices that I’d have to modify to get the water temperature and probably use a power relay to switch on when needed or a fancy wifi + app boilers that are probably overkill and there is no info about integration with any system except promise of “controlling from your phone”.
I’ll be running that boiler in a house with solar panels, so I want to run it when the sun shines, not on a clock scheduler.
I did search the forum - please point me in the right direction if there is a topic about electric boilers.
I’m OK with DIY, but I’m starting from empty space for a boiler, so I’d rather buy a solution than make a solution.

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My idea for control of the boiler, if I have to go DIY:
Hardware: thermostat on the boiler, additional temperature sensor for openHAB, power relay to control the boiler externally. The goal is to maximise use of the solar energy, without compromising the comfort.

  1. The thermostat on the boiler set to maximum temperature, probably 70 deg C
  2. Current solar power info available for use needs to be known to openHAB
  3. Current water temperature needs to be known to openHAB
  4. All taps need to be be with thermostat to avoid burns with 70 deg C water (not sure if this is required)


  1. If the water temperature is below comfort temperature (probably 45 deg C) - turn the heating on, no conditions
  2. If the water temperature is above comfort temperature (probably 45 deg C) - turn the heating on if the available solar power is higher than the boiler power (+ a margin of 10-20%).
  3. If there is plenty of solar power the boiler will reach the max. temperature and them the built-in thermostat cuts off the power
  4. Some hysteresis to be added to avoid rapid on-off cycles
  5. Anti-legionella heating to 65 deg C, independent of available solar power to be executed once a week, if the boiler didn’t reach the treatment temperature in the last week. This is probably not required, but I like to use the system to the max :smiley: Time and temperature of treatment to be confirmed.
  6. Guest mode (expected higher water usage): bump comfort temperature to 55 deg D
  7. Holiday mode (expected zero usage): drop comfort temperature to 15 deg D, so effectively only solar power would be used.

I kindly ask for comments from more experienced openHAB users.