With Two Heat Exchangers

With two fixed heat exchangers, a vertical cylinder connects two heat sources without their circuits mixing. The lower heat exchanger, in the cooler part of the tank, usually takes the solar collectors, so the sun is used even at low temperatures. The upper one connects to the boiler or heat pump and heats only the top of the tank when the sun is not enough.

Together with the electric immersion heater you have three sources, hence triple energy. It is the logical choice for new boiler rooms, even if the solar collectors will be added later.