With One Heat Exchanger

This is the most common boiler-room cylinder in homes. The fixed heat exchanger, a coil at the bottom of the tank, connects to one heat source: the central heating boiler, a heat pump or a forced circulation solar system. The electric immersion heater tops up when that source is off, which is why it is called dual energy.

Capacity is chosen by the number of people and the outlets (bathrooms, kitchen) it will serve. If you are thinking of adding solar collectors later, choose a model with two heat exchangers.