Buffer tank with a heat exchanger + internal storage tank
A buffer tank is a heat exchanger and an enameled domestic hot water tank built into the tank without insulation.
The steel tank is not treated on the inside (it is not necessary, since it will contain oxygen-free water, so it will not rust), and it has a single-layer primer paint on the outside.
In this buffer tank, you store the hot water that you also circulate in your heating system.
You can heat the water in this closed circuit directly with a wood-burning boiler or a water-filled fireplace.
It can also be heated with an electric heating element.
You can only connect a solar collector to buffer tanks with an internal (or external) heat exchanger, because antifreeze circulates in them, so a lower heat exchanger tank is required for this. A heat pump can be connected directly to it, if your heating circuit does not operate with antifreeze.
Heated bath water is prepared and stored in the built-in internal tank, which is enameled.

One heat exchanger and tank in the tank NADO v2 500/140