The tank in tank solar water heater is one pressurized solar water heater. The system has three tanks: heating tank, storage tank and small filling tank.
The heating tank is a non-pressurized tank which can heat the water inside of the storage tank by heat transfer fluid.
The Storage Tank, inside of the heating tank, is a pressurized tank which can provide hot water for daily use.
The small filling tank could fill heat transfer fluid in the gap between the heating tank and storage tank when fluid vaporized.
Technical Data:

Max. Working pressure: 6 bar
Storage Tank: Enameled SPCC d=2mm
Heating Tank (inner layer): Stainless Steel SUS304-2B d=0.6mm
Heating Tank (outer layer): Painted Steel d=0.426mm
Length of vacuum tube (mm): 1500mm/ 1800mm
Tank/ filling tank capacity: 150L / 5.5L