The Tank in Tank container by MasterSol combines a boiler room boiler with a buffer tank and connects to the boiler or solar collectors. The installation of the tank-in-tank system achieves better performance with simultaneous fuel economy and hot water production. This results in the burner operating even fewer hours, minimizing the energy requirement for space heating. The insulating material surrounding the buffer tank maintains the water temperature for a long period, as there is no need for the boiler or heat pump to operate when the hot water circulation is done by the water stored in the buffer tank.