A total of five different mantle tanks have been tested in the indoor storage test facility of the Institute. The tanks have different volumes, height/diameter ratios, mantle widths and mantle heights. The mantle tanks were tested under different typical working conditions.· The tests showed that it is advantageous both to the building and keeping of the temperature stratification of the domestic water volume and to the cooling of the solar collector fluid through the tank if the mantle is wide. The reason is that the mixing of the solar collector fluid at the top of the mantle is larger in a slim mantle. The fact is that a wider mantle implies that the natural work-ing powers in the mantle become more predominant.· The tests showed that it is advantageous both to the building and the keeping of the temperature stratification of the domestic water volume and to the cooling of the solar collector fluid through the tank if the height/diameter ratio is large.· The dynamic tests showed that the temperature stratification is very easily built up in the domestic water volume. When the temperature stratification is built, it is very difficult to destroy again - even under "hard working conditions".The flow and the thermal conditions of the domestic water in the tank and of the solar collector fluid in the mantle through the tests have been calculated by means of detailed CFD-models for the tested mantle tanks. The calculations were validated against the measurements, and the results showed that the CFD-models calculate the temperatures in the mantle tanks correctly.In the subsequent investigations the CFD-calculations have been used for giving a larger and more detailed insight into how the heat transfer from for instance the solar collector fluid to the hot water tank takes place.Further, a number of detailed parameter analyses have been carried out, which are impossible to carry out experimentally. The analyses in question clarify how the height/diameter ratio of the tank, the mantle width of the tank and the volume flow rate in the mantle respectively, influence the con-vective heat transfer from the solar collector fluid to the tank wall.· The results showed that the height/diameter ratio of the domestic water tank does not influence the convective heat transfer from the solar collector fluid to the tank wall.· Further, the investigations showed that in typical working situations the natural convection in the mantle would be predominant.· Finally, the investigations showed that the width of the mantle has a direct influence on the con-vective heat transfer from the solar collector fluid to the tank wall.The investigations made it possible to draw up correlations to describe the convective heat transfer between the solar collector fluid and the tank wall.