Experimental and theoretical investigations of vertical mantle tanks for solar domestic hot water systems have been carried out. Differently designed mantle tanks have been evaluated in a laboratory test facility and a transient three-dimensional CFD-model of one of the mantle tanks is developed in the CFD-program CFX 4.1. The model is validated against the experimental tests and good agreement between measured and calculated results are achieved. The results from the CFD-calculations are used to illustrate the thermal behaviour and the fluid dynamics in the mantle and in the inner tank The CFD-calculations are used to carry out a detailed analysis of the heat transfer from the solar collector fluid to the wall of the inner tank. The analysis has resulted in a local Nusselt-Rayleigh correlation for the heat transfer between the solar collector fluid and the wall of the inner tank.