Manufacturers have during the last few years started marketing of newly developed natural gas burner/solar tank- and oil fired boiler/solar tank-units.These units make it possible to reduce the costs of solar heating systems, because an electric heating element is not built into the solar tank and because the installation is easier.Further, the use of well designed units in stead of traditional solar tanks can result in increased thermal performance of the solar heating systems and in increased energy savings, since the use of a unit can result in reduced heat loss from the solar tank, from the pipes connecting the natural gas burner/oil fired boiler and the top spiral of the solar tank and from the natural gas burner/oil fired boiler. Furthermore, the unit with a fixed design of the natural gas burner/oil fired boiler - top spiral loop open up the possibility of an optimum control of the natural gas burner/oil fired boiler and of the circulation pump of the loop so that the heat left in the loop outside the solar tank after every period of operation is reduced to a minimum.Measurements have been carried out in practice for three solar heating systems based on the new units. Additional measurements have been carried out for a traditional solar heating system based on a separate solar tank and a separate natural gas burner.The measurements showed that the thermal performance and the energy savings for the solar heating systems based on the new units are smaller than the thermal performance and the energy savings for the traditional designed solar heating systems. That is: The investigations showed that there is a large need to improve the design of the units. By means of the investigations design rules for the units were worked out.