This simulation study investigates both different boiler specifications and various kinds of boiler integration in solar thermal combi systems with the help of a new developed fossil fuel boiler model. For each variant, the simulation outcomes to be discussed are the annual boiler efficiency, the cycling rate and, as the main indicator, the primary energy savings of the complete system. The results indicate that the effects of a solar thermal system on the annual boiler efficiency are small. Besides the system layout the annual amounts of hot water and space heating demand and their temperature levels affect the impact of the solar thermal system. Each additional storage tank, which is heated by conventional energy, should be analyzed critically due to their additional heat losses, which reduce the system efficiency. The latter is particularly true for a boiler buffer storage. This study points out that the savings of primary and final energy are the most important indicators for the assessment of solar thermal combi systems. Subsystem indicators like the annual values of collector yield and boiler efficiency give additional information, but they are not sufficient for a full evaluation of a complete system.