There are two basic differences in the system configuration of solar combi-systems in a way how the boiler and the thermal solar collectors are integrated in the system with a lot of variations. With a serial integration of the thermal solar system as shown in figure 2 only the solar collectors charge the buffer store. If the temperature of the buffer store is too low the boiler delivers the remaining necessary heat.

With a parallel integration (see figure 3) the boiler is charging e. g. the upper third of the buffer store and keeps it on a certain temperature level if the solar collectors do not deliver enough heat. An advantage of the parallel integration is that the boiler does not start as often as in serial integration which improves its efficiency. However the higher temperatures in the buffer store can diminish the solar fraction and lead to higher heat losses of the buffer store.

Which of the two systems is better depends on certain criteria, e. g. the heat load of the connected housing area. This will be considered in the project as well. Both serial and parallel integration can be found in the chosen systems. An overview of these systems with the main features is given in table 1.