Figure 4 shows the various volumes that a relevant in a combistore. They are ordered by the increase of temperature from the bottom to the top of the tank. All heights in the tank are referred to as relative heights between 0 and 1.

Figure 4 Different volumes in a combistore for auxiliary heat for external (left) and internal (right) heater

In Figure 5 the dependency of fsax, ext on the relative heights of inlets/outlets and heat exchangers in the stores is shown.

Figure 5 Dependency of fsav, ext on specific parameter change of storage geometry

Most relevant seems to be the collector outlet position. As higher it becomes as less volume of the store can be heated. Additionally there may be heat load parts not delivered with heat any more. There were only two systems in the simulation were this parameter was altered. The standard variation shows, that the results differed widely. So no clear statement of this influence can be drawn.

Of less importance is the height of the boiler inlet.

For the heat sinks, the DHW heat exchanger inlet position, that often determines the not useable volume of the store is most significant followed by the position of the inlet tube for the SH system. The latter defines the preheating zone for the DHW, which represents the coldest part of the store and therefore the volume for the solar collector where its highest efficiency occurs.

The (upper) SH system outlet, that also represents the position if there is only one SH outlet, seems to be not so important, as most of the systems do not fully cover the SH demand during the heating season. Therefore the after heating volume for the DHW production is mostly not delivered by the solar collector in the heating season. In summer the SH system is not relevant.

The positions of the boiler inlet and outlet were varied for 4 systems. As the boiler outlet height defines the auxiliary volume and therefore the volume that can be preheated by the collector, this value is very significant. This volume should be kept as small as possible in order to have a high volume exclusively charged by the solar collector. The minimum value is determined by the maximum possible temperature in the auxiliary volume, the power of the heating device and its required minimum running time, and the maximum load. The higher the maximum temperature and the higher the power of the auxiliary heater the smaller can this volume be. If, on the other hand, a longer running time (i. e. wood log boiler) is required, the volume has to be big enough assuring this running time. A higher load needs also a higher auxiliary heated volume.

For the system shown in Figure 6 these effects can be clearly seen for an internal heat exchanger. fsav is increasing with decreasing auxiliary volume until the user demand can not be satisfied any more (fsi is decreasing) due to too high load or too low heating capacity of the auxiliary heater. This happens at a volume below 0.09 m3.