The complete solar combisystem consists of two units, the “Technical Unit” and the “Solar Store Unit” (see Fig 1 and Fig. 2).

In the technical unit all components like boiler, pumps, mixing valve, switching valves, heat exchangers, hot water preparation unit, expansion vessels, etc. are pre-installed. The main difference of this concept compared to existing ones, is the kind of integration of the condensing natural gas boiler. Due to the fact that the boiler is powerful enough for direct domestic hot water preparation, it can be avoided to heat the standby volume of the solar tank up to high temperatures for hot water preparation (typically 70°C or more). Therefore, the standby volume is only used at the temperature level needed for space heating operation, which results in much lower average temperature of the complete system and leading to higher overall performance of the heating system thanks to reduced heat losses of the tank and the pipes as well (detailed simulation results are presented in [2]). If hot water demand occurs and the temperature in the top of the solar tank is not high enough, the gas boiler immediately starts running in hot water preparation mode at high temperature level for direct hot water preparation in combination with the flat plate heat exchanger (see Fig. 3).

Further advantage for the condensing natural gas boiler is the very low return temperature during domestic hot water preparation leading to higher condensation rate and higher efficiency. During periods where space heating load is less than the minimum power of the boiler which can be reached by modulation (5.7 kW) the use of the standby volume reduces significantly the start/stop frequency leading to less start/stop emissions and longer life time of the ignition unit in the boiler.

Due to this operation concept also the top of the solar tank (Tc1) never is heated to high temperatures by the boiler resulting in a higher heat storage capacity of the tank for the solar heating system.