Error: "The model is structurally singular" Dymola Modelica Solar thermal coupled with hot water tank and floor heating

Here I want to heat the floor heating by using in oder of solar thermal collectors, hot water tank and heat exchanger as long as the global radiation is higher than 100 W/m^2. How can I set this expression (if the global radiation is => 100 W/m^2, open the valve.) For that I used realExpression. On the building side I have a constant mass flow. I want see the temperature change of Boundry_pT in a whole year.