Simscape thermal model of office space - temperature drop

In the model I simulate the thermal behavior of office space. To realise that i use a mixture of simulink and simscape. For that reason I only want to work with pure power input and temperatures. Therefor I use some controlled Heat flow Sources (solar radiation, heating, people) and controlled Temperature Sources (environment and connected rooms). My problem is connected with the controlled Heat flow Source of the heating. The Power Input of the heater varies between two values: 0W and 800W and is not connected to pipes or something else only with the airmass of the room. I tryed to turn it off by a relay, a embedded matlab function and a stateflow chart an the result in every case is the same. Only the overshoot is diffrent. The picture is the result of the matlab function, if I use stateflow the overshoot is nearly 1K.

My porblem is a fast temperature drop after turning the heater off. When I only calculating the room behavior without the heating it is a quite good match with the meassured temperatures but if the heating is included the results look like the attached picture. The main problem is the temperature droop because the increase in temperature is in real nearly as fast as the simulation because of the small room volume. The temperature drop like the same between the small intervals but on the flanks it is more obvious.

Is the problem caused by a numeric issue and how can I solve it?