The control quality of thermostatic valves is influenced by design parameters of the valves as well as by design parameters of the overall system. Both of these influences should also be taken into account by the energetic evaluation of thermostatic control valves. The following article presents a closed analytical approach for calculating the room temperature control loop and the comparison of this model to measurements in a well-known test room. In a second chapter sensitivity analysis are carried out with the numerical model under steady state conditions for relevant parameters. The results of these investigations were compared to current normative specifications.