The representation of functions of Environment Control and Life Support Systems in ESATAN Models requires the implementation of automatic control features. Mainly the air temperature and air mass flow are subject to automatic control.This paper shows - as an example - the implementation of a PI- (Proportional- and Integral-) Controller to represent the effects of automatic control of HERMES ECLSS.Implementing these functions in ESATAN requires different approaches for steady state and transient solutions. The control algorithm is implemented in the model so that it is passed through in each timestep. Hence the setting of the active components is updated in each timestep. For the calculation of steady state cases the control algorithm had to be implemented so that the controller parameters are not timestep dependent.The paper shows that active control features can be implemented in ESATAN Models by adding some subroutines. The models can give valuable information on the control strategy to choose and on the layout of the controller. Furthermore transient effects can be studied in more detail due to the fact that the transient effects of automatic control features are accounted for.