In this project model identification of a thermal closed loop system is investigated. The plant is analyzed using first principles and the behavior of the controller is investigated. This leads to nominal model for plant and controller which is used for model identification. The Hansen Scheme method of identifying a plant is explained. The Hansen Scheme necessitates coprime factorizations of the system which are not widely available for PI-controllers with reference feedforward. Coprime factorizations for a such a controller are analytically found and a model identification technique using these factorizations is given. Using this technique on data from the "Styr din varmepumpe"-project a closed loop identification is performed.