The R&D laboratory of Gorenje d. d. develops complex cooling appliances. To develop optimal-performance appliances, regarding all functionalities, also engineers from other areas are participating in a developing process. To allow for an easier and faster development, we used mathematically-based simulation and optimization tools. A cooling appliance operates optimally when it is able to cool to the desired temperature at the lowest possible power consumption. For the purpose of our investigation we developed a simulator for simulating temperatures inside the appliance cabinets at different control parameters. By using simulations a considerable part of measurements was replaced, and as they are much time consuming due to the slow thermal processes, a lot of time was saved which in the end also minimized the development manufacture cost of the appliance.