The thesis at hand is a mandatory part of the fourth semester master program in Building Energy Design at Aalborg University. The research’s objective is to investigate and evaluate the influence of occupancy variation and flow reduction on a two-pipe system for simultaneous heating and cooling, in a large-scale office building; and how performance of energy flexibility is affected by these variations. Studies are carried out in order to determine the range of flow and occupancy variations, and four main cases are established including the baseline model. For each case three control strategies are implemented and a total of 12 cases are simulated using EnergyPlus software. The results will show that occupancy variation has a clear impact on the performance of the two-pipe system, and a 15% flow reduction actually improves the energy savings with no considerable effect on thermal comfort. From the control algorithms analyzed a control strategy based on electricity price levels will be proven to give the best results.