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To meet the rising comfort requirements of non-residential buildings like office buildings a mechanical ventilation in the form of e.g. air handling units (AHU) is needed. As of today, fans in AHUs are usually mounted in a free-running configuration, meaning that almost all kinetic energy present at the exit of the fan impeller is treated as a loss. This is due to the fact, that only the total-static pressure rise is taken into account to rate the fan performance. Depending on the operating point the ratio of dynamic and static pressure rise at impeller outlet can be unfavourable, leading to suboptimal efficiency of the whole system. This leads to higher electrical consumption than needed and therefore higher CO2 emissions. The present work investigates the behaviour of a generic radial fan in a free-running configuration by conducting 3D numerical flow simulations. In a first configuration, the radial fan blows into an open air duct channel. The kinetic losses as well as the static and total pressures at the impeller outlet are then evaluated over the operating range of the fan. In total three lines of constant fan speeds with eight operating points per speed line are investigated. By defining areas of occurring backflow downstream the fan, the areas of disadvantageous flow are identified. To enhance the conversion of dynamic into static pressure and therefore increase the total-static efficiency an additional so-called pressure regain unit-body (PRU) is implemented in the flow downstream of the fan. The design is kept as simple as possible to enable an easier implementation in a real application. Starting from an initial design of the PRU the design is further improved to reveal possible limits of the efficiency increase by such unit. By designing the PRU in a specific way, it might be possible to shift the peak efficiency of the system towards lower or higher volume flows and thus better tune the system to the requirements.