Abstract In this paper, a free running centrifugal fan inside a rectangular duct is investigated via measurements and CFD. A rectangular cuboid-shaped body is mounted downstream of the free running centrifugal fan, covering most of the space next to the fan wheel, leaving only a small channel for the air near the duct walls to pass through. We call this cube a ‘pressure regain unit’ (PRU). The cube increases the fan efficiency by reducing vortices downstream of the fan and tranfering a larger part of the kinetic energy into static pressure. Experiments are conducted with several PRU geometry variations. The measurements show that an increase in efficiency of 10% is possible compared to a free running fan in an empty duct. Subsequently, a numerical analysis is performed to analyze the effect which leads to efficiency increase. Comparing the installation with and without PRU shows that the main difference is a recirculation area behind the fan wheel when no PRU is installed. Higher level of turbulence and strong shear layers in the empty duct cause efficiency losses. The turbulence level and the amount of shear layers are reduced by the PRU. Both experimental and numerical data show an energy saving potential by optimizing the downstream region of the free running centrifugal fan.