Replacement of an old twin belt-drive fan in a roof-mounted air handling unit in the office building of an automotive supplier with a supply air volume flow of 27,800 m³/h. The fan drove two impellers simultaneously with one motor. Both bearings were permanently lubricated. Over time, the entire system became unbalanced. This created a large gap between the impeller and cone, which led to a significantly reduced efficiency of the fan. The replacement optimised not only the energy efficiency but also the entire system in terms of pressure losses, cleanliness, maintenance and balancing. The system was replaced in only 2 days. The payback period is 3.1 years.