Presents a study showing that pumps in most office building heating and cooling systems are oversized. Gives the reasons for this. Uses the study to estimate the potential energy savings from trimming the pump's impeller to match the actual system head requirements. The study was based on data collected from 14 large commercial office buildings in Madison. Discusses the results and concludes that substantial savings can be realised by trimming the pump impeller to the proper size. For the study shows a payback period of 1.3 years. In addition, impeller trimming is easy to implement and can be done without disrupting the building's normal operation.