Variable-frequency drives (VFDs) have been widelyapplied to chilled-water pumps and heating hot-water pumpsto meet variable loads in building heating, ventilation, and airconditioning(HVAC) systems. However, pumps with VFDshave a minimum speed near 30% of rated speed due to themotor overheating at lower speeds. Thus, they have to run atthis minimum power and flow rate to cover a large percentageof hours when the system loads are below 30% of the designedcondition. This results in a minimum pump power of ~10% ofrated power when the pump power could be as low as 1% ofrated power during these hours if the pump were capable ofoperating effectively at lower speeds. A new configurationcalled multiple pump sizes with VFD is proposed. A lowercapacity pump with a VFD is added to cover the operatinghours with lower loads and maintain a constant-loop differentialpressure in the pumping system. It can significantlyreduce pump energy consumption. A laboratory building anda hospital building both running for 24 hours per day, 7 daysper week are analyzed as case studies, and results show 10.5%to 32.1% pumping power energy savings with the new configurationand 1.5 to 7.1 year simple payback.