Currently, the domestic hot water (DHW) system in building 90 at Lawrence Berkeley National Laboratory uses a recirculation system and a pump that is constantly running. This way, the supply pipe always contains hot water and it is directly delivered upon request. This talk will describe the field instrumentation that was installed and how the performance of the DHW system at building 90 was determined. The amount of hot water used per day, gas energy, and efficiencies will also be discussed.