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Recirculation loops (RL) in multi-family central domestic hot water (DHW) systems bring hot water close to dwelling units to provide tenants with quick access to hot water. However, long RL pipes make them a major source of system heat loss. Previous studies assessed energy savings by different RL control technologies based on measurement of overall system energy consumptions, and they did not reach definite conclusions. Overall energy consumption is affected by many operational parameters. Without evaluating RL pipe heat loss directly, energy savings cannot be accurately assessed and energy savings mechanisms cannot be well understood. This paper presents an energy flow analysis method to assess RL pipe heat loss based on field measurements of pipe temperatures and water flows. This method further separates the RL pipe heat loss into two components, one associated with the recirculation flow heat loss and one associated with hot water draw flow heat loss, so that the direct impact of controls to RL pipe heat loss was revealed. The energy flow analysis method also provided a way to characterize central DHW system performance by based on percentage weighting of four major energy flow components, including water heater loss, RL pipe heat loss, branch pipe heat loss and hot water draw energy, in the overall system energy consumption. Our field study results indicated that RL pipe heat loss represented about one third of system fuel energy consumption and the average overall system efficiency was only about 34%.