One of the crucial elements to ensure the efficiency of building operations is to understand the dynamics of energy flows, control strategies, and occupant behaviour in buildings. Currently, abundant sensors and sub-meters are installed in modern buildings to measure resource consumption at various levels of spatial and temporal resolution to help track and reduce energy use and greenhouse gas emissions. These sensors and sub-meters provide many data sources at the building level; however, this data is not necessarily in a readily comprehensible format. In addition, the current data availability and visualization tools have some limitations in identifying system inefficiencies and possible solutions. This paper proposes a method to estimate and visualize energy and mass flows in addition to the corresponding cost through different components of heating, ventilation, and air-conditioning (HVAC) system using Sankey diagrams. The aim of this study is to facilitate accessibility of data and to identify system inefficiencies. A major contribution of the paper is to convert sparse sensor data into estimated energy flows for each major AHU and plant loops component. The proposed method is then applied to a large Canadian university building. This paper concludes by discussing the challenges in obtaining data from sensors and sub-meters. The proposed method helps in understanding the performance of the whole system - not just individual components. Moreover, it helps identify operational problems and quantify their impact. (C) 2017 Elsevier B.V. All rights reserved.