Waternomics is an innovative project aimed at improving water management within municipalities, corporations and domestic users by providing water managers and consumers with timely and actionable information relating to water usage, water availability and the state of systems within a buildings water network infrastructure. A novel aspect of Waternomics is to apply Fault Detection and Diagnostics (FDD) methods and techniques to building water networks in order to identify potential operational issues including leaks, malfunctioning equipment and inefficient operation. FDD is a measurement science which has traditionally been used to identify and rectify faults in Heating Ventilation and Air-Conditioning (HVAC) systems in buildings. The results have been reduced maintenance costs, increased efficiency and energy savings of between 10 and 30% in the HVAC industry. To date, these FDD methods and tools have not been applied systematically to water infrastructure in buildings and thus it could provide the basis for significant innovation in managing water infrastructure. This paper outlines how a water wasting fault was found in a Rainwater Harvesting System (RWHS) within an engineering building at the National University of Ireland, Galway (NUIG) and how a concept FDD methodology may be applied to the RWHS to prevent such faults and water waste occurring in the future.