Hydraulic heritage systems, both underground and exposed, have been known to be sustainable for millennia. Persian and also Roman aqueducts are examples of such hydrosystems. Their value is often overlooked but they have undeniable advantages: they have functional interconnectedness with their surrounding society and ecology, which sometimes leads to revitalization plans. By using the notion ‘qualifying role’, this paper will raise questions concerning the disregarded functions and early and historical positions of hydraulic heritage systems. This article illustrates the qualifying role of Qanats in urban drainage by describing the skill in their planning and construction. This is shown by a problematic case study in Iran, where the construction of a drainage system modelled on bygone Qanat techniques resulted in a dramatic drawdown in the water level of the area soon after construction.