Historical Turkish baths still keeping their functional systems represent their original architectural and building technologies. Studies on the functional systems of historical baths are therefore needed to discover such technologies and to maintain them in working order. This study was conducted on a 15th century historical Turkish bath, Seng&uuml / l Hamami, in Ankara for assessment of its water supply and drainage systems. These systems comprised of hot and cold-water supply, wastewater and rainwater drainage, were examined in terms of their adequacy, capacity and faults. Comparisons were made with certain other historical Turkish baths to determine their original water storage and consumption capacities. The investigations were made by using non-destructive methods. Among them, the calculation methods used for discharge capacity assessment of drainage systems in Seng&uuml / l Hamami and consumption capacities of its water supply system were adapted and developed from the calculation methods used for contemporary buildings. Serious dampness problems arising from certain roof drainage faults were identified in the building. The wastewater collection and discharge system was found to have serious problems due to improper grading and inadequate flow dimensions of drains and wastewater channels. The ideal situation for the improvement of discharge systems was discussed together with some suggestions on the urgent remedial interventions, preventive measures and future improvements. The methods developed in this study were considered useful for the calculation of adequacy and capacity of wastewater and roof drainage systems and of water storage and consumption capacities of water supply system for the other historical baths.