Afyon geothermal district heating system (AFJET) provides heating to 4519 residences, covering an area of 513,683 m(2). Due to limitations in reinjection capacity, geothermal waters are released to the Akarcay Stream, detrimentally affecting the environment. Optimum heating load of the system was determined for a given ambient conditions with respect to different outdoor temperatures. Usage of AFJET was found to be higher than the optimum consumption rates. Optimizing the usage of geothermal water will decrease operational cost, increase equipment life-span, and reduce environmental pollution. Published by Elsevier Ltd.