This paper deals with an energy and exergy evaluation and modeling of geothermal district heating systems for their system analysis, performance evaluation and optimization. As a comprehensive case study, the Afyon geothermal district heating system (AFJET) in Afyon, Turkey is considered and actual thermal data are collected and employed for analysis. Using actual system data, an evaluation of the district heating system performance, energy and exergy efficiencies, and exergy destructions in the system are conducted in this regard. This study is also conducted to show how energy and exergy efficiencies of the GDHSs will change with the reference temperature and how exergy losses will affect by the temperature difference between the geothermal resource and the supply temperature of the district heating distribution network. In addition, the negative effects of discharge waters of the AFJET are presented. The energy and exergy efficiencies of the entire AFJET are found to be 37.59% and 47.54%, respectively. The results are expected to be helpful to researchers and engineers in the area. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.