Following the international Paris agreement (United Nations FCCC, 2015) and the Dutch derivative “Climate Agreement” (Dutch Central Government, 2019) various sectors, including the heating sector, need to improve on sustainability. Currently 90% of the Dutch households are heated by individual natural gas boilers, causing CO2 emissions, while only ~4% is connected to a district heating system (DH-system). DH-systems can be based on several types of heat sources, not all equally sustainable. The sustainability of geothermal heat, aquathermal heat and residual heat is explored in this study. Through literature study and expert judgement, the impact on ecology, emissions and security of supply of these three heat sources have been inventoried, leading to a set of sustainability criteria. Literature based classification of these criteria is combined in a Decision Support System: a model designed to determine the sustainability of a heat source when used in a DH-system. The model also takes the environmental situation and the characteristics of a chosen heat source into account. The usability of the model is tested and verified by applying it to practical examples: heat sources in the regions of Leeuwarden, Twente and Helmond (NL) and was found to be workable and useful for actual situations. The model can be modified, fine-tuned or extended as desired.