Cities need to become independent from finite resources, among which fossil fuels. There is great potential to save energy is many cities, however the generation of power is insufficient due to the limited space as a result of urban densities. An energy source less commonly explored is reuse of waste heat from various urban functions. In that matter, central heat grids, powered by industrial waste heat at high temperatures, are well-known, but the lower-caloric heat from buildings as offices, supermarkets, swimming pools and also dwellings is abundant and seldom utilised. This has a two-sided negative effect: energy is lost and the city is heated up, which in summer can aggravate the urban heat island effect, which statistically leads to higher mortality rates. The Rotterdam Energy Approach & Planning (REAP) was developed to support energy-neutral urban planning in a structured, incremental way. The approach was used in several studies and has received worldwide acclaim. Especially the second step of the REAP method, regarding tuning, exchanging, cascading of energy at various scales, is relatively new, offering new opportunities for cities. Nevertheless, REAP needed to be elaborated further, for practical application in actual urban redevelopment. This was done for the transformation of the Rotterdam city harbours, in a study called REAP2. The research project, culminated in the proposition of five principles for a heat system in the urban district, including the introduction of novel techniques for the exchange of different temperature levels of heat and involving the spatial plan based on the energy system chosen. The paper will discuss REAP, the REAP2 research project, the methodology involved, proposition of energy system principles, novel concepts for heat exchange, and spatial consequences.