Reliable and cost effective operation of district heating system led the designers to the decision to select three sorts of primary fuel as raw materials for the production of heat and electric power. In combined production in Termoelektrarna - toplarna Ljubljana (TETOL), different coals are used, whereas middle weight fuel oil is used in peaksources. Such selection of fuels prevents us from various crisis situations in fuel supply emerging in individual periods as well as from sporadic price incongruity between various fuel sorts (world energy crisis - high oil prices) .
The diagram shows the increase of heat production from the CHP station Termoelektrarna - toplarna Ljubljana (coal). Such an increase of cheaper source provided the possibility of building a "transitory" primary hot-water network connecting both heating stations.