One of the most fundamental decisions to be made by a designer of a new district heating system is the selection of design operating temperatures. Lower operating temperatures will reduce the cost of heat production from a CHP plant, but to achieve lower temperatures requires additional investment in the heating systems within the buildings. The cost of the district heating network is reduced if the temperature difference between flow and return is maximised. There is therefore a need to establish the optimum design temperatures to achieve the most cost-effective scheme.

The results are presented in a series of graphs for each case study analysed showing the cost of heat against the design temperature difference for different flow temperatures. It was found that, for all cases, it was not worthwhile to reduce the design flow temperature below 90°C as this leads either to a smaller temperature difference and therefore higher network costs or, if the temperature difference is maintained, additional costs for larger radiators. Both of these cost penalties are more significant than the small reduction in heat production cost obtained with using lower flow temperatures.

However, there are many other potential benefits from using lower temperatures, in particular the ability of the district heating network to utilise low grade heat sources available from industry, solar heat and heat pumps.