Plate heat exchangers (PHEs) are an important part of a geothermal heating system. If titanium is the material used for the PHE, then the latter will cost from about 25-30% of the total cost of the heating system. This paper discusses the integrated optimal design of the materials, placement, size and flow-rate of a plate heat exchanger. With an input of data on the application technology, and economic and environmental parameters, a computer can be used to design the optimal placement, size and flow-rate of the PHEs. With this type of calculation, the computer can provide values for PHE size, flow-rate, flow paths and flow channels, with a minimum heat transfer area for a basic heat requirement. The paper also illustrates how the choice of PHE size, and the placement of the flow paths and flow channels influences the heat transfer area, producing a different design from that of a practical application (Tianjin, China). By optimizing its design, the PHEs will effectively be smaller than the real example given, with a consequent reduction in cost. (C) 2003 CNR. Published by Elsevier Ltd. All rights reserved.