The optimal configuration, that which achieves the minimum steady-state rates of heat loss from, or gain by, the supply pipe for a district-heating (DH) or district-cooling (DC) system under the considered conditions has been determined experimentally. By employing the displacement ratios of the two pipes as the experimental variables, the optimal arrangement of the supply and return pipes was deduced, for each set of performance requirements, to occur at displacement ratios of 0.70 and - 0.05 for the DH system and , 0.08 and 0.67 for the DC system. For both systems, the warmer pipe should be in the upper region of the atmospheric-pressure air-filled trench and the cold pipe vertically below it, the pipes being symmetrically placed with respect to the side walls. This optimal configuration differs significantly from the traditionally adopted side-by-side configuration of district heating and cooling (DHC) pipelines and should prompt designers to become more aware of the prospective energy and financial savings that can be achieved by choosing the optimal thermal design.

Units: Dual