Factors influencing the heat loss behaviours of horizontal [`]supply' and [`]return' hot-water pipes, within a rectangular trench, are considered (see Fig. 1). The use of such atmospheric-pressure air channels for district-heating systems is prevalent in the UK in order to facilitate drainage and evaporation. By optimising the location of the two pipes within the rectangular-sectioned trench, it is possible to achieve considerable reductions in the steady-state rate of heat loss from the supply pipe compared with conventional practice. Recommendations for future designs are presented.