While major reductions in pumping energy requirements can be achieved by adding drag reducing surfactant solutions in recirculating district heating and cooling (DHC) systems, special techniques are needed to enhance the reduced heat-transfer coefficients. In this paper, the use of ultrasonic energy to enhance the heat-transfer ability of drag reducing surfactant solutions is described. The surfactant solutions tested include two viscoelastic drag reducing surfactant solutions: Ethoquad T13-50/NaSal (5 mM/8.75 mM) (S1) and SPE98330/HCHO/Trilon A (1500 ppm/150ppm/500ppm) (S2) and a drag reducing surfactant solution Arquad S-50/NaSal (5 mM/12.5 mM) (S3) whose viscoelasticity cannot be detected by our rheometer. Significant heat-transfer enhancements were observed for the two viscoelastic drag reducing surfactant solutions, S 1 and S2, with ultrasonic energy while surprisingly, only small effects on enhancing the heat-transfer ability of S3 were observed. The effectiveness of ultrasonic energy in enhancing the heat-transfer ability of viscoelastic drag reducing surfactant solutions depends on the solution temperature, ultrasonic intensity and also the microstructure of the surfactant solutions. The relationships among solution viscoelasticity, vortex inhibition and heat-transfer reduction of the surfactant solutions are also discussed. (C) 2003 Elsevier B.V. All rights reserved.