Experiments were conducted on a vertical flow packed bed sensible thermal storage system. The packing consisted of hollow high density polyethylene spheres filled approximately 95% with water, and water was also used as the working fluid. Experimental results show the effect of altering D-E (the equivalent sphere diameter of the packing) on the degree of axial dispersion in thermally short packings. The significance of thermally short versus thermally long packings to designers of sensible storage packed bed systems is discussed. Experimentally measured average temperature wave velocities in the packing are presented The significance of natural convection during heat exchanger operation is also demonstrated experimentally. Natural convection may explain instabilities in packed bed convection coefficients reported in the literature. It is shown that one-dimensional temperature profiles in the packing can be obtained using rectangular storage tanks in conjunction with flow distributors.