This paper presents an experimental study to develop a new control algorithm to replace the traditional on-off control algorithm widely used by direct expansion (DX) air conditioning (A/C) systems, which are widely used in small to medium sized buildings. The new control algorithm, which enables both the compressor and the supply air fan in a DX A/C system to operate at high speeds when the indoor air dry bulb temperature setting is not satisfied and at low speeds otherwise, is termed the H-L control. Extensive experiments have been conducted with an experimental DX A/C system at simulated different climates under both H-L and on-off control. The results suggest that the use of H-L control would result in better control performance in terms of an improved indoor humidity level and a higher energy efficiency of the DX A/C system when compared to the use of the traditional on-off control. The H-L control is also simpler, and its associated hardware cost is much lower than that of the control algorithms based on variable speed technology. (c) 2007 Elsevier Ltd. All rights reserved.