The purpose of this study is to realize energy efficient features for a dedicated desiccant air-conditioning system throughout the season, and propose a new high-efficiency application of this system. Many advanced air-conditioning systems are intended to reduce energy and/or space only for sensitive heat conditioning. During the dehumidification season, the results show that a latent heat conditioning system using a desiccant air-conditioner achieves over 20% higher energy efficiency than conventional systems. A new humidification system that combines a desiccant outdoor air-conditioning unit and heat-pump is proposed for the winter season. The experimental results show that the humidification performance satisfies the minimum demand for an outdoor air conditioner in many areas of Japan, but the energy efficiency is slightly inferior to that of traditional boiler systems. Some improvements in the structure and operation of this system are suggested to improve the higher energy efficiency.