This paper presents our implementation experience in building an energy efficient HVAC system for cooling and air conditioning. The system exercises the 'low exergy' theory and leverages high temperature water (18°C) cooling for better energy efficiency. In order to achieve this, the system decomposes the cooling and dehumidification functionalities, and employs decentralized air control for on-demand dehumidification and ventilation. The system comprises two control modules, namely, radiant cooling module and distributed ventilation module, cooperating with each other to provide the HVAC control. Abundant sensors and embedded control devices are customized and instrumented, and we develop a wireless sensor network to support control data exchange among those devices. Our experimental evaluation demonstrates that the system achieves accurate control targets and promptly responses to environment dynamics. The wireless sensor network effectively supports the system needs with long system lifespan. Compared with traditional HVAC systems, our system is of much higher energy efficiency, as measured by the standard Coefficient of Performance (COP) metric.