One of the most important simulation systems in the Building Energy and Environments labs is a precisely controlled 6 × 4.5 × 3 m environmental chamber. This chamber serves as a test room where certain groups of indoor parameters are controlled while others are measured. The airtight and insulated walls isolate the chamber from external influences, and provide a precise control of mass and energy balances in the chamber space. One external wall contains a hydronic cooling / heating system that mimics windows and impacts of the outdoor environment. The reconfigurable digital control system can precisely control airflow, temperature and relative humidity for steady-state and variable heating/cooling loads in the chamber.