Since the energy crisis of 1973, engineers have endeavoured to implement energy conservation in buildings. Unfortunately, their efforts have often resulted in energy saving which ignores the fundamental delivery of indoor satisfaction. Hence, either energy is conserved sacrificing indoor environmental quality (IEQ), or additional energy is consumed for the cooling of occupancy space. These misconceptions have prompted the development of an integrated design and operation protocol based on a so-called Building Environmental Performance Model (BEPM). The Building Environmental Performance Model links the IEQ and the building energy consumption together. It treats a building as a system. Energy consumption in the building services systems is the input to this system with the IEQ as the output. The BEP model incorporates two main modules: an adaptive comfort temperature control module (ACT) and a new CO2 demand control module (nDCV). These two modules (ACT and nDCV) take an innovative approach and help maintain satisfaction levels as well as optimum energy consumption. The BEP model was built into an algorithm for implementation in a Direct Digital Control system. It was used in a longitudinal monitoring study in a typical Hong Kong office, so that the usefulness of the model could be verified. It was found that the BEPM was able to save up to 15% of total energy used, and maintain the occupant dissatisfaction rate of thermal comfort and indoor air quality at around 20%. The incentive of the BEPM can be deduced from a simple life cycle costing analysis. It assumes an inflation rate and bank interest rate of 8 and 10%, respectively, the total net present value (NPV) for the original airside system is higher than that employing the BEPM by HK$ 19,700,000 for 20 years of operation. (c) 2004 Elsevier B.V. All rights reserved.