Owing to their complexity, the design and efficient operation of HVAC systems in commercial buildings can be greatly enhanced by using accurate thermal simulation models. The DOE-2 model was validated using monitored data for a large (28,000 m(2)) commercial building. The model provided a means to evaluate some conservation measures which included window glazings, occupancy sensors, cold deck temperature set point and reduced ventilation air. Reductions in cold deck temperature and ventilation air were attractive as no-cost options which could be easily returned to original settings if problems arose. The potential energy savings for occupancy sensors was substantial and could prove economical.