The electrical power has been applied in many applications extensively since the industrial revolution and becomes the main energy source in the 21th century. Because of the fast economic growth, the electrical power demands have increased extensively in the past 30 years. In recent years, number of semiconductor factories have been built in Taiwan and bring the electrical power not enough in the summer. Because the climate is hot and humid in Taiwan, energy consumption of air conditioning is half of total consumption of the building, and the energy consumption of chiller plant is more than 30% of total consumption of HVAC system. Based on field measurement data, the air-conditioning system consumes more than 40% of the building total energy use, followed by 30% of lighting, and 30% of miscellaneous pumping systems.. Among them, 60% power was consumed by chiller plants, while the chilled water pumping and air side equipment each accounts for another 20%. It is no doubt, that the energy efficiency improvement of chillers is one of the most important items in building energy conservation. The capacity of HVAC system in many Taiwan¡¦s big buildings has the situation that the designed capacity is much larger than the actual required, and this Oversize Design of HVAC system caused waste of extra equipment and cost. In this content, we offer improvement methods that are predicated on the HVAC system waste situation, and provide the prediction of efficient. As most of HVAC system in existing building have Oversize Design problem, it caused HVAC engines are with low efficiency and loading, and energy wasting is getting worse and worse. Besides, some existing engines may keep running over 10 years; the performance is far behind the new type engine. According to the existingsystem¡¦s running status, we can have the precise HVAC¡¦s loading, and redesign the conformable HVAC¡¦s engine type to make it is under efficient operation; furthermore, in order to be energy-saving, we design the best proper sets based on loading capacity, and change to new type HVAC engine with high performance, also, we improve the design of water system.