Chiller Plants in Hong Kong consumed over one-eighth of the energy consumption. Passive designed chiller plant with excess cooling capacity or redundant chiller(s) in small to medium-sized commercial buildings has always existed because of concern of reliability and the buildings aiming for basic operating criteria only. Since air-cooled chiller plants were widely used in commercial buildings in Hong Kong in past two decades, the situation of excess cooling capacity and opportunities of energy saving at chiller plant of small to medium sized commercial buildings are worth of studying. Four small to medium-sized commercial buildings in Hong Kong were selected and operating log sheets of the air-cooled chiller plants were collected. The cooling load profiles of the four buildings were developed and compared with design data. From the results, the phenomenon and level of excess cooling capacity of air-cooled chiller plant in small to medium-sized commercial buildings in Hong Kong was demonstrated. Once the situation of excess cooling capacity or redundant chiller(s) in small to medium sized commercial buildings was identified, some questions that came in mind were how to reduce the excess cooling capacity, increase the energy conservation and then maintain the reliability of the chiller plant at the same time. It involved some changes in design, operation and maintenance. There are a lot of correlations amongst the actions taken to support the changes for achieving the objectives. Therefore, an optimal and life cycle model for design, operation and maintenance (OLDOM) of air-cooled chiller plant for small to medium-sized commercial buildings in Hong Kong - for optimization of energy saving of the chiller plant in its entire life was developed. The existence of excess cooling capacity is not because of the wrong calculation; it is mainly due to provision of too much safety margin to maintain the reliability. In this research, making use of the physical characteristics of air-cooled screw chillers to size the chiller plant is introduced and discussed. It reduces the level of excess cooling capacity and maintain reasonable safety margin in sizing of chiller plant. The saving in capital investment of redundant chiller is used to adopt integrated design and install metering and monitoring devices for energy management in operation. Developers, electrical and mechanical consultants, operation and maintenance professionals, and commissioning authority have to participated and work together at the design stage in application of the OLDOM model. The communication and commitment amongst them to enhance the operation and maintenance is important and highlighted. Integrated design is discussed and recommended to increase the energy efficiency in air-cooled chiller plant, including buying efficient and reliable chillers at the very beginning, applying unequally sized chillers in series chiller plant configuration at low flow design with variable primary flow and chiller plant control at the system level. The additional costs spent in integrated design can be offset by savings in capital investment of redundant chiller(s). It enables developers and consultants to adopt the OLDOM model to their future development without a budget concern. The change in design concept enriches the requirements in operation and maintenance. The operators focus on maintaining the chiller plant in normal operation rather than effective and efficient operation is observed during the data collection. With the availability of appropriate metering and monitoring devices, and chiller plant control or building management system, the energy efficiency of chiller plant can be continuously optimized throughout its life time with assistance of experienced operators or Energy Manager. The life cycle cost savings are significant and important in energy conservation. The approach is more effective than the ad hoc and top down energy saving projects. It also enables the systems of the buildings to be reviewed and re-commissioned from time to time. To further support the reliability of the chiller plant without redundant chiller, a risk-based preventive maintenance strategy is applied. The specific risk level of each component of the air-cooled screw chillers is evaluated and developed by conducting a survey with the local industry. The developed risk levels are compared and high risk components are highlighted for better planning of the maintenance schedule or optimal inspection period. It will reduce the frequency and possibility of failure of air-cooled chillers for maintaining adequate cooling capacity of the chiller plant, at or above the defined threshold level, without regard to the time of cooling required. With the risk-based preventive maintenance, all maintenance and services will be well scheduled and the frequency of emergency shut down of the chillers will be minimized. In the long run, the repair cost will be lowered. This optimal and life cycle model for design, operation and maintenance (OLDOM) of air-cooled chiller plant would be an integrated and cost effective approach in comparison to the current discrete practices.