This paper suggests an operation method for the latest fixed-speed and variable-speed chillers based on characteristics that include energy conservation. The effects of new planning and operation method suited for the load of air conditioning heat source systems are shown in the cases of the variable-speed chillers and the combination cases of the variable-speed and fixed-speed chillers. High-performance data based on actual measurements are used to evaluate the energy conservation. In addition, this paper provides a new planning and operating method for a whole heat source system, including a new estimation method of cooling tower performance, which is a very important element.