ABSTRACT The COP of the latest fixed-speed centrifugal chiller is more than 6.4 and that of the latest variable-speed centrifugal chiller driven by an inverter reaches a 21.9 COP. This tremendous high performance is very effective for advanced heat source systems. Many energy conservation studies have been conducted on industrial heat source systems in Japan. But there have been few reports on the heat source systems for business use. This paper suggests an operation method for the latest fixed-speed and variable-speed chillers based on those unique high-performance characteristics. The effects of new planning and operation method suited for the heat source system for business use. 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.