With the recent advancement in microprocessor technology, many programs have been developed in the area of hydronic systems for heating and cooling applications. Most of them are meant for pipe and valve sizing and cost estimating. Primarily these programs are based on data manipulations and are lacking in overall system design objectives. Hydronic systems are almost invariably designed based on maximum design conditions. In reality, however, a system operates at the design condition for only a fraction of the total operating time. Hence, it would be prudent to establish design criteria enabling a system to operate at maximum efficiency under part load conditions. The program discussed establishes such criteria and carefully analyzes controls and balancing requirements for a given hydronic system.