Hydronic solar systems with copper tube collectors or vacuum glass tube collectors are commonly used thermal-solar systems. These systems run well in residential buildings or in small scales for domestic water heating. These systems can be scaled up and applied to commercial buildings. The solar energy collected by these systems can be used to drive absorption chillers in summer and heat the feed water for boiler in winter. Properly designed and installed systems can save energy by utilizing free solar energy. However, operation and maintenance do have an important impact on whether these systems truly save energy. This paper presents a technical review of a large scale hydronic solar system installed in Carbondale, Illinois and compares this system with a photovoltaic system which was proposed to retrofit the hydronic solar system in energy conversion efficiency, system components, initial cost, operation and maintenance etc.