The purpose of this study is to utilize the concept of exergy to discuss the meaning of well water 'coolness' applied to a ceiling radiant cooling system. A house in Chiba was equipped with a ceiling radiant cooling system using well water, and with outdoor sun-shading on the South-facing window. We conducted measurements of indoor thermal environment and electric power consumption in the summer of 2001. The results showed the following : 1. The ceiling radiant cooling system consumed cool thermal exergy from well water and emitted a small part of it as cool radiant exergy from the ceiling radiant panel surface. 2. When cooling system was used, not only the ceiling radiant panel but also the surrounding walls and floor contributed to the radiant cooling effect by emitting cool radiation exergy. 3. The well water pump consumed about 56 times more exergy than the ceiling radiant panel and the circulation pump consumed about ten times more exergy.
