As in many other countries in the world, Japan has witnessed an increased focus on low-energy buildings.For testing different engineering solutions for energy-efficient buildings, a low-energy building was builtat the University of Tokyo as an experimental pilot project. In this building, a radiant heating/coolingceiling panel system is used. However, no standard exists for the in situ performance evaluation of radiantheating/cooling ceiling systems; furthermore, no published database is available for comparison. Thus,this study aims to not only clarify the system performance but also to share our experience and our resultsfor them to serve as a reference for other similar projects. Here, the system performance in relation toits heating/cooling capacity and thermal comfort has been evaluated. The heat transfer coefficient fromwater to room was 3.7 W/(m2K) and 4.8 W/(m2K) for heating and cooling cases, respectively. The upwardheat flux from the panels was found to be as large as 30–40% of the water heating/cooling capacity; thiswould translate into heat loss in certain operating modes. Several proposals for reducing the upwardheat flux were discussed. The measurements also showed that a category B thermal environment wasobtained using the radiant ceiling heating/cooling system.© 2014 Elsevier B.V. All rights reserved.