Lab Evaluation of Downward Capacity of Radiant Ceiling Panel Systems: U.S. Department of Energy (DOE), Energy Efficiency & Renewable Energy (EERE)

This project investigated the cooling delivery effectiveness of radiant ceiling panels as a function of attic insulation level using multiple laboratory testing and analytical methodologies. Delivery effectiveness is the heating or cooling energy delivered to a conditioned space divided by the total heating or cooling energy added or removed by the space conditioning system. The lower the losses of the heating or cooling delivery method, the higher the delivery effectiveness. For ducted systems, delivery effectiveness is reduced by both air leakage and thermal losses (especially if the ducts are installed in attics), while the delivery effectiveness of a radiant system supplied by hot and cold water is only reduced by thermal losses, which can be mitigated by sufficient insulation above, or at the "back" of the panel. Being installed at or below the ceiling plane, sufficient back insulation should be provided by default in the form of the attic insulation above the radiant ceiling panels. Site-built radiant ceiling panels were evaluated at Frontier Energy’s Building Science Research Laboratory (BSRL) in a uniquely designed environmental test chamber with independently controllable indoor and attic spaces and a height-adjustable ceiling. Unfortunately, the assembly used for raising and lowering the attic was imperfectly sealed and insulated around the perimeter of the ceiling, resulting in potential energy losses that would not be as significant in actual residential radiant ceiling panel applications. The performance of these panels was evaluated in cooling at steady state with a range of insulation levels from R-19 to R-109. Test conditions were selected based on Frontier’s decades of field experience with radiant ceiling systems. Key data points included the air temperature in the indoor and attic space, radiant ceiling panel total heat flow, surface temperatures of all indoor-facing surfaces, and heat flux through the indoor-facing ceiling surface. This data was used to calculate the heat transfer rate between the radiant ceiling panels and the indoor space (the "downward" heat flux, including both radiative and convective components) using three methods: 1. The area-weighted average of the heat flux sensors. 2. The equations and methods provided in the ASHRAE Handbook (ASHRAE, 2020). 3. A simplified calculation model developed by Birol Kilkis, which is frequently cited in radiant system research (Kilkis, Sager, & Uludag, 1994). Multiple methods of determining the downward heat flux were used due to the potential for application uncertainty in heat flux sensors. This project successfully evaluated radiant ceiling panel delivery effectiveness in cooling mode at multiple insulation levels. Challenges were encountered when attempting to quantify edge effects around the radiant ceiling panels, which were much more impactful in a small test chamber than they would be in a real house with larger rooms. However, these edge effects were accounted for through a combination of lab tests and calibrated modeling using THERM.