Performance and indoor environment study of convective-radiant coupled terminals: Insights from start-up and steady-state control modes

The efficient design of heating terminals that ensure thermal comfort is critical for advancing the built environment. Convective/radiant (C/R) coupled terminals offer a promising solution by combining the rapid response of convective heating with the enhanced comfort provided by radiant systems. However, current research has primarily focused on development and manufacturing, with limited investigation into operational steady-state and start-up control strategies. This study investigates the indoor environmental and thermal performance of C/R terminals in a controlled climate chamber. Experiments were conducted to assess multiple start-up modes, including convective-only heating, C/R with fixed condensing temperature, variable frequency, and low-temperature start-up mode. Different steady-state control strategies were also analyzed, focusing on return air temperature, central air temperature, central and local operative temperature controls. Key findings indicate that in variable frequency mode, heating output decreased by 11.1 %, while energy efficiency improved by 4.1 % compared to the fixed condensing temperature mode. The time required to reach the set-point temperature increased by 10 min, with a time-matching coefficient of 0.932 of fixed condensing temperature mode, reflecting efficient heat output utilization. Compared to convective-only mode, the C/R terminal demonstrated superior performance during the start-up stage in both time response and efficiency. Additionally, local operative temperature control resulted in a 16.5 % reduction in heat output compared to central operative temperature control. This study provides actionable insights into optimizing operational control strategies of C/R terminals, highlighting their potential to improve indoor comfort and energy performance in intermittent heating scenarios.