Experimental Investigation of Dynamic Operation and Performance Limits of ASHP-Driven Radiant Floor and Fan Coil Heating System

This study investigates the operation of an air source heat pump (ASHP) working with combined radiant floor (RF) and fan coil unit (FCU) heating systems in hot summer and cold winter (HSCW) regions. Intermittent heating demands and ASHP sensitivity to supply water temperature in these regions lead to insufficient steady-state assumptions, while experimental evidence on transient heating behavior, thermal comfort development, and operational limits remains limited. In this study, experiments were conducted to analyze six supply water temperatures (ranging from 35 °C to 45 °C) with respect to the system’s dynamic thermal response, vertical air temperature difference, floor surface temperature, power consumption, and coefficient of performance (COP). The results show that start-up heating is dominated by FCU convection, causing pronounced vertical temperature stratification, while radiant heat becomes dominant as the system approaches steady operation. A good vertical air temperature difference with respect to breathing zones and ankle-level temperature differences below 2 °C was achieved after sufficient operating time. Increasing the supply water temperature accelerated the heating response, where the time required for the average indoor temperature to reach 18 °C decreased from 5.5 h at 35 °C to 2.2 h at 45 °C. However, this improvement was accompanied by reduced energy efficiency, with the mean ASHP unit COP declining from 2.5 to 2.3. Excessively high supply temperatures further induced premature indoor overheating and the frequent start–stop cycling of the heat pump, thereby limiting thermal benefits and increasing power demand. These findings provide experimentally grounded insight into the operation and performance limits of ASHP RF–FCU heating systems.