Field investigation of the impact of different terminal devices on thermal comfort and ventilation uniformity

JC STEM Lab of Healthy Built Environment, Department of Building Environment and Energy Engineering, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, China

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

Indoor thermal comfort and air quality in buildings significantly depend on heating, ventilation, and air conditioning (HVAC) system performance, particularly at the final stage through terminal units. Current studies have not comprehensively evaluated advanced terminal unit performance regarding indoor thermal comfort and air quality under both heating and cooling modes. To address this gap, this study presents a field investigation of three terminal systems in an office building located in Suzhou's hot summer and cold winter climate: radiant surface systems (RSS), gravity cabinet units (GCU), and fan coil units (FCU). Thermal performance is evaluated through vertical air temperature stratification, while ventilation uniformity was assessed using local air change rates (ACH) determined by the CO₂ decay method. The results indicated that all three terminal systems provided satisfactory thermal comfort, maintaining vertical temperature differences between ankle and head levels below 2 °C in both cooling and heating seasons. However, pronounced seasonal performance asymmetry was observed. The FCU exhibited the most consistent performance across seasons. The GCU achieved superior thermal uniformity during cooling but experienced the largest vertical stratification during heating. In contrast, the RSS demonstrated outstanding heating performance with minimal stratification, while slightly increased stratification occurred during cooling. Ventilation measurements revealed that ACH uniformity was higher at lower heights during cooling and higher at upper heights during heating, reflecting buoyancy-driven airflow behavior. Remarkably, the RSS delivered the most uniform air distribution during heating, whereas its ventilation uniformity was reduced during cooling relative to the GCU and FCU. These findings highlight the importance of accounting for seasonal performance differences when selecting terminal devices and provide practical guidance for HVAC system design in hot summer and cold winter climate regions.