Optimizing supply conditions and use of return air in UFAD system: Assessment of IAQ, thermal comfort and energy performance

•There is a need to reduce energy consumption in UFAD systems while maintaining acceptable IAQ and thermal comfort.•A parametric study was performed using CFD simulations to optimize the supply conditions as well as the return air ratio.•Results may be put in practice to give recommendations on UFAD optimum design. This study is the first to optimize the use of return air, along with other supply conditions, in underfloor air distribution (UFAD) systems to achieve acceptable indoor air quality (IAQ) and thermal comfort while minimizing energy consumption. A comprehensive 3D computational fluid dynamics (CFD) model has been developed to simulate airflow patterns, thermal fields, and CO2 distribution in a standard office environment. The CFD approach used in this research has been validated against experimental data found in the literature. A parametric study was conducted, varying supply diffuser size, supply air velocity and temperature, and the return air ratio to identify the optimal UFAD system settings. These settings aim to minimize energy use while keeping CO2 levels within the recommended 1100 ppm in the breathing zone and ensuring that the percentage of people dissatisfied (PPD) remains at 10%. The findings indicate that using a 20 cm × 20 cm supply diffuser, setting the supply air temperature to 17°C, supply velocity to 1.2 m/s, and recirculating 29% of the return air will result in optimal performance.