Establishing multi-criteria optimization of return vent height for underfloor air distribution system

Return vent height significantly influences energy consumption, indoor air quality (IAQ), and thermal comfort of rooms ventilated by underfloor air distribution (UFAD) systems. Previous research took the optimal return vent height as the one requiring the least cooling load among alternatives meeting requirements on IAQ and thermal comfort presented by standards (e.g., ASHRAE Standard 55, ISO 7730). However, the rankings among cases satisfying standards were determined solely by the cooling load, ignoring differences in other indices. Moreover, the supply rate and supply temperature were fixed among various alternatives in previous research, while they might be adjustable for demand-controlled systems. This study aims to optimize the return vent height of a UFAD system by a multi-criteria ranking preference method, technique for order preference by similarity to ideal solution (TOPSIS), based on evaluation indices covering cooling load, IAQ, and thermal comfort. The results are listed as follows: a) predictive mean vote (PMV) in the occupied zone (OZ) under all return vent heights are lower than −0.5 when the supply temperature is 18 °C, exceeding the suggested range for general thermal comfort; b) the optimal return vent height is 1.5 m ranked by the TOPSIS method; c) the optimal return vent height changes to 2.3 m when the supply temperature is adjusted to keep the occupied zone thermoneutral (i.e., |PMV|<0.5).